The fourth and fifth digits of the ZIP Code represent the area of the city or town. ZIP Codes are not drawn according to state boundaries. In fact, since they are designed only to increase mailing efficiency, ZIP Codes can and do cross county and state boundaries.
For example:. For example ZIP Codes that start with are assigned to the Navy, and therefore have no geographic location. Similarly, areas without regular postal routes rural route areas or areas with no mail delivery undeveloped areas , are not assigned ZIP Codes.
They are based on sparse delivery routes, so the boundary between ZIP Code areas there is indefinite. You can see our instructions on how to find a county by ZIP Code. 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. 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: 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.
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 sides by portions of city streets that each have their own name and addresses.
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 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. Using full ZIP Codes ensure the fastest, most accurate mailing possible.
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