The additional 4 digits help USPS more precisely group mail for delivery.
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 codesthe 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.
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 Find city and state by zip code appearance that ZIP codes are defined by a clear geographic boundary.
However, some ZIP codes have nothing to do with geogaphic areas. 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. The main issue is discussed above: 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.
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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.
Their purpose is to convey statistical data about regions that are familiar to most citizens. 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.
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 Find city and state by zip code 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, because the census block is the area that is precisely measured. If you are getting very precise usually a matter of meters, not milescensus 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, and are good examples. 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.
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.
Search by ZIP, address, city, or county: 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. Look Up a ZIP Code™.
Search by Address · Search by City/State · Search for a City by ZIP Code · Find USPS Locations Near this Address; Get Shipping Prices. If you know the address but not the ZIP Code Click here.