What Are ZIP Codes?

Everything you need to know about the US Zip Code system
  • By Doaa Kurdi
  • 2022-05-24

When you are looking for a way to sort mail or segment mailing lists within the United States, using the US Zip Code system can be valuable. US zip codes are postal codes the US Postal Service (USPS) uses to sort mail efficiently. ZIP stands for Zone Improvement Plan.

Zip Codes are 5-digit numbers created by the USPS that represent post offices all over the US. Each 5-digit zip code comprises the national area, the region or city, and the delivery area. The US is divided into ten zip code areas, with numbers beginning in the Northeast and crossing the country to the far West.

Assigning Zip Code Numbers

  • The first number of the zip code indicates a group of states assigned to a central mail processing facility, also called the sectional center facility or "sec center," used to process and sort mail

  • The following two numbers represent the codes of a central post office within that region.

  • The final two numbers designate small or local post offices or postal zones.

  • The ZIP+4 code is not essential, but it helps the post office with additional mail sorting. A ZIP+4 code could correspond to a city block, an apartment complex, or an individual who receives vast mail. It is also used for each PO Box number to conform with a unique ZIP+4 code.

  • Zip codes usually span several states to make mail routing and delivery easier.

The original zip code zoning address system began during the second world war in 1943. Many post office employees were called to serve in the military, and post offices were understaffed. The system needed to be simplified to accommodate the decreasing employee circumstances. Zip codes represent approximate areas but are not entirely accurate.

ZIP Code Tabulation Area (ZCTA)

All zip code maps use Zip Code Tabulation Areas as determined by the United States Census Bureau. The US Census Bureau developed Zip Code Tabulation Areas to collect statistical data about the citizens living in specific geographic areas, including insight into the area's demographics, among other data. The US Census is updated every ten years.

Correlating ZIP Codes with States, Counties, and Cities

Zipcodes were not created to relate to existing cities, counties, or states. It is more efficient for a mail carrier to drive across a state line to deliver mail most of the time.

It gets even more complicated when assigning a ZIP code to a county, congressional district, metro area, time zone, area code, etc. The edges of the boundaries commonly overlap.

Here is a list of zip codes and the states included in each.

ZIP Codes Beginning With States / Territories
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
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
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

When you segment, create customer lists, or segment customers by similar demographics, zip codes can often make your task easier.