U.S. Postal Service / Zip Code Information
Through recent discussions with the United States Postal Service (USPS), all zip codes assigned to Wildwood are now identified with Wildwood as the default city. This represents a big change for Wildwood, which has a total of 7 zip codes, 5 of which are shared with nearby municipalities.
On May 2, 2017, Wildwood obtained agreement from USPS to take the following actions that will identify Wildwood as the corresponding city for its zip codes:
• First, the two zip codes that serve only Wildwood locations, 63038 and 63040, will now automatically identify Wildwood as the default city.
• Second, the remaining five zip codes that are shared with other nearby municipalities will be identified as Wildwood when the ZIP+4 exchanges assigned to Wildwood are used. These include 63005-xxxx, 63011-xxxx, 63021-xxxx, 63025-xxxx and 63069-xxxx.
Customers who wish to lookup their ZIP+4 can go to the United States Postal Service website and select “Look up a ZIP Code” under the “Mail and Ship” dropdown menu. Using a complete address (including the 9-digit ZIP Code) on all correspondence will not only better identify your City name, but will also increase efficiency and reduce delivery times.
Beginning on June 15, 2017, businesses and other senders likely began updating their respective databases to include the default Wildwood address. However, it can take up to a year before certain industry mailing lists are updated, depending on the reprocessing frequency they choose.
Changing Postal ZIP Code Boundaries
Constituents often turn to members of Congress for assistance in securing changes to ZIP Code boundaries, usually because their mailing addresses do not correspond to the geographic and political boundaries of their municipalities' jurisdictions. This report explains why ZIP Code boundaries often are not aligned with geographic political jurisdiction boundaries, describes some problems that may occur because of the misalignment, and discusses efforts by the U.S. Postal Service (USPS) and Congress to address these problems.
The Post Office Department (now the U.S. Postal Service) began dividing large cities into delivery zones in 1943, inserting two digits between the city and the state in the lower address line. In 1963, the whole country was divided into five-digit postal delivery codes—termed ZIP Codes by the Post Office. These codes corresponded to the post offices where final sorting of mail was done and from which letter carriers were dispatched to make deliveries. The term ZIP Code, originally trademarked and always capitalized, was an acronym for "Zoning Improvement Plan." Mass mailers were first required to use ZIP Codes in 1967, and today their use is ubiquitous.
Almost all mail is sorted by machines, and the basis for this sorting is a ZIP Code. ZIP Codes have expanded through the years to 9 digits (ZIP+4) in 1983 and to 11 digits in 1991. Most customers know only their five-digit ZIP Codes. The first number in the ZIP Code represents a general geographic area of the nation—moving from a "0" for places in the east to a "9" for locations in the west.1 The second and third numbers indicate regions of the United States, while the fourth and fifth digits route the mail to specific post offices. For example, the ZIP Code for Alturas, the county seat of Modoc County in the northeastern corner of California, is 96101. The 9 directs the mail to the west. The 61 directs mail to the processing facility in Reno, NV, which is the distribution point for some California post offices such as Alturas, Cedarville (96104), Fort Bidwell (96112), and Likely (96116). Reno is also the processing facility for ZIP Codes in Nevada beginning with 894, 895, and 897. The four final ZIP Code numbers, which were added in 1983 "allow mail to be sorted to a specific group of streets or to a high-rise building."2
ZIP Codes Are Widely Used Outside USPS
The Postal Service has contended that the ZIP Code system's only purpose is to facilitate the efficient and orderly delivery of the mail. Nevertheless, ZIP Code information is readily available to the public, and both private and governmental entities have found it a convenient and accessible tool for many purposes unrelated to mail delivery. Postal Service competitors like FedEx and UPS use the ZIP Code. The ZIP Code also has been adopted for non-delivery purposes, such as providing a convenient, yet sometimes imperfect means of targeting populations for performing demographic research, setting insurance rates, estimating housing values, remitting state tax revenues back to localities, and directing advertising messages. USPS works with state and local authorities as well as private companies to better align ZIP Codes with both postal and non-postal needs.
Because ZIP Codes are based on the location of delivery post offices, they often do not correspond to political jurisdiction boundaries. This means that millions of Americans receive their mail from post offices in adjacent towns, villages, or neighborhoods. Their mailing addresses may not reflect the name and ZIP Code of the jurisdictions where they actually live. This situation was not uncommon when ZIP Codes were first assigned nearly 50 years ago, and it has become more common since then—particularly in rapidly growing suburban areas. The boundaries of many jurisdictions have changed with growth, annexation, and the incorporation of new communities. At the same time, USPS has sought to reduce rather than expand the number of post offices as its retail business model has changed.
Problems Caused by Misalignment with Municipal Boundaries
The widespread use of ZIP Codes for non-postal purposes has exacerbated problems for those postal patrons whose mailing addresses do not match their actual towns or cities of residence. The following is a sample of the problems that have been brought to congressional attention:
- higher automobile insurance rates for drivers who live in the suburbs but are charged city rates based on their ZIP Codes;
- residents who are confused about where to vote in municipal elections because they do not distinguish between their voting and mailing addresses;
- sales tax revenues rebated by states to the cities where they are collected often being misdirected because they are collected by merchants with ZIP Codes in different jurisdictions, or by merchants who mail their products to customers knowing only their ZIP Codes;
- individuals being sent jury duty notices when they are not eligible to serve based on their actual residences;
- emergency service vehicles being misdirected by confusion over what town a call has come from, based on mailing address information; and
- homeowners in expensive neighborhoods complaining that their housing values are diminished because their mailing addresses place them in less prestigious communities.
In addition, a community may lack a delivery post office and complain that the need to use mailing addresses from neighboring towns robs them of their community identity. For example, even though Haddon Township, NJ, is an incorporated municipality with a 2009 estimated population of 14,368 people,3 it has no delivery post office, and its residents receive mail from the Camden, Haddonfield, Gloucester City, and Mount Ephraim post offices—each with a different ZIP Code.
Congressional Hearing Registers Concern
A host of ZIP Code misalignment problems were aired in a 1990 hearing of a House postal subcommittee.4 Ten members of Congress described ZIP Code alignment problems in their districts, and similar statements were received from many local governments, as well as the National League of Cities. The hearing in the 101st Congress considered three bills (H.R. 2380, H.R. 2902, and H.R. 4827) that would have allowed local governments, rather than the Postal Service, to determine local addresses or ZIP Code boundaries as a solution to the widespread problems.
USPS expressed strong opposition to these bills and said that depriving USPS of control over "the most basic tool of the postal trade—the mailing address" would be "disastrous."5 A USPS boundary survey found that more than 11 million deliveries6 were served by carriers who cross municipal boundaries, and estimated that if delivery boundaries were realigned to match municipal boundaries, 1,600 new postal facilities and 10,500 new carriers would be needed.7 Also to be considered was the availability of additional ZIP Codes in certain large areas. At of the end of 1989, 924 of the 1,000 possible three-digit combinations already had been assigned; in 20 areas, 90 or more of the 100 possible ZIP Codes already had been assigned; and in Houston, all 100 possible ZIP Codes had been used.8
These arguments may have proved persuasive because the legislation never advanced, and neither have similar bills introduced in later Congresses. At the hearings, however, USPS also earned some criticism because of its "peremptory denials" of local suggestions or requests for ZIP Code changes that were variously characterized as "cold and haughty," "cursory," "unresponsive," "stonewalling," and "uncaring."9 The Government Accountability Office (GAO, then the General Accounting Office) examined postal case files on 26 municipal requests for ZIP Code changes, only 2 of which were approved by USPS. GAO reported that USPS not only could do a better job of providing facts and reasoning to explain its decisions in individual change requests, but also could "do more to ... resolve problems caused by conflicts between municipal and ZIP Code boundaries."10
Postal Service Attempts to Resolve Problems
Current USPS Process for Realigning ZIP Codes
In the years since the 1990 hearing and GAO's investigation, USPS has made a concerted effort to develop a process for the regular review of ZIP Code boundaries. Under Section 439 of the Postal Operations Manual,11 the manager of the District Office's Address Management System (AMS) is responsible for reviewing "and monitoring delivery growth patterns, facilities planning, and any other factors" that may affect ZIP Code boundaries.12 Increased growth in a geographic area is the most common precipitating factor in such USPS-initiated ZIP Code changes. USPS has established criteria and thresholds for ZIP Code changes, which include, but are not limited to, the establishment of 25,000 new deliveries13 or more than 55 carrier routes.14 ZIP Code changes are invariably sensitive locally, and often involve considerable coordination and investment, so USPS requires approval from the district manager, the manager of operations programs support, the manager of processing and distribution, and the district manager of customer service and sales before a proposal can be sent to the Area (regional) Office for approval.
Most of the required ZIP Code change request analysis is based on operational considerations internal to USPS. One of the questions a manager of the District Office's AMS must address, however, is whether municipal boundaries will be crossed. The manager must also consider whether municipal officials have been asked to comment on the revised boundaries. The new boundary review process requires that "officials should consider municipal boundaries and customer interests in all zone splits. If a ZIP Code that is being considered for adjustment crosses municipal boundaries, consult municipal offices before submitting the proposal, and consider all reasonable solutions."15
Process for Considering Requests from a Community or Municipality
The process for considering requests from municipalities and community groups for ZIP Code changes dates to March 1991—not long after the congressional hearing referenced above. It has taken some time for the process to become a settled practice, and for USPS to adopt a willingness to consider requests for boundary adjustments that are based solely on "community identity" concerns. A key event was a November 18, 1999, directive to the vice presidents in charge of each of the nine postal areas from John E. Potter (who later served as Postmaster General, but then served as senior vice president for operations) and Deborah Wilhite, senior vice president for government relations and public policy. The memorandum noted that a review of correspondence with the public on the issue of ZIP Code changes "has indicated a need for general improvement." The memorandum then emphatically reemphasized the expectation that USPS would give careful, objective consideration to community wishes, even if they were based solely on "identity" considerations.
As indicated when the Review Process was first implemented in 1991, "just saying no" does not make identity issues go away. In fact, growth and the increasing use of ZIP Codes as database links and demographic tools tend to make them worse over time. If you receive a municipal identity request and a reasonable means of full or partial accommodation can be identified, offer it, apply the customer survey process, and move on. Requests can be denied, but only based on appropriate, objective reasons that are consistent with the Review Process....
(P)ostal policy is to offer any reasonable administrative or operational accommodation that can correct, or alleviate, the municipal identity concerns. The objective is to find ways to say "yes," not excuses for saying "no." Do not deny a request out of concern that "other communities will want the same thing." Others will make requests.... In the case of identity, customers measure the Postal Service by its impact on their daily lives. When mailing identities generate negative effects on our customers' properties, households and associations, even when caused by third-party actions, they are perceived as "bad service" and intrusive bureaucracy (emphasis in original).16
What the Process Requires
The boundary review process requires any municipality and community group seeking a ZIP Code change to submit the request in writing to the manager of the district, with any rationale and justification. After a community has submitted a ZIP Code request change to USPS, "the District Office forwards the request to the Area Office for review and approval." If the request is approved at the area level, "the proposal is sent to Headquarters Address Management System (AMS) for review and approval." The local postmaster is not the decision maker in this process. The district manager is to identify all relevant issues and potential solutions to them, quantify the specific operational impacts and feasibility of the request, meet with the group of proponents to discuss issues and explain potential alternatives, and provide a determination within 60 days.
The district manager must notify the proponent group in writing if their ZIP Code change request was denied. The notification must include specific justifications for the denial, must be based on the results of the analysis, and must advise the proponent group of the appeal process.
If the request is feasible, the process then requires a formal survey of all of the customers who would be affected by the proposed change. This is an important step, because it might reveal that the proponent group was an activist minority and most customers would prefer not to notify correspondents, change magazine subscriptions, replace stationery, go to a different post office to pick up left-notice mail, or perhaps to adopt a different "community identity." A simple majority of the survey respondents is adequate for approval.
Finally, there is a process in place for customers to appeal to headquarters if USPS determines it will not change ZIP Code boundaries in a case prompted by "municipal identity" issues. Any proponent may appeal an adverse decision to the manager of delivery operations, except in cases where a potential accommodation was not implemented because a majority of affected customers did not support it in the survey.
Within delivery operations at headquarters, an operations specialist who works full time on boundary review appeals determines whether the district provided "reasonable accommodation" to the proposed change. Having knowledge of situations all over the country, and of various accommodations that have been implemented, the operations specialist is in a unique position to judge whether the district manager has fully applied the spirit and letter of the 1999 guidance (made available to a proponent on request) to "find ways to say 'yes.'" The manager of delivery operations must make a final decision on the appeal within 60 days.
There is some evidence that the boundary review process is having some positive effect. USPS has not kept statistics on resolutions in recent years, but it did report that in 1991, the first year of the new policy's implementation, accommodations were reached in 64% of the first 28 reviews completed.17
Possible Accommodations to Resolve ZIP Code Complaints
The most common form of request to the Postal Service (and to members of Congress) is for "a new ZIP Code" for a specific area. Most postal patrons may not realize that a new, unique ZIP Code usually accompanies the creation of a new delivery post office. They also may not realize that a delivery post office (as opposed to a retail station) is a major investment, requiring substantial space, loading docks, sorting equipment, access to major transportation routes, and negotiations with several unions over work assignments. USPS, however, believes that such requests "are fundamentally identity issues" and are made because customers perceive a new ZIP Code as "the only means of achieving postal identity."18 In fact, other options are often available and much simpler to achieve. Sometimes fairly minor adjustments in carrier routes can be made that will solve at least part of a community's boundary problem.
A compromise solution that does not involve changing USPS delivery structure is to allow customers to use an alternative city name in the last line of their addresses, while not changing the ZIP Code. This situation most often occurs when one or more communities fall within the boundaries of a single ZIP Code.
When a large portion of the mail was sorted manually, the use of an alternate city name could have caused mis-sorting and delayed mail. Today, however, almost all mail is sorted by computerized processing equipment. This alternative can help ameliorate community identity issues, but may not address whether certain non-USPS services—like ambulances—can properly locate a home. USPS advises that an alternate city name should not be written in an address until USPS has added it into the AMS. USPS sorting technology currently reads all lines of the address to obtain the delivery point barcode, and use of an unapproved alternative city name could hinder delivery.
USPS routinely has worked with large-scale mailers to improve their address files, sorting—in most cases—to 11 digits rather than five digits. As noted earlier in the report, in 1983, the ZIP Code was expanded to nine digits (ZIP +4). The 10th and 11th ZIP Code numbers, created in 1991, allow mail sent by large-scale mailers to be sorted "directly to a residence or business."19 If a mailer seeks such USPS assistance, USPS may refine municipal mailing lists to conform to political jurisdictions and eliminate errors based on the less sophisticated use of the five-digit code.
What Can a Member of Congress Do?
When a member's office receives a request for assistance in persuading USPS to create a new ZIP Code, it may be helpful to ascertain at the outset the underlying reason for the request. If the constituents are complaining about poor delivery service, then the Postal Service is more likely to address the complaints expeditiously, determine if they have merit, and seek solutions. If population growth or obsolescence of a delivery facility is leading to service problems, USPS will attempt to resolve the problems, including those prompted by confusion over ZIP Code boundaries.
Often, the ZIP Code modification request may have little to do with delivery service, but stems from community identity issues. Constituents are frequently unaware of the boundary review process. In many cases, constituents or municipal officials may have approached a letter carrier or local postmaster and been told that an adjustment would be disruptive, costly, and impractical.
USPS internal policies (as described above) quite firmly state that a cursory, negative response to a request for a ZIP Code modification is no longer permissible. Even if an accommodation cannot be reached, USPS officials are required to explain fully the reasons for the refusal, based on a comprehensive review of operational and cost data.
Occasionally, members will be asked to introduce legislation to force USPS to establish ZIP Code boundaries in statute. Only once has such piece of legislation become law. The Postal Accountability and Enhancement Act of 2006 (P.L. 109-435; 120 Stat. 3261) required USPS to assign "a single unified ZIP Code to serve, as nearly as practicable, each of the following communities:
- 1. Auburn Township, Ohio
- 2. Hanahan, South Carolina
- 3. Bradbury, California
- 4. Discovery Bay, California"
Those ZIP Codes are currently active, according to USPS.
Finally, USPS advises that a constituent should not substitute the preferred city name before the ZIP Code in an address line, without receiving USPS approval to do so. USPS mail processing equipment has internal checks that compare the ZIP Code with the proper city name; if the two do not match, default sequences come into play, and mail very likely will be directed to the wrong delivery post office, certainly causing delay and possibly causing the mail to be returned as undeliverable.
This report originally was written by [author name scrubbed], who has retired from CRS. Readers may contact [author name scrubbed] with questions on ZIP Code issues.
U.S. Postal Service, "Postal Facts 2010," p. 15, http://www.usps.com/strategicplanning/_pdf/PostalFacts_03_17_2010.pdf.
U.S. Census Bureau, "Population Finder," http://factfinder.census.gov/servlet/SAFFPopulation?_event=ChangeGeoContext&geo_id=06000US3400728740&_geoContext=&_street=&_county=haddon&_cityTown=haddon&_state=04000US34&_zip=&_lang=en&_sse=on&ActiveGeoDiv=&_useEV=&pctxt=fph&pgsl=010&_submenuId=population_0&ds_name=null&_ci_nbr=null&qr_name=null®=null%3Anull&_keyword=&_industry=.
U.S. Congress, House Committee on Post Office and Civil Service, Subcommittee on Postal Operations and Service, ZIP Code Boundaries, hearing on H.R. 2380, H.R. 2902, and H.R. 4827, 101st Cong., 2nd sess., June 7, 1990 (Washington: GPO, 1990). Hereafter cited as "ZIP Code Boundary Hearing."
Ibid., p. 105.
A "delivery" occurs when the object sent through the mail is brought to its designated destination.
Zip Code Boundary Hearing, p. 92.
ZIP Code Boundary Hearing, pp. 3, 38, 49, 95, and 97.
U.S. General Accounting Office, Conflicts Between Postal and Municipal Boundaries, GAO/T-GGD-90-47, June 7, 1990, pp. 14-16 and 23.
The Postal Operations Manual is a rulebook that contains a variety of internal policies and operations that guide USPS employees on a variety of topics, from closing post offices to changing post office names. The Manual is kept internally by USPS, but various editions of it are available online. The online versions, however, are on websites hosted by private entities and the publically available versions may not be up-to-date with the most recent USPS modifications. For the most recent USPS policies, contact USPS directly at 202-268-7225.
Information provided electronically to the author by USPS on June 2, 2009.
Deliveries are a fraction of the population growth in an area because most delivery points are households with multiple occupants.
These thresholds for possible ZIP Code changes are rough guidelines rather than absolute cutoff levels.
U.S. Postal Service, Postal Operations Manual, Section 439.211.
USPS has continued efforts to notify its employees of the new ZIP Code policy, which also was posted on the USPS internal website in 2006. In December 2006, USPS sent an additional e-mail reminder of the new policy to the service's delivery and retail departments.
U.S. Postal Service, Comprehensive Statement on Postal Operations, 1991 (Washington: 1992), p. 47.
USPS Internal Memorandum to Vice Presidents, Area Operations, "Proper Treatment of Appeals, ZIP Code Boundary Review Process," November 18, 1999, p. 2.
U.S. Postal Service, "Postal Facts 2010," p. 15, http://www.usps.com/strategicplanning/_pdf/PostalFacts_03_17_2010.pdf.
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About US ZIP Codes
US ZIP codes are a type of postal code used within the United States to help the United States Postal Service (USPS) route mail more efficiently. ZIP codes near me are shown on the map above. Some still refer to ZIP codes as US postal codes. The term ZIP stands for Zone Improvement Plan. The basic 5-digit format was first introduced in 1963 and later extended to add an additional 4 digits after a dash to form a ZIP+4 code. The additional 4 digits help USPS more precisely group mail for delivery. Though ZIP codes were originally developed for USPS, many other shipping companies such as United Parcel Service (UPS), Federal Express (FedEx), DHL, and others make use of ZIP codes for sorting packages and calculating the time and cost of shipping a package (the shipping rate).
Types of Zip Codes
- Unique/single high volume address (ex. 20505 for the CIA in Washington, DC)
- PO Box only (ex. 22313 for the PO Boxes of Alexandria, VA)
- Standard (all other ZIP codes)
Assignment of ZIP codes and Mail Sorting
The first digit of a USA ZIP code generally represents a group of U.S. states. The map of the first digit of zip codes above shows they are assigned in order from the north east to the west coast. The first 3 digits of a ZIP code determine the central mail processing facility, also called sectional center facility or "sec center", that is used to process and sort mail. All mail with the same first 3 digits is first delivered to the same sec center where it is sorted according to the last 2 digits and distributed to local post offices. The sec centers are not open to the public and usually do most sorting overnight. As you can see from the map of the first 3 digits of zip codes, the digits after the first are also generally assigned from east to west. In the map, 0 is closer to white and 9 is much more vivid. It's easy to follow the gradient across each of the zones even though there are a few exceptions (such as the southwest tip of Georgia which uses 39XXX like central Mississippi).
The ZIP+4 code is not required, but it aids the post office in additional sorting of mail. A ZIP+4 code may correspond to a city block, group of apartments, or an individual high-volume receiver. It is also common for each PO Box number to correspond to a unique ZIP+4 code. Sometimes, several PO Box numbers are grouped into the same ZIP+4 code by using the last several digits of the PO Box number. This method isn't a universal rule though so the ZIP+4 must still be looked up for each PO Box.
Places in the US so Remote, They Don't Have a ZIPAs you can see from the map, not everywhere in the US is assigned a ZIP code. Remote and especially rural areas of the country do not have enough deliverable addresses to create a mail route. Without mail delivery, a ZIP is not needed. If you are looking to get off the grid, these areas are some of the most remote places within the country.
USA ZIP Code Boundaries
Despite the fact that ZIP codes seem to be geographic in nature, that wasn't their intended purpose. They are intended to group mail to allow the USPS to deliver mail more efficiently. Some ZIP codes will span multiple states in order to make mail routing and delivery more efficient. In most cases, addresses in close proximity to each other are grouped in the same ZIP code which gives the appearance that ZIP codes are defined by a clear geographic boundary. However, some ZIP codes have nothing to do with geogaphic areas. For instance, a single ZIP code is used for all US Navy mail. When ZIP codes appear to be geographically grouped, a clear shape cannot always be drawn around the ZIP code because ZIP codes are only assigned to a point of delivery and not the spaces between delivery points. In areas without a regular postal route or no mail delivery, ZIP codes may not be defined or have unclear boundaries.
US ZIP Code Map
No official ZIP code map according to actual USPS data exists. The main issue is discussed above: there simply isn't always a clear geographic boundary for a ZIP code. The Census Bureau and many other commercial services will try to interpolate the data to create polygons (shapes using straight lines) to represent the approximate area covered by a ZIP code, but none of these maps are official or entirely accurate.
On this site, all ZIP code maps use the ZIP Code Tabulation Areas (ZCTAs) as specified by the United States Census Bureau in 2010 (or newer) and discussed below. They provide a very close approximation of the area covered by a ZIP code. You can easily notice some of the boundary issues when viewing our maps. Very rural areas aren't labeled as belonging to a ZIP code (such as much of Nevada and Utah) where there are few, if any, addresses to deliver mail. If the address is on the same street as a ZIP code boundary on the map, be sure to search for the full street address to determine the ZIP code instead of relying on the map.
ZIP Code Tabulation Areas (ZCTAs)
ZIP code tabulation areas were developed by the United States Census Bureau. Their purpose is to convey statistical data about regions that are familiar to most citizens. However, ZCTAs are not exactly the same as ZIP codes. As discussed above, it is difficult to precisely define a geographic area covered by a ZIP code. ZCTAs were developed to account for some of the difficulties in assigning an area to a ZIP code and to precisely define a geographic area. Also, ZCTAs are not updated as frequently as ZIP codes. In general, they are updated once every 10 years for the Census.
The Census assigns an area to a ZCTA according to census blocks (the smallest geographic unit used by the census). Imagine a city block that makes up a typical census block as pictured to the right. It is bounded on all 4 sides by portions of city streets that each have their own name and addresses. The issue is that census blocks almost always split down the middle of the street. ZIP codes rarely do because that would require two postal workers delivering mail to that street - one for each side of the street. In the example, one mail carrier may deliver to 3 sides of the block via one ZIP code while another mail carrier delivers mail on the other street in a different ZIP code. When this happens, the Census Bureau will assign the entire block to a single ZCTA (in this case, 21044) because the census block is the area that is precisely measured. If you are getting very precise (usually a matter of meters, not miles), census block boundaries near the edge of a ZIP code almost always split ZIP codes.
The statistics provided by the Census Bureau can give insight into the demographics within the ZIP code. For instance, see our ZIP code rankings.
Matching ZIP Codes with States, Counties, and Cities
Remember that ZIP codes were made to make mail delivery easier. They weren't made to correspond to existing boundaries such as cities, counties, or even states. If it is more efficient for a mail carrier to drive across a state line to deliver mail, the ZIP code "boundary" will cross the state lines. ZIP codes don't usually cross state lines, but some do (65733, 71749, and 73949 are good examples). The complete list of ZIP codes that cross state lines is available as part of our US ZIP code list.
It gets even more complicated when trying to assign a ZIP code to a specific county (as much as 25% cross county lines), congressional district, metro area, time zone, area code, etc. The edges of the boundaries commonly overlap. For the purposes of our free zip code database by county downloads, we will commonly list either the most common region for the ZIP code or list multiple regions if several exist in the ZIP code.
For cities, the assignment is somewhat more complicated. USPS does not always use the city in which the ZIP code is located. The assignment of cities to ZIP codes is more general. The city is usually the name of the main post office. For instance, almost all ZIP codes in St. Louis County in Missouri have a city of Saint Louis when they may be more accurately described as the name of a smaller city where they are located.
Click here to learn more about matching ZIPs to cities and counties
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1963 was a momentous year in America: President John F. Kennedy was assassinated, Martin Luther King Jr. delivered his “I Have a Dream” speech at the March on Washington and, somewhat less heralded by all but the most fervent postal historians, the ZIP code was introduced.
Since its founding in 1775, the post office relied on hand sorting based on local addresses to get mail where it was supposed to go. A piece of mail often went through 10 postal workers before making it to its recipient. But by the 1940s, the then-named Post Office Department realized its sorting system was not keeping pace with the growing population of the country it served.
In 1943, as a way to streamline mail sorting for the biggest American cities, the post office began placing one and two digit numbers between the city and the state to help clerks wade through the increasing volume of mail, which was then around 20 million pieces per year.
(MORE: How to Fix the Post Office: Keep the ‘Last Mile,’ Outsource the Rest)
By the early 1960s, the post-war population boom and continued western growth led to even greater use of the postal service. Mail volume doubled between 1943 and 1962, putting further pressure on the post office to sort mail efficiently. On July 1, 1963, on the recommendation of an internal advisory board, the post office introduced the Zone Improvement Plan Code, which divided the entire country into coded delivery areas. The first two or three numbers told carriers to which states mail was being sent. More populous regions like New York were given five digit numbers starting with 10-14, for example, whereas less populous areas like Montana received five-digit numbers. These new ZIP codes helped the post office better pinpoint where mail was headed while allowing it to expand machine-based sorting systems that could quickly read digits. But many Americans were reluctant to adopt the new system.
“People were concerned they were being turned into numbers,” says Jennifer Lynch, a U.S. Postal Service historian. “They thought it was depersonalizing them.”
To get people on board, the post office began an extensive marketing campaign centered around Mr. ZIP, a friendly looking cartoon mail carrier. A folk group called The Swingin’ Six sang about ZIP Code usage in a lengthy public service announcement video. “Put ZIP in your mail” ran in magazines across the country, including TIME, while a series of short TV ads showed postal workers drowning in a sea of letters and used slogans like “Only you can put ZIP in your postal system.”
Evidently the ads were persuasive. In 1966, three years after ZIP Codes were introduced, 50% of Americans said they used ZIP Codes. By 1969, 83% said they did, according to a 1969 study conducted by Roper Research Associates.
In 1983, the post office expanded the ZIP Code to nine digits to identify which side of the street the mail was being delivered to, as well as particular office buildings. Today, ZIP codes are translated into “automation-readable barcodes” that are placed on pieces of mail when sorted and contain 31 digits of information that tell the post office everything from whether it was presorted, if the mail is first-class or a periodical, and even which business sent it. It also allows the U.S.P.S. to track virtually every letter and package around the country.
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The post office estimates that increased efficiencies for both large mailers and the postal service itself add almost $10 billion of value to the U.S. economy a year.
Today, 50 years after the ZIP code debuted, the postal service’s Office of the Inspector General is recommending that they be linked to digital geographic information systems based on latitude and longitude to further increase delivery accuracy. This time, however, Mr. ZIP will stay in retirement.
MORE: Why Los Angeles Is a Bad City to Be a Letter Carrier
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ZIP+4 Codes | How to find yours and what it means
ZIP+4 Codes (or ZIP Plus 4 Codes) are the final 4 digits of a full nine-digit ZIP Code. The 9-digit ZIP Code consists of two sections. The first five digits indicate the destination post office or delivery area. The last 4 digits represent specific delivery routes within delivery areas.
ZIP plus 4 codes assist the USPS in effectively sorting and distributing mail.
Full 9-digit ZIP Code Lookup
We make it easy to perform USPS ZIP+4 Code searches. Choose your option below to lookup a ZIP+4.
Table of contents:
How to Find My ZIP+4 Code
You can also easily find out your own ZIP+4 last 4-digits by watching this video which shows you how to do it in under a minute.
How Full ZIP Codes Are Used
Knowing what the last four digits of a ZIP Code are all about requires knowing what ZIP Codes themselves are all about. The Zone Improvement Plan was something that the USPS came up with to make it easier to ship letters and packages across the country. It helped divide the country into different "zones" according to how mail was distributed, which accelerated sorting and delivery.
As the US population has increased and scattered, it's been necessary to expand on the system, to make room for everybody and their dog. That's where the "plus 4" part comes in. We're getting ahead of ourselves, though, so let's start with the basics.
USPS Five-Digit ZIP Codes
These are the codes you're familiar with.
They look like this…
…and most commonly indicate a destination post office. Here's why:
If you're mailing a letter from Boston to Seattle, the mail carrier in Massachusetts doesn't really care what the street address of the destination is. It's what you might call "outside his jurisdiction." He just needs to know which mail carrier to send it to so the other carrier can get it to that address.
A postal worker can only cover so much ground on a given day. And since the USPS has a standard of delivering in rain, sleet or snow, that rules out the possibility of doing the service of delivering in stages (some today, some tomorrow). That means that any given post office is only servicing what it can reach in a day. ZIP Codes reflect that.
Typically, a ZIP Code is tied to a post office; by that we mean, every one of the latter has one of the former. You might think of ZIP Codes as a mailing address for a particular post office. Some do handle more than one ZIP Code, but a one-on-one game plan is the norm.
It's very important to note that ZIP Codes aren't "boundaries." They're a collection of delivery routes. They don't follow geographic or administrative boundary lines; they can cross city, county, or even state lines. They follow where the delivery trucks go.
Some ZIP Codes are special cases. Among them are "military" ZIP Codes, which include everything from military bases (domestic or otherwise) to battleships at sea. Then there are "unique" codes. Businesses and organizations sometimes get their own ZIP Codes, due to the volume of mail they send and receive. These are frequently benefiting from bulk mailing discounts, since the organization usually has a mail department that (1) presorts mail before giving it to the USPS, and (2) distributes mail internally so the USPS doesn't have to. Like standard ZIP Codes, "military" and "unique" ZIP Codes circumscribe their own delivery area.
Using full ZIP Codes ensure the fastest, most accurate mailing possible. They're only provided for an address once the address has been standardized, validated and proven real. These codes indicate a specific delivery route, meaning the actual path the mail truck would travel in a single drop-off. Usually this comprises ten to twenty homes or locations. ZIP+4 Codes are also assigned to PO Boxes. Typically, each PO Box gets its own +4 Code, which often matches the box number.
Because ZIP codes plus 4 extra digits are based on delivery routes instead of more permanent boundaries, the last 4 digits of a complete ZIP Code can change often. Five-digit ZIP Codes also change, but they do so infrequently; it's a lot less likely that you will be living in a ZIP Code when it changes. Not so for the full 9-digit zip code. The +4 on a ZIP Code can be changed as frequently as once a month, based on things like how many postal employees are working, or who is working what route, etc.
Benefits of Full ZIP+4 Codes
There are a number of reasons that full ZIP+4 Codes are good omens for your shipping. For one, ZIP+4 Codes require validation; that means you know for sure an address is real if it has a ZIP+4 attached to it. For another thing, they can help get you those bulk mailing discounts.
Another important benefit to using the last four digits of ZIP Codes is delivery speed. Using complete ZIP+4 on your mail can speed up processing and delivery, sometimes by as much as two days. That's right, your mail can show up faster if you label things right. Bet you're scrambling for those codes now, huh? Get those extra 4 digits.
Conclusion | What is My 9 Digit ZIP Code?
Three things will be on the quiz: (1) ZIP+4 Codes indicate delivery routes, (2) using ZIP+4 Codes gets your mail there faster and more accurately, and (3) we at SmartyStreets can get you those codes. If you want to try it on a single address, you can use our single address verification tool now. Our blindingly fast USPS address verification API provides the appropriate ZIP+4 Code for every address we process. Go ahead and perform some postal address verification. Or you could call us instead if you'd rather talk to a real person. (We'd offer a fake person, but we don't have one of those on staff.) Either way, we can help you ZIP your address, and ZIP it good.
Look Up a ZIP Code™
Click here to Look up ZIP Codes™ for corporate and residential addresses.
ZIP Code by Address
Enter street address, city, and state to see a specific ZIP Code. Note:
- A ZIP Code result does not confirm that a person or company is at that address.
- If you searched for a company and did not get the results you expected, search again either without the company name or with a different version of the company name (e.g., full name or acronym).
ZIP Codes by City and State
Enter city and state to see all the ZIP Codes for that city.
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Numeric postal code used in the United States and its territories
For postal codes outside the United States, see Postal code.
A ZIP Code is a postal code used by the United States Postal Service (USPS). Introduced in 1963, the basic format consisted of five digits. In 1983, an extended ZIP+4 code was introduced; it included the five digits of the ZIP Code, followed by a hyphen and four digits that designated a more specific location.
The term ZIP is an acronym for Zone Improvement Plan; it was chosen to suggest that the mail travels more efficiently and quickly (zipping along) when senders use the code in the postal address. The term ZIP Code was originally registered as a service mark by the USPS; its registration expired in 1997.
Early history and five-digit ZIP Codes
The early history and context of postal codes began with postal district/zone numbers. The United States Post Office Department (USPOD) implemented postal zones for many large cities in 1943.
Mr. John Smith
3256 Epiphenomenal Avenue
Minneapolis 16, Minnesota
The "16" is the number of the postal zone in the specific city.
By the early 1960s, a more organized system was needed, and non-mandatory five-digit ZIP Codes were introduced nationwide on July 1, 1963. The USPOD issued its Publication 59: Abbreviations for Use with ZIP Code on October 1, 1963, with the list of two-letter state abbreviations which are generally written with both letters capitalized. An earlier list, publicized in June 1963, had proposed capitalized abbreviations ranging from two to five letters. According to Publication 59, the two-letter standard was "based on a maximum 23-position line, because this has been found to be the most universally acceptable line capacity basis for major addressing systems", which would be exceeded by a long city name combined with a multi-letter state abbreviation, such as "Sacramento, Calif." along with the ZIP Code. The abbreviations have remained unchanged, with the exception of Nebraska, which was changed from NB to NE in 1969 at the request of the Canadian postal administration, to avoid confusion with the Canadian province of New Brunswick.
Robert Moon is considered the father of the ZIP Code; he submitted his proposal in 1944 while working as a postal inspector. The post office credits Moon with only the first three digits of the ZIP Code, which describe the sectional center facility (SCF) or "sec center". An SCF is a central mail processing facility with those three digits. The fourth and fifth digits, which give a more precise locale within the SCF, were proposed by Henry Bentley Hahn, Sr.
The SCF sorts mail to all post offices with those first three digits in their ZIP Codes. The mail is sorted according to the final two digits of the ZIP Code and sent to the corresponding post offices in the early morning. Sectional centers do not deliver mail and are not open to the public (although the building may include a post office that is open to the public), and most of their employees work the night shift. Items of mail picked up at post offices are sent to their own SCFs in the afternoon, where the mail is sorted overnight. In the case of large cities, the last two digits as assigned generally coincided with the older postal zone number.
Mr. John Smith
3256 Epiphenomenal Avenue
Minneapolis, MN 55416
In 1967, these became mandatory for second- and third-class bulk mailers, and the system was soon adopted generally. The United States Post Office used a cartoon character, which it called Mr. ZIP, to promote the use of the ZIP Code. He was often depicted with a legend such as "USE ZIP CODE" in the selvage of panes of postage stamps or on the covers of booklet panes of stamps. Mr. ZIP was featured prominently alongside musical group "The Swingin' Six" in a variety show that the post office used to explain the importance of using ZIP codes.
In 1971, Elmira (NY) Star-Gazette reporter Dick Baumbach found out the White House was not using a ZIP Code on its envelopes. Herb Klein, special assistant to President Nixon, responded by saying the next printing of envelopes would include the ZIP Code.
In 1983, the U.S. Postal Service introduced an expanded ZIP Code system that it called ZIP+4, often called "plus-four codes", "add-on codes", or "add-ons". A ZIP+4 Code uses the basic five-digit code plus four additional digits to identify a geographic segment within the five-digit delivery area, such as a city block, a group of apartments, an individual high-volume receiver of mail, a post office box, or any other unit that could use an extra identifier to aid in efficient mail sorting and delivery. However, initial attempts to promote universal use of the new format met with public resistance and today the plus-four code is not required. In general, mail is read by a multiline optical character reader (MLOCR) that almost instantly determines the correct ZIP+4 Code from the address—along with the even more specific delivery point—and sprays an Intelligent Mail barcode (IM) on the face of the mail piece that corresponds to 11 digits—nine for the ZIP+4 Code and two for the delivery point.
For Post Office Boxes, the general (but not invariable) rule is that each box has its own ZIP+4 code. The add-on code is often one of the following: the last four digits of the box number (e.g. PO Box 107050, Albany, NY 12201-7050), zero plus the last three digits of the box number (e.g., PO Box 17727, Eagle River, AK 99577-0727), or, if the box number consists of fewer than four digits, enough zeros are attached to the front of the box number to produce a four-digit number (e.g., PO Box 77, Juneau, AK 99750-0077). However, there is no uniform rule, so the ZIP+4 Code must be looked up individually for each box (e.g. using the USPS's official ZIP Code Lookup tool, and being sure to enter just city and state, not the 5-digit ZIP).
Postal bar code
The ZIP Code is often translated into an Intelligent Mail barcode that is printed on the mailpiece to make it easier for automated machines to sort. A barcode can be printed by the sender (some word-processing programs such as WordPerfect include the feature), but this is not recommended, as the address-to-ZIP lookup tables can be significantly out of date. It is better to let the post office put one on when it processes the piece. In general, the post office uses OCR technology, though in some cases a human might have to read and enter the address.
Customers who send bulk mail can get a discount on postage if they have printed the barcode themselves and have presorted the mail. This requires more than just a simple font; mailing lists must be standardized with up-to-date Coding Accuracy Support System (CASS)-certified software that adds and verifies a full, correct ZIP+4 Code and an additional two digits representing the exact delivery point. Furthermore, mail must be sorted in a specific manner to an 11-digit code with at least 150 mailpieces for each qualifying ZIP Code and must be accompanied by documentation confirming this. These steps are usually done with PAVE-certified software that also prints the barcoded address labels and the barcoded sack or tray tags.
The assignment of delivery point digits (the 10th and 11th digits) is intended to ensure that every single mailable point in the country has its own 12-digit number. The delivery-point digits are calculated based on the primary or secondary number of the address. The USPS publishes the rules for calculating the delivery point in a document called the CASS Technical Guide. However, when confronted with two addresses like 18 and 18C, often CASS will assign the same 12-digit number to two distinct mail delivery points. The last digit is always a check digit, which is obtained by summing all 5, 9 or 11 digits, taking the residue modulo 10 of this sum (i.e., the remainder after dividing by 10) and finally subtracting this from 10. (Thus, the check digit for 10001-0001 00 would be 7, since 1+1+1=3, 3≡3(mod 10) and 10–3=7.)
Structure and allocation
Scope and international mail
ZIP Codes designate delivery points within the United States (and its territories). There are generally no ZIP Codes for deliveries to other countries, except for the independent countries of the Federated States of Micronesia, the Republic of the Marshall Islands, and the Republic of Palau, each of which is integrated into the U.S. postal system under a Compact of Free Association. Another exception is ZIP Codes used for overseas stations of USA armed forces.
Mail to U.S. diplomatic missions overseas is addressed as if it were addressed to a street address in Washington, D.C. The four-digit diplomatic pouch number is used as a building number, while the city in which the embassy or consulate is located is combined with the word "Place" to form a street name. Each mission uses a ZIP+4 Code consisting of 20521 and the diplomatic pouch number. For example, the mailing address of the U.S. Embassy in New Delhi, India would be:
Embassy of the United States of America
9000 New Delhi Place
Washington, DC 20521-9000
However, individuals posted at diplomatic missions overseas are now assigned a Diplomatic Post Office address and unique box number. The Zip Code identifies the diplomatic mission destination and is different from the diplomatic pouch number in the example above. While delivered through the pouch system, mail to such addresses are not considered "Diplomatic Pouch" materials, and as such must adhere to the mailing regulations of the host country. An example address is:
UNIT 8400 BOX 0000
DPO AE 09498-0048
By type and use
There are four types of ZIP Codes:
- Unique: assigned to a single high-volume address
- Post Office Box-only: used only for PO Boxes at a given facility, not for any other type of delivery
- Military: used to route mail for the U.S. military
- Standard: all other ZIP Codes.
Unique ZIP Codes are used for governmental agencies, universities, businesses, or buildings that receive such extremely high volumes of mail that they need their own ZIP Codes. Government examples include 20505 for the Central Intelligence Agency in Washington, D.C.; 81009 for the Federal Citizen Information Center of the U.S. General Services Administration (GSA) in Pueblo, Colorado. An example of a university specific ZIP is 21252, which serves Towson University. Examples of private address unique ZIP Codes include the headquarters of Walmart (72716).
An example of a PO Box-only ZIP Code is 22313, which is used for PO Boxes at the main post office in Alexandria, Virginia, such as those used by the United States Patent and Trademark Office. In the area surrounding that post office, home and business mail delivery addresses use ZIP Code 22314, which is thus a Standard ZIP Code.
In certain exceptional cases, a nominally Standard-type ZIP Code may, in practice, be used as one of the three other types. For example, the ZIP Code 43210 in Columbus, Ohio is listed as a Standard-type ZIP Code. However, since its geographic boundaries are essentially coterminous with those of the Ohio State University's main campus, it is effectively exclusive to the university even though it is not officially a "Unique"-type ZIP Code.
Primary state prefixes
ZIP Codes are numbered with the first digit representing a certain group of U.S. states, the second and third digits together representing a region in that group (or perhaps a large city) and the fourth and fifth digits representing a group of delivery addresses within that region. The main town in a region (if applicable) often gets the first ZIP Codes for that region; afterward, the numerical order often follows the alphabetical order. Because ZIP Codes are intended for efficient postal delivery, there are unusual cases where a ZIP Code crosses state boundaries, such as a military facility spanning multiple states or remote areas of one state most easily serviced from a bordering state. For example, ZIP Code 42223 serves Fort Campbell, which spans Christian County, Kentucky, and Montgomery County, Tennessee, and ZIP Code 97635 includes portions of Lake County, Oregon, and Modoc County, California.
In general, the first three digits designate a sectional center facility, the mail sorting and distribution center for an area. A sectional center facility may have more than one three-digit code assigned to it. For example, the Northern Virginia sectional center facility in Merrifield is assigned codes 220, 221, 222, and 223. In some cases, a sectional center facility may serve an area in an adjacent state, usually due to the lack of a proper location for a center in that region. For example, 739 in Oklahoma is assigned to Liberal, Kansas; 297 in South Carolina is assigned to Charlotte, North Carolina; 865 in Arizona is assigned to Gallup, New Mexico; and 961 in California to Reno, Nevada.
In terms of geographic location, many of the lowest ZIP Codes, which begin with '0', are in the New England region. Also in the '0' region are New Jersey (non-contiguous with the remainder of the '0' area), Puerto Rico, the U.S. Virgin Islands, and APO/FPO military addresses for personnel stationed in Europe, Africa, Southwest Asia, and aboard vessels based in the waters adjoining those lands. The lowest ZIP Code is in Holtsville, New York (00501, a ZIP Code exclusively for the U.S. Internal Revenue Service center there). Other low ZIP Codes are 00601 for Adjuntas, Puerto Rico; 01001 for Agawam, Massachusetts, and the ZIP Codes 01002 and 01003 for Amherst, Massachusetts; 01002 is used for mail in town, while 01003 is reserved for the University of Massachusetts Amherst. Until 2001, there were six ZIP Codes lower than 00501 that were numbered from 00210 to 00215 (located in Portsmouth, New Hampshire) and were used by the Diversity Immigrant Visa program to receive applications from non-U.S. citizens.
The numbers increase southward along the East Coast, such as 02115 (Boston), 10001 (New York City), 19103 (Philadelphia), 21201 (Baltimore), 20008 (Washington, D.C.), 30303 (Atlanta), and 33130 (Miami) (these are only examples, as each of these cities contains several ZIP Codes in the same range). From there, the numbers increase heading westward and northward east of the Mississippi River, southward west of the Mississippi River, and northward on the West Coast. For example, 40202 is in Louisville, 50309 in Des Moines, 60601 in Chicago, 63101 in St. Louis, 77036 in Houston, 80202 in Denver, 94111 in San Francisco, 98101 in Seattle, and 99950 in Ketchikan, Alaska (the highest ZIP Code).
The first digit of the ZIP Code is allocated as follows:
- 0 = Connecticut (CT), Massachusetts (MA), Maine (ME), New Hampshire (NH), New Jersey (NJ), New York (NY, Fishers Island only), Puerto Rico (PR), Rhode Island (RI), Vermont (VT), Virgin Islands (VI), Army Post Office Europe, Central Asia, and the Middle East (APO AE); Fleet Post Office Europe and the Middle East (FPO AE)
- 1 = Delaware (DE), New York (NY), Pennsylvania (PA)
- 2 = District of Columbia (DC), Maryland (MD), North Carolina (NC), South Carolina (SC), Virginia (VA), West Virginia (WV)
- 3 = Alabama (AL), Florida (FL), Georgia (GA), Mississippi (MS), Tennessee (TN), Army Post Office Americas (APO AA), Fleet Post Office Americas (FPO AA)
- 4 = Indiana (IN), Kentucky (KY), Michigan (MI), Ohio (OH)
- 5 = Iowa (IA), Minnesota (MN), Montana (MT), North Dakota (ND), South Dakota (SD), Wisconsin (WI)
- 6 = Illinois (IL), Kansas (KS), Missouri (MO), Nebraska (NE)
- 7 = Arkansas (AR), Louisiana (LA), Oklahoma (OK), Texas (TX)
- 8 = Arizona (AZ), Colorado (CO), Idaho (ID), New Mexico (NM), Nevada (NV), Utah (UT), Wyoming (WY)
- 9 = Alaska (AK), American Samoa (AS), California (CA), Guam (GU), Hawaii (HI), Marshall Islands (MH), Federated States of Micronesia (FM), Northern Mariana Islands (MP), Oregon (OR), Palau (PW), Washington (WA), Army Post Office Pacific (APO AP), Fleet Post Office Pacific (FPO AP)
Secondary regional prefixes (123xx) and local ZIP Codes (12345)
See also: List of ZIP Code prefixes
The second and third digits represent the sectional center facility (SCF) (e.g. 477xx = Vanderburgh County, Indiana), and the fourth and fifth digits represent the area of the city (if in a metropolitan area), or a village/town (outside metro areas), e.g. 47722 (4=Indiana, 77=Vanderburgh County, 22=University of Evansville area). When a sectional center facility's area crosses state lines, that facility is assigned separate three-digit prefixes for the states that it serves.
In some urban areas, like 462 for Marion County, Indiana, the three-digit prefix will often exist in one county, while, in rural and most suburban areas, the prefix will exist in multiple counties; for example, the neighboring 476 prefix is found in part or entirely in six counties: Gibson, Pike, Posey, Spencer, Vanderburgh, and Warrick. In some cases, an urban county may have more than one prefix. This is the case with Allen (467, 468), Lake (464, 463), St. Joseph (465, 466), and Vanderburgh (476, 477) counties. Cities like Chicago, Houston, Los Angeles, and New York City have multiple prefixes within their city limits. In some cases, these may be served from the same SCF, such as in San Diego County, California, where the three-digit prefixes 919 and 920 are used for suburban and rural communities, and 921 for the city of San Diego itself, although all three are processed through the same SCF facility.
Despite the geographic derivation of most ZIP Codes, the codes themselves do not represent geographic regions; in general, they correspond to address groups or delivery routes. As a consequence, ZIP Code "areas" can overlap, be subsets of each other, or be artificial constructs with no geographic area (such as 095 for mail to the Navy, which is not geographically fixed). In similar fashion, in areas without regular postal routes (rural route areas) or no mail delivery (undeveloped areas), ZIP Codes are not assigned or are based on sparse delivery routes, and hence the boundary between ZIP Code areas is undefined. For example, some residents in or near Haubstadt, Indiana, which has the ZIP Code 47639, have mailing addresses with 47648, the ZIP Code for neighboring Fort Branch, Indiana, while others living in or near Fort Branch have addresses with 47639. Many rural counties have similar logistical inconsistencies caused by the aforementioned sparse delivery routes, often known as Rural Routes or by some other similar designation.
Almost all U.S. government agencies in and around the capital are assigned ZIP Codes starting with 20200 to 20599, which are Washington, D.C., ZIP Codes, even if they are not located in Washington itself. While the White House itself is located in ZIP Code 20006, it has the ZIP Code 20500. The Nuclear Regulatory Commission is located in Rockville, Maryland, at ZIP Code 20852, but has been assigned by the Postal Service the address "Washington, DC 20555".
In similar manner, the ZIP Code for the Metropolitan Washington Airports Authority, a federally chartered independent authority, is 20001–6000, even though the physical address of the Authority's office, "1 Aviation Circle", is in Arlington, Virginia.
One current exception to this rule is the United States Patent and Trademark Office (USPTO). When the USPTO was located in the Crystal City neighborhood in Arlington, Virginia, it was assigned by the Postal Service the address "Washington, DC 20231" despite being physically located in ZIP Code 22202. However, the USPTO now uses ZIP Codes assigned to its current locations in Alexandria, Virginia. The patents side of the USPTO uses a PO box assigned the ZIP+4 code of 22313-1450 for all postal communications. The trademarks side of the USPTO uses the same ZIP+4 code for most of its mail, but the office of the Commissioner for Trademarks has its own PO box with the ZIP+4 code of 22313–1451, and some offices within that part of the USPTO use the 22314 ZIP Code.
In rare circumstances, a locality is assigned a ZIP Code that does not match the rest of the state. In even rarer cases a ZIP Code may cross state lines. Usually, this occurs when the locality is so isolated that it is most conveniently served from a sectional center in another state. Examples:
- Fishers Island, New York, bears the ZIP Code 06390 and is served from Connecticut because the only ferry service is to Connecticut – all other New York ZIP Codes (excepting those at Holtsville for the IRS) begin with "1".
- Returned government parcels from Washington, D.C. are sent to ZIP Codes beginning with "569" so that returned parcels are security checked at a remote facility (this was put into place after the 2001 anthrax attacks).
- Some Arkansas roads north of Bull Shoals Lake can best be accessed by the Protem, Missouri, delivery unit (ZIP Code 65733), as they are accessible by road only through Missouri.
- El Paso, Texas, in addition to 798xx and 799xx (Texas has the 75xxx-79xxx ZIP codes), also uses ZIP code 885xx (which is right after the 870xx-884xx used in New Mexico).
- Fort Campbell (ZIP Code 42223), is primarily in Tennessee, but the main entrance is in Kentucky.
- The Kentucky Bend geographical anomaly in far western Kentucky, where a small portion of land is separated from the rest of the state by a bend in the Mississippi river, shares ZIP Code 38079 with Tiptonville, Tennessee.
Preferred place names: ZIP Codes and previous zoning lines
A ZIP Code's address and the city name written on the same line do not necessarily mean that address is within the boundaries of that city. The Postal Service designates one preferred place name for each ZIP Code. This may be an incorporated town or city, a subentity of a large city or an unincorporated census-designated place, or a small unincorporated community. Additional place names may be recognized as acceptable for a certain ZIP Code. Still, others are deemed not acceptable, and if used may result in a delay in mail delivery.
Preferred place names are generally the city or town in which the address is located. However, for many cities that have incorporated since ZIP Codes were introduced, the city name is not the preferred place name. Many databases automatically assign the preferred place name for a ZIP Code, without regard to any acceptable place names. For example, Centennial, Colorado is divided among seven ZIP Codes assigned to Aurora, Englewood, or Littleton as its preferred place names; none of these seven ZIP Codes carries "Centennial" as a preferred name, and in the ZIP Code directory, Centennial addresses are listed under those three cities. Since it is acceptable to write "Centennial" in conjunction with any of the seven ZIP Codes, one can write "Centennial" in an address in Aurora, Englewood, or Littleton, as long as it is in one of the shared ZIP Codes.
Acceptable place names are usually added to a ZIP Code in cases where the ZIP Code boundaries divide them between two or more cities, as in the case of Centennial. However, in many cases, only the preferred name can be used, even when many addresses in the ZIP Code are in another city. People sometimes must use the name of a post office rather than their own city.
One extreme example is ZIP Code 85254; it was assigned the place name Scottsdale, Arizona, because it is served by the Scottsdale post office, but 85% of its territory is inside the city limits of neighboring Phoenix. Another notorious example is an entire neighborhood of the city of Los Angeles known as Beverly Hills Post Office. Its residents prefer the more glamorous Beverly Hills address and 90210 ZIP Code, but this regularly causes problems with emergency response when dispatchers have to sort out whether a given home in 90210 is in Los Angeles or Beverly Hills.
Similarly, Missouri City, Texas, straddles Harris and Fort Bend counties. The portion within Harris County is within the ZIP Code 77071, which must use the city name of Houston instead of Missouri City. At the same time, a small portion of the city of Houston is in Fort Bend County in the ZIP Code 77489, and residents there must use the name Missouri City for their address even though they are in Houston.
This also occurs in some rural areas where portions of one town have their mail delivered to other post offices. For example, while most of the town of Plainfield, Massachusetts is in ZIP Code 01070, some sections of town are in the ZIP Code area for the neighboring town of Cummington with ZIP Code 01026. Only the preferred name of Cummington is allowed in ZIP Code 01026, so residents of parts of Plainfield must list their address as being in Cummington.
This phenomenon is repeated across the country. The previously mentioned Englewood, Colorado is an inner-ring suburb that was built out by the 1960s. Its post office served the area that is now the high-growth southern tier of the Denver metropolitan area, and ZIP Codes in this area were assigned Englewood as their preferred place name. A business community as large as downtown Denver has grown in this area, with headquarters for many internationally recognized corporations. These companies indicate Englewood as their location (the preferred postal place name), although they are located in other cities. As a result, there are really two Englewoods – the city, small and with a largely working-class residential population, and, a number of miles away, the postal Englewood, a vast suburban area of upscale subdivisions and office parks that have nothing to do with the city of Englewood yet share a split identity with it solely because of ZIP Codes. People who say they live or work in Englewood and identify closely with it may rarely enter the city. In Indiana, the ZIP Code for a town usually indicates the ZIP Code for its corresponding township, as nearly all of Indiana's small town post offices have rural routes.
Acceptable place names also come into play in areas where citizens identify more strongly with a particular urban center than their own municipality. For example, Allegheny County, Pennsylvania, has 130 distinct municipalities, yet many of the county's residents, and even some residents of adjacent counties, commonly use Pittsburgh, Pennsylvania as their postal address. On the opposite end of the spectrum, in some urban areas, neighborhood names may be acceptable even though they have no legal standing, such as La Jolla, California, which is the preferred place name for ZIP Code 92037, despite the fact that La Jolla is a district of San Diego, California and not a separate legal entity. This ZIP Code is also in the 919/920 sequence used by San Diego County's suburban and rural areas, not in the 921 sequence used in the remainder of the City of San Diego, even though La Jolla has always been part of San Diego.
Many ZIP Codes are for villages, census-designated places, portions of cities, or other entities that are not municipalities. For example, ZIP Code 03750 is for Etna, New Hampshire, but Etna is not a city or town; it is a village district in the town of Hanover, which itself is assigned the ZIP Code 03755. Another example is ZIP Code 08043, which corresponds to the census-designated place of Kirkwood, New Jersey, but serves the entirety of Voorhees Township. This is also the case in LaGrange, New York, a portion of which is served by the 12603 ZIP Code based in the neighboring town of Poughkeepsie. The rest of LaGrange is served by the LaGrangeville Post Office. LaGrangeville is itself not a town at all, but a section of LaGrange. Willow Grove, Pennsylvania, served by the 19090 ZIP Code, is a village that straddles the border of Upper Moreland Township and Abington Township, and that post office also serves a small portion of Upper Dublin Township. Furthermore, non-municipal place names may also share ZIP Codes with municipal place names. For example, West Windsor Township, New Jersey, is commonly referred to in most mailing databases as Princeton Junction, a census-designated place within West Windsor. Silver Spring, Maryland, (20815, 20901–20912) is neither a city nor a town, but simply the common name for an unincorporated area consisting of a large part of the lower southern portion of Montgomery County.
Postal designations for place names become de facto locations for their addresses, and as a result, it is difficult to convince residents and businesses that they are located in another city or town different from the preferred place name associated with their ZIP Codes. Because of issues of confusion and lack of identity, some cities, such as Signal Hill, California (an enclave located entirely inside the separate city of Long Beach), have successfully petitioned the Postal Service to change ZIP Code boundaries or create new ZIP Codes so their cities become the preferred place name for addresses within the ZIP Code.
Postal designation confusion may have financial implications for local governments because mail volume is one factor used by the U.S. Census Bureau to estimate population changes between decennial census enumerations. Sometimes local officials in a community that is not the preferred place name for a ZIP Code but is an acceptable place name will advise residents to always use the name of the community, because if the census estimate of that town's population is low they may receive fewer funds that are computed based on population. A typical example is Paddock Lake, Wisconsin, whose preferred place name is Salem. Paddock Lake is incorporated as a village within the town of Salem, even though there are more people in the village of Paddock Lake than there are in the unincorporated parts of the town of Salem. Further confusion is caused because Silver Lake, Wisconsin, which is also a village in the town of Salem and is of similar size and status to Paddock Lake, has its own ZIP Code and post office.
In another case, the U.S. Federal Communications Commission (FCC) denied a radio station (now WNNX FM) a move requiring a change in its city of license to Sandy Springs, Georgia, largely because it was not a city (until municipal incorporation in late 2005), despite being the seventh-largest place in the state by population. The FCC cited the use of "Atlanta" on letters of support from local organizations, even though the USPS forced them to use Atlanta for 30328 until well after incorporation took effect. Currently "Sandy Springs" is only acceptable, despite none of 30328 being in Atlanta, or anywhere else outside the Sandy Springs city limit. This even applies to the ZIP Code used only for PO boxes at the Sandy Springs main post office.
Because ZIP Codes and their associated place names can ignore county lines, problems may occur where street addresses are based on quadrant location within a county. For example, the area served by 30339 straddles the Cobb County–Fulton County line in Georgia. The Cobb County portion of this area includes Vinings in the southeast of that county; the Fulton County portion lies within the city limits of Atlanta. Every street address in Vinings is labeled SE, and has a house number on the Cobb County grid (according to the distance from the town square in the county seat of Marietta). However, because the USPS demands the use of Atlanta, Vinings addresses are written such that they appear to be in southeast Atlanta, instead of in the opposite (northwest metro Atlanta) side.
Division and reallocation of ZIP Codes
Like area codes, ZIP Codes are sometimes divided and changed, especially when a rural area becomes suburban. Typically, the new codes become effective once announced, and a grace period (e.g., one year) is provided in which the new and old codes are used concurrently so that postal patrons in the affected area can notify correspondents, order new stationery, etc.
In rapidly growing communities, it is sometimes necessary to open a new sectional center facility, which must then be allocated its own three-digit ZIP-code prefix or prefixes. Such allocation can be done in various ways. For example, when a new sectional center facility was opened at Dulles Airport in Virginia, the prefix 201 was allocated to that facility; therefore, for all post offices to be served by that sectional center facility the ZIP Code changed from an old code beginning with 220 or 221 to a new code or codes beginning with 201. However, when a new sectional center facility was opened to serve Montgomery County, Maryland, no new prefix was assigned. Instead, ZIP Codes in the 207 and 208 ranges, which had previously been assigned alphabetically, were reshuffled so that 207xx ZIP Codes in the county was changed to 208xx codes, while 208xx codes outside that county were changed to 207xx codes. Because Silver Spring (whose postal area includes Wheaton) has its own prefix, 209, there was no need to apply the reshuffling to Silver Spring; instead, all mail going to 209xx ZIP Codes was simply rerouted to the new sectional center facility.
On the other hand, depopulation may cause a post office to close and its associated ZIP Code to be deallocated. For example, Centralia, Pennsylvania's ZIP Code, 17927, was retired in 2002, and ZIP Codes for Onoville (14764), Quaker Bridge (14771) and Red House (14773) in New York were prevented from going into use in 1964 in preparation for the Kinzua Dam's completion.
Elkins Park, Pennsylvania, was originally issued the 19117 ZIP Code, although it lies in Montgomery County, Pennsylvania. Because of the 191 prefix, which is found only in Philadelphia apart from that lone exception, auto insurance companies charged higher city premiums to residents of that suburban location. For that reason, residents petitioned the USPS for a 190-prefix ZIP Code, which is common to the inner-ring Pennsylvania suburbs of that city, and, after several attempts that were initially disapproved by the USPS, Elkins Park was finally reassigned to the 19027 ZIP Code.
ZIP Codes also change when postal boundaries are realigned. For example, concurrent with the above-noted change in Montgomery County, Maryland, and under pressure from the then-mayor of Washington, D.C., Marion Barry, the USPS realigned the postal boundaries between Washington, D.C. and Maryland to match the boundary. Previously, many inner suburbs, such as Bethesda and Takoma Park, Oxon Hill, Temple Hills, Suitland, and Capitol Heights had been in the Washington, D.C., postal area. As a result of the change, ZIP Codes in Maryland beginning with 200 were changed to new ZIP Codes beginning with 207, 208, or 209, depending on their location, and ZIP Codes straddling the D.C.–Maryland line were split. For example, 20016 (Bethesda) became 20816, while the Maryland portion of 20012 (Takoma Park) became 20912.
Delivery services other than the USPS, such as FedEx, United Parcel Service, and DHL, require a ZIP Code for optimal internal routing of a package.
As of October 2019[update], there are 41,702 ZIP Codes in the United States. ZIP Codes are used not only for tracking of mail, but also in gathering geographical statistics in the United States. The U.S. Census Bureau calculates approximate boundaries of ZIP Codes areas, which it calls ZIP Code Tabulation Areas (ZCTAs). Statistical census data is then provided for these approximate areas. The geographic data provided for these areas includes the latitude and longitude of the center-point of the ZCTAs. There are approximately 32,000 ZCTAs. The reason that there is not one ZCTA for every ZIP Code is that PO Boxes are excluded, since only populated areas are included in the Census data.The Census Bureau provides many statistical data sets for ZIP Codes, but does not keep up-to-date datasets of all ZCTAs. Complete datasets providing a similar approximate geographic extent are commercially available. ZIP Codes are inherently discrete or point-based data, as they are assigned only at the point of delivery, not for the spaces in between the delivery points. The United States Census Bureau then interpolates this discrete data set to create polygons, or areal features representing the approximate extent of the ZIP Code to use for mapping and data presentation. ZCTAs are not to be confused with ZIP Codes, and they are not updated as frequently as ZIP Codes. However, for many research and planning purposes, they are very useful and can be used with ZIP Code data.
The data is often used in direct mail marketing campaigns in a process called ZIP-code marketing. Point-of-sale cashiers sometimes ask consumers for their home ZIP Codes. Besides providing purchasing-pattern data useful in determining the location of new business establishments, retailers can use directories to correlate this ZIP Code with the name on a credit card to obtain a consumer's full address and telephone number. ZIP-Coded data are also used in analyzing geographic factors in risk, an insurance and banking industry practice pejoratively known as redlining. This can cause problems, e.g., expensive insurance, for people living near a town with a high crime rate and sharing its ZIP Code, while they live in a relatively crime-free town. (See Elkins Park, Pennsylvania, above.)
California outlawed this practice in 2011.
ZIP Codes may not currently be used to identify existing legislative districts. Although the website of the United States House of Representatives has a "Find Your Representative" feature that looks up congressional districts based on ZIP Codes alone, it often returns multiple districts corresponding to a single ZIP Code. This is because different parts of one ZIP Code can be in different districts. One proposal to eliminate the possibility of extreme partisan gerrymandering calls on using ZIP Codes as the basic units for redistricting.
A 1978 proposal for a nationwide system of community networks suggested using ZIP Codes for routing.
ZIP Code data is an integral part of dealer / store locator software on many websites, especially brick-and-click websites. This software processes a user-input ZIP Code and returns a list of store or business locations, usually in the order of increasing distance from the center of the input ZIP Code. As the ZIP Code system is confined to the U.S. Postal network, websites that require ZIP Codes cannot register customers outside the United States. Many sites will purchase postal code data of other countries or make allowances in cases where the ZIP Code is not recognized.
ZIP Codes are regularly used on the Internet to provide a location in situations where an exact address is not necessary (or desirable) but the user's municipality or general location is needed. Examples (in addition to the store locator example listed above) include weather forecasts, television listings, local news, and online dating (most general-purpose sites, by default, search within a specified radius of a given ZIP Code, based on other users' entered ZIP Codes).
Credit card security
Main article: Address Verification System
ZIP Codes are used in credit card authorization, specifically Address Verification System (AVS). When a merchant collects the entire address, the ZIP Code is an important part of AVS. In some cases, the ZIP Code is the only thing used for AVS, specifically where collecting a signature, or other information is infeasible, such as pay at the pump or vending machines.
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