Understanding and mapping ZIP Codes

Caitlin Dempsey


If you live or frequently send mail to the United States, then you probably understand that one of the most important components of the address is getting the ZIP code correct in order to have your mail successfully arrive at the intended address. 

In terms of mail delivery, the city or community that you list is not as important as long as the ZIP code is correct. 

Standing 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.

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The ZIP code is a postal code specific to the United States that was created in 1963 as a measure to increase mail sorting efficiency. Mail processing by ZIP code was improved by having the base five-digit ZIP code tied to geography. Philadelphia Postal Inspector Robert Moon came up with the idea of using a national code system for processing mail in 1944.

Moon believed that a digit-based mail delivery system would aid in the post-World War II surge in mail volume. Moon’s original plan called for a three-digit system that would route mail to regional centers based on location.

In 1963, the ZIP code concept was introduced to the public as an attempt to increase mail sorting efficiency. The ZIP code system used Robert Moon’s three-digit proposal and added two more digits for local zones established by Postmaster General Edward Day.

The first digit in the ZIP code designates the geographic region of the ZIP Code. For example, a ZIP code starting with “3” is for mail intended for the states of Alabama (AL), Florida (FL), Georgia (GA), Mississippi (MS), or Tennessee (TN). A ZIP code starting with 3 also covers military designations of: Army Post Office Americas (APO AA), or Fleet Post Office Americas (FPO AA).

The Regions for Each ZIP Code

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)

ZIP Codes Vary in the Geography Covered

While most people associate ZIP codes with a specific geographic area, they can also refer to a single site, such as a building.

For example, one ZIP code (48222) is solely for the J.W. Wescott, a mail boat operated by the United States Postal Service out of Detroit. The only floating ZIP code in the United States, the 45-foot contract postal boat distributes mail to passing ships along the Detroit River.

ZIP codes can designated a region of a city, a group of high-volume apartments, or a single building. The ZIP+4 code is used to designated a subarea within a ZIP code such as PO boxes or a city bock.

A black and white dot map showing ZIP codes in the continental United States.
The center of each ZIP code in the United States is depicted on this map. Centroids derived from the U.S. Census Bureau’s 2010 ZIP Code Tabulation Areas (ZCTAs). Map: Caitlin Dempsey 

Accessing GIS ZIP Code Data

There are some free sources for accessing ZIP Code data in GIS format.

The US Census Bureau offers what is known as ZIP Code Tabulation Areas (ZCTA) which is a shapefile format polygon file of the generalized ZIP Code service areas.  

The ZIP Code Database Project offers public domain data based on the 200 census for ZIP Codes in either CSV or MySQL dump format.  The databases include geographic coordinates.


Some have looked at the numbering scheme used for ZIP Codes.  Ben Fry created an basic application called zipdecode that shows the location of all the ZIP codes in the United States. 

You can type in a ZIP code and the location and name of that location will show up on the map.  If you hold down the shift key and type the first digit (or number) of the ZIP code, the region of the United States is highlighted. 

For example the number “9” highlights California, Oregon and Washington. 

Highlighting through the entire set of nine numbers sequentially (1, 2, 3 etc.) provides an interesting manual animation of how the ZIP codes are laid out across the country.

Ben Fry's ZIP Code Map: Zipdecode.
Ben Fry’s ZIP Code Map: Zipdecode.

US ZIPScribble Map

Robert Kosara took this idea a step further and created the US ZIPScribble Map.  His premise was to “connect the dots”.  He connected all the ZIP code point locations in ascending order and color coded the map to see if the ZIP codes adhered to state boundaries.  He found that some ZIP codes did adhere to state boundaries and others did not.

Robert Kosara's Zip Scribble Map
Robert Kosara’s ZIP Scribble Map

This article was originally published on November 17, 2011 and has since been updated.


The Untold Story of the ZIP Code. (2013, April 1). U.S. Postal Service Office of Inspector General. https://www.uspsoig.gov/sites/default/files/reports/2023-01/rarc-wp-13-006_0.pdf


USPS Making the Case for GIS
With the fiftieth anniversary of the ZIP Code, the Office of the Inspector General at the USPS has release a paper exploring ways to improve the ZIP code. The report, entitled “The Untold Story of the ZIP Code”, recommends linking the USPS address database with GIS to create a more efficient ZIP Code system.

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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.