Zip Code

About US ZIP Codes

About US ZIP Codes

The United States Postal Service (USPS) uses US ZIP codes as a type of postal code to more effectively route mail within the country. The map above shows ZIP codes close to me. Some still use the term “US postal codes” to describe ZIP codes. Zone Improvement Plan is what the acronym ZIP stands for. In order to create a ZIP+4 code, the initial 5-digit format was later expanded to include an additional 4 digits following a dash. This was first adopted in 1963. USPS is able to more precisely arrange mail for delivery with the use of the extra 4 digits. Although the US Postal Service (USPS) is the organization for whom ZIP codes were initially created, many other shipping businesses, including United Parcel Service (UPS), Federal Express (FedEx), DHL, and others, use ZIP codes for package sorting and determining the time and cost of transporting a product (the shipping rate).

Types of Zip Codes

  1. Unique/single high volume address (ex. 20505 for the CIA in Washington, DC)
  2. PO Box only (ex. 22313 for the PO Boxes of Alexandria, VA)
  3. Military
  4. Standard (all other ZIP codes)

Assignment of ZIP codes and Mail Sorting

A group of American states are often represented by the first digit of a USA ZIP code. The first digit of the zip codes are assigned in order from the north coast to the west coast, as can be seen on the map above. The central mail processing facility—also known as a sectional center facility or “sec center”—that is used to handle and sort mail is identified by the first three digits of a ZIP code. The same sec center receives all mail with the same first three digits before sorting it based on the final two digits and distributing it to neighborhood post offices. The sec centers often sort most items overnight and are not accessible to the general public. The zip code’s first three digits are shown on a map, and you can see that the next digits are typically assigned from east to west as well. In the map, 9 is much more vibrant and 0 is closer to white. Even though there are a few exceptions (such the southwest point of Georgia, which uses 39XXX like middle Mississippi), it’s simple to trace the gradient across each of the zones.

Although it is not necessary, the ZIP+4 code enables the post office to further sort mail. A city block, a collection of apartments, or a single high-volume receiver may all have the same ZIP+4 code. Additionally, it is typical for every PO Box number to correlate to a different ZIP+4 code. The last few digits of a PO Box number can be used to group many PO Box numbers into the same ZIP+4 code. However, since this approach is not a general rule, it is still necessary to look up the ZIP+4 for each PO Box.

Places in the US so Remote, They Don’t Have a ZIP

Places in the US so Remote, They Don't Have a ZIP

As 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

Although ZIP codes appear to have a geographic focus, it wasn’t their intended use. They are designed to organize mail into groups so that the USPS can deliver mail more effectively. To improve mail delivery and routing, some ZIP codes may cross multiple states. The majority of the time, addresses that are adjacent to one another are put into the same ZIP code, giving the impression that ZIP codes have a distinct geographic limit. Some ZIP codes, however, have nothing to do with geographic regions. For instance, the US Navy uses a single ZIP code for all mail. Because ZIP codes are only given to a point of delivery and not the areas between delivery locations, even when ZIP codes appear to be geographically clustered, a distinct shape cannot always be formed around the ZIP code. ZIP codes may not be well defined or have ambiguous limits in places where there is no mail delivery or no regular postal route.

US ZIP Code Map

Using genuine USPS data, there is no official ZIP code map. The fundamental problem has already been mentioned: a ZIP code’s geographic range isn’t always well-defined. None of these maps are official or completely accurate, but the Census Bureau and many other commercial services will attempt to interpolate the data to construct polygons (shapes using straight lines) to approximate the area covered by a ZIP code.

All ZIP code maps on this website use the ZIP Code Tabulation Areas (ZCTAs) as described below and provided by the United States Census Bureau in 2010 (or a later version). They give a pretty accurate representation of the territory a ZIP code covers. When looking at our maps, you can immediately see some of the boundary problems. Very remote regions (such as a large portion of Nevada and Utah) with few, if any, addresses to transport mail to are not designated as belonging to a ZIP code. Be sure to look up the entire street address to find the ZIP code rather than depending on the map if the address is on the same street as a ZIP code boundary on the map.

ZIP Code Tabulation Areas (ZCTAs)

ZIP Code Tabulation Areas (ZCTAs)

The United States Census Bureau created the ZIP code tabulation areas. Their objective is to disseminate statistical information about areas that are well-known to most citizens. ZCTAs are somewhat similar to ZIP codes, but not quite. As was previously mentioned, it might be challenging to pinpoint the particular geographic area that a ZIP code applies to. ZCTAs were created to precisely describe a geographic area and address some of the issues with allocating an area to a ZIP code. ZCTAs are not updated as often as ZIP codes, as well. For the Census, they are typically updated once every 10 years.

Census blocks—the lowest geographic unit utilized by the census—are used to allocate an area to a ZCTA. As shown on the right, visualize a city block that corresponds to a typical census block. Parts of city streets with their own names and addresses encircle it on all four sides. Census blocks frequently split down the middle of the street, which is a problem. Because it would take two postal workers to deliver mail to the street—one for each side of the street—ZIP codes rarely do. In the example, one mail carrier may use one ZIP number to carry mail to three sides of the block while a second mail carrier uses a different ZIP code to send mail to the other street. Because the census block is the area that is accurately measured, the Census Bureau will in this scenario give the entire block to a single ZCTA (in this case, 21044). Census block boundaries close to the edge of a ZIP code almost always split ZIP codes if you are getting really accurate (typically a matter of meters, not miles).

The Census Bureau’s statistics can provide information on the demographics of a ZIP code.

Matching ZIP Codes with States, Counties, and Cities

Keep in mind that the purpose of ZIP codes is to facilitate mail delivery. They weren’t designed to line up with established boundaries like those of cities, counties, or even states. The ZIP code “boundary” will cross state lines if it is more practical for a mail carrier to do so in order to deliver mail. Although few ZIP codes (65733, 71749, and 73949 are notable examples) do so, some do. Our US ZIP code list includes a comprehensive overview of ZIP codes that span state boundaries.

When attempting to assign a ZIP code to a certain county (up to 25% cross county lines), congressional district, metro area, time zone, area code, etc., it becomes much more challenging. The boundaries frequently overlap at their edges. In our free zip code database by county downloads, we typically either provide the ZIP code’s most populous area or, if the ZIP code contains multiple regions, list them all.

The task is a little trickier when it comes to cities. In some cases, USPS does not use the city where the ZIP code is located. More broadly, cities are assigned to ZIP codes. The primary post office is typically identified by its city. For instance, practically all ZIP codes in St. Louis County, Missouri, list Saint Louis as their city even though it may be more accurate to refer to them by the name of a nearby smaller city.