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. 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).
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.
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.
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 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.
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).
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 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.