How Zip Codes Are Assigned

Every address in the United States has a zip code, but few people understand the logic and system behind how these five-digit numbers are assigned. The zip code system is a carefully designed hierarchy that encodes geographic information into a simple numerical format. This guide explains how it all works.

The Origin of Zip Codes

The ZIP Code system was introduced on July 1, 1963 by the United States Postal Service (then the Post Office Department). ZIP stands for Zone Improvement Plan, reflecting the system's purpose of speeding up mail delivery by organizing the country into numbered zones.

Before Zip Codes

Prior to 1963, large cities used postal zone numbers — a simple one- or two-digit number appended to the city name (e.g., "New York 10, NY"). Rural areas and smaller cities had no numbering system at all, relying entirely on city and state names for mail sorting.

The 1963 Launch

Postmaster General Edward Day announced the zip code system in 1963. The original system used five digits, and compliance was voluntary. It became mandatory for second- and third-class bulk mailers in 1967.

How the Five Digits Work

Each digit in a zip code carries specific geographic meaning, moving from broad region to specific delivery area.

First Digit: National Region

The first digit represents one of ten geographic regions of the United States:

0 — Northeast (Connecticut, Massachusetts, Maine, New Hampshire, New Jersey, New York, Puerto Rico, Rhode Island, Vermont, Virgin Islands)
1 — Northeast (Delaware, New York, Pennsylvania)
2 — Mid-Atlantic (District of Columbia, Maryland, North Carolina, South Carolina, Virginia, West Virginia)
3 — Southeast (Alabama, Florida, Georgia, Mississippi, Tennessee)
4 — Midwest (Indiana, Kentucky, Michigan, Ohio)
5 — Upper Midwest (Iowa, Minnesota, Montana, Nebraska, North Dakota, South Dakota, Wisconsin)
6 — Central (Colorado, Illinois, Kansas, Missouri, Nebraska)
7 — South Central (Arkansas, Louisiana, Oklahoma, Texas)
8 — Mountain West (Arizona, Colorado, Idaho, Nevada, New Mexico, Utah, Wyoming)
9 — Pacific (Alaska, American Samoa, California, Guam, Hawaii, Oregon, Washington)

Second and Third Digits: Sectional Center

The second and third digits together identify a Sectional Center Facility (SCF) — a main mail processing and distribution center. There are roughly 900 SCFs across the country. For example:

100 through 102 — New York City processing centers
900 through 908 — Los Angeles area processing centers
770 through 772 — Houston area processing centers

Fourth and Fifth Digits: Delivery Area

The last two digits identify the specific delivery office or post office within the sectional center area. This is typically a local post office or delivery zone.

ZIP+4: The Extended Code

In 1983, the USPS introduced ZIP+4, which adds a hyphen and four additional digits to the basic five-digit code. The additional digits narrow the location further:

Sixth and seventh digits — identify a specific delivery sector (a group of streets, a large building, or a group of PO boxes)
Eighth and ninth digits — identify a specific delivery segment (one side of a street, a floor of a building, or a specific group of PO boxes)

For example, a ZIP+4 code like 10001-2345 identifies not just the Midtown Manhattan area (10001) but a specific block or building within it.

Special Zip Code Categories

Not all zip codes represent geographic delivery areas.

Unique Zip Codes

Some large organizations receive their own zip codes:

20500 — The White House
10048 — formerly the World Trade Center
12345 — General Electric in Schenectady, New York
Various military APO/FPO codes — for Armed Forces mail

PO Box Zip Codes

Some zip codes are assigned exclusively to Post Office Box sections and do not correspond to street delivery areas.

Standard vs. Non-Standard Codes

Standard zip codes — represent street delivery areas
PO Box zip codes — serve only PO Box customers
Unique zip codes — assigned to single organizations
Military zip codes — APO and FPO codes for overseas military addresses

How New Zip Codes Are Created

The USPS creates new zip codes when:

New communities develop — rapidly growing suburbs and planned communities may need new codes
Existing codes become too large — when a delivery area grows too large for efficient sorting, it may be split
Processing centers change — reorganization of mail processing can trigger new assignments
Annexation or boundary changes — when cities grow or merge, zip code boundaries may change

Zip Codes and People Search

Zip codes are valuable tools for people search:

What Zip Codes Reveal

Geographic precision — narrows location to a specific neighborhood or area
Demographic context — each zip code has associated income, age, and education data
Record filtering — adding a zip code to a name search dramatically reduces results

Using Zip Codes Effectively

Search our people search tool with a known zip code to narrow results
Use our address lookup to find residents within a specific zip code
Browse state directories for California, Texas, New York, and other states
Combine zip code with name for the most targeted search results

Fun Zip Code Facts

The lowest zip code is 00501 (Holt, New York — an IRS processing center)
The highest zip code is 99950 (Ketchikan, Alaska)
There are approximately 41,683 zip codes in the United States
The most populated zip code (77449 in Katy, Texas) has over 120,000 residents
Some zip codes cross state lines, though this is rare
Beverly Hills 90210 is the most famous zip code in pop culture, thanks to the TV show

Understanding how zip codes work gives you a useful framework for navigating American geography and improving the precision of your people searches.