ZIP Codes 101
The "ZIP" in ZIP Code stands for "Zone Improvement Plan". A ZIP Code indicates the destination post office or delivery area to which a letter will be sent for final sorting for delivery. A ZIP Code is simply the name for a postal code in the US. Therefore, all ZIP Codes are postal codes but not all postal codes are zip codes.
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ZIP Codes designate delivery routes and areas. That's their whole shtick. For further information on how they work and how to read them, look below.
What ZIP Codes Are and How they Work
There are three main parts of the 5-digit ZIP Code—the national area, the region or city, and the delivery area. The United States Postal Service (USPS) has segmented the country into 10 ZIP Code areas. Starting in the northeast, they are numbered 0-9. The map below shows each ZIP Code's region:
The first digit of the ZIP Code is allocated as follows:
|ZIP Codes Beginning With||States|
|0||Connecticut, Massachusetts, Maine, New Hampshire, New Jersey, Puerto Rico, Rhode Island, Vermont, Virgin Islands, Army Post Office Europe, Fleet Post Office Europe|
|1||Delaware, New York, Pennsylvania|
|2||District of Columbia, Maryland, North Carolina, South Carolina, Virginia, West Virginia|
|3||Alabama, Florida, Georgia, Mississippi, Tennessee, Army Post Office Americas, Fleet Post Office Americas|
|4||Indiana, Kentucky, Michigan, Ohio|
|5||Iowa, Minnesota, Montana, North Dakota, South Dakota, Wisconsin|
|6||Illinois, Kansas, Missouri, Nebraska|
|7||Arkansas, Louisiana, Oklahoma, Texas|
|8||Arizona, Colorado, Idaho, New Mexico, Nevada, Utah, Wyoming|
|9||Alaska, American Samoa, California, Guam, Hawaii, Marshall Islands, Federated States of Micronesia, Northern Mariana Islands, Oregon, Palau, Washington, Army Post Office Pacific, Fleet Post Office Pacific|
Reading a ZIP Code
After the first number in a ZIP Code is assigned, the USPS assigns the next two numbers according to city. If a region has a main town or city, the USPS will often assign it the first ZIP Codes. After that, the ZIP Codes will proceed alphabetically.
The first three digits of a ZIP Code together usually indicate the sectional center facility to which that ZIP Code belongs. This facility is the mail sorting and distribution center for a zone or area. Some sectional center facilities have multiple three-digit codes assigned to them. For example, the Northern Virginia sectional center facility in Merrifield is assigned ZIP Codes beginning in 220, 221, 222, and 223.
The fourth and fifth digits of the ZIP Code represent the area of the city or town. For example, if a letter is received with a ZIP Code of 47722, the USPS can know that it's in Indiana (4), it's in Vanderburgh county (77), and it's in the area of the University of Evansville (22).
ZIP Codes are Lines, Not Shapes
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:
- 42223 spans Christian County, Kentucky, and Montgomery County, Tennessee
- 97635 includes portions of Lake County, Oregon, and Modoc County, California
- 51023 and 51001 are in both Iowa and Nebraska
Although most ZIP Codes can be roughly assigned to a geographic area, ZIP Codes do not represent geographic regions; ZIP Codes denote address groups or delivery routes. Because of this, ZIP Code "areas" can overlap, be contained within another ZIP Code, or have no geographic area. For example ZIP Codes that start with 095 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 county by ZIP Code. In cases where ZIP Codes cross county lines, SmartyStreets displays the default county in the line labeled ‘County Name’ and include an additional line labeled as ‘Alternate Counties’.
In 1983, the USPS changed its ZIP Code system to include the new ZIP+4. A ZIP+4 Code uses the basic five-digit code plus four additional digits to for a full 9 digit ZIP Code. The full ZIP Code identifies a small delivery segment such as a street, a city block, a group of apartments, or even an individual address that receives a high volume of mail. The ZIP+4 Code is not required and is usually calculated automatically when the mail is sorted and processed.
As a rule, each Post Office Box has its own ZIP+4 Code. The +4 on the ZIP Code is often the last four digits of the box number, zero plus the last three digits of the box number, or, if the box number is less than 4 digits, zeros followed by the box number. Since there is this variance, the ZIP+4 Code must be looked up individually for each Post Office Box.
Not all USPS deliverable addresses have a ZIP+4 Code assigned to them. For those addresses, geocoding lookups or address validation that require a ZIP+4 may not succeed. Even though the USPS might not provide accurate geocodes for those addresses, SmartyStreets can still provide roof-top level geocodes for most addresses in the US.