ZIP Codes in the United States

Caitlin Dempsey

Updated:

It’s likely that if you routinely send mail to a location within the United States, you know how critical it is to get the ZIP code right in order for your mail to arrive at its intended destination.

What Does ZIP Code Stand For?

The ZIP stands for “Zone Improvement Plan.”

The Census Bureau defines ZIP code as:

A Zone Improvement Plan (ZIP) Code is the numerical code assigned by the U.S. Postal Service to designate a local area or entity for the delivery of mail. ZIP Codes may consist of 5, 7, 9, or 11 digits, and may refer to a street section, a collection of streets, an establishment, a structure, or a group of post office boxes.

While most people think of ZIP codes as covering a specific area of geography, ZIP Codes can also represent just one location such as a building. In fact, one ZIP code (48222) is just for a U.S. Postal Service mail boat out of Detroit called the J.W. Wescott. The 45-foot contract postal boat distributes mail to passing ships on the Detroit River.

A subarea within a ZIP code, such as PO boxes or a city block, is represented by the ZIP+4 code.

In 1944, Philadelphia Postal Inspector Robert Moon, dubbed “Mr. Zip,” proposed using a national code system for mail processing. Moon felt that a digit-based mail delivery system would help with the increase mail volume after World War II. Moon’s original proposal was envisioned as a three-digit system that would send mail to regional hubs based on geography.

The ZIP code concept was finally introduced in 1963 as a way to improve the efficiency of mail sorting. The ZIP code system included the three digits proposed by Robert Moon and added an additional two digits for local zones devised by Postmaster General Edward Day.

The advent of the ZIP code also necessitated the development of a standardized way to abbreviate all of the individual states. Prior to 1963, states were abbreviated to the first three or four letters of their names.

Addressing equipment at the time could only fit 23 characters including spaces on the bottom line. So, in order to all of the needed information on that line (town, state, and ZIP code), all states were abbreviated to two characters.

How Many ZIP Codes are There in the United States?

According to the United States Postal Service, there are currently 41,683 ZIP codes in the United States.

A black and white dot map of all the centers of ZIP codes in the continental United States.
A map showing the center of each ZIP code for the United States. Centroids generated from the 2010 ZIP Code Tabulation Areas (ZCTAs), U.S. Census Bureau. Map: Caitlin Dempsey.

Geography of ZIP Codes

The digits in a ZIP code are tied to the country’s geography. The base ZIP code has five digits.

The very first number represents the general geographic area of the country and range from 0 to 9. Each first digit represents:

0

Connecticut (CT), Massachusetts (MA), Maine (ME), New Hampshire (NH), New Jersey (NJ), New York (NY, Fishers Island only), Puerto Rico (PR), Rhode Island (RI), Vermont (VT), Virgin Islands (VI), Army Post Office Europe, Central Asia, and the Middle East (APO AE); Fleet Post Office Europe and the Middle East (FPO AE)

1

Delaware (DE), New York (NY), Pennsylvania (PA)

2

District of Columbia (DC), Maryland (MD), North Carolina (NC), South Carolina (SC), Virginia (VA), West Virginia (WV)

3

Alabama (AL), Florida (FL), Georgia (GA), Mississippi (MS), Tennessee (TN), Army Post Office Americas (APO AA), Fleet Post Office Americas (FPO AA)

4

Indiana (IN), Kentucky (KY), Michigan (MI), Ohio (OH)

5

Iowa (IA), Minnesota (MN), Montana (MT), North Dakota (ND), South Dakota (SD), Wisconsin (WI)

6

Illinois (IL), Kansas (KS), Missouri (MO), Nebraska (NE)

7

Arkansas (AR), Louisiana (LA), Oklahoma (OK), Texas (TX)

8

Arizona (AZ), Colorado (CO), Idaho (ID), New Mexico (NM), Nevada (NV), Utah (UT), Wyoming (WY)

9

Alaska (AK), American Samoa (AS), California (CA), Guam (GU), Hawaii (HI), Marshall Islands (MH), Federated States of Micronesia (FM), Northern Mariana Islands (MP), Oregon (OR), Palau (PW), Washington (WA), Army Post Office Pacific (APO AP), Fleet Post Office Pacific (FPO AP)

The next two digits represent the region within each of the above areas. The last two digits in the five digit ZIP code represent specific Post Offices or postal zones in larger cities. 

ZIP+4 Code includes four additional digits after the first five digits and a dash (e.g 10005-1234). These four digits narrow down the location of the ZIP+4 Code to to specific streets, buildings, houses and businesses.

A comic style map on an orange background with the outline of the United States showing how ZIP codes work.
A map of the contiguous United States from a 1968 promotional comic book created to encourage the use of ZIP codes when sending mail. Source: Smithsonian.

ZIP codes are not static. The US Postal Service makes frequent updates and changes to ZIP codes.

Fun Geography Facts About ZIP Codes

The highest ZIP Code number is 99950 in Ketchikan in Alaska.

The lowest ZIP Code number is 00501 which is designated for the Internal Revenue Service in Holtsville, New York.

Cathedral Station, NY, 10025 had the most street deliveries in 2012, with 46,395 deliveries.

The largest ZIP code by geography is located around Tonopah, Nevada. ZIP code 89049 covers 10,000 square miles and is located between Las Vegas and Reno.

ZIP code 48222 is the only floating ZIP code in the country. As a postal ship, the ZIP code doesn’t have a fixed geographic location. The postal ship, J.W. Westcott II, moves around the Detroit River delivering mail to members of ship vessels in the area. Mail intended for ship crew members is addressed as “Vessel Name, Marine Post Office, Detroit, Michigan, 48222.”

Read next: Using Giant Arrows to Guide Airmail Flights

References

Pope, N. A. (2013). Promotional ZIP code comic book. National Postal Museum. https://postalmuseum.si.edu/collections/object-spotlight/zip-code-comic-book

State Abbreviations. (2019, May). USPS.com. https://about.usps.com/who/profile/history/state-abbreviations.htm

Stiles, M. (2013, July 1). The ZIP code turns 50 today; Here are 9 that stand out. NPR.org. https://www.npr.org/sections/thetwo-way/2013/07/01/197623129/the-zip-code-turns-50-today-here-are-9-that-stand-out

The Untold Story of the ZIP Code. (2013, April 1). U.S. Postal Service Office of Inspector General. https://www.uspsoig.gov/reports/white-papers/untold-story-zip-code#:~:text=In%201963%20the%20Post%20Office,an%20organizing%20and%20enabling%20device.

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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.

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