ZIP codes, ZCTAs, P.O. boxes and ZIP+4
Published
How ZIP codes work, how the Census Bureau maps them as ZCTAs, why some ZIP codes have no area on a map, and what the four extra digits of ZIP+4 are for.
What a ZIP code is
The Postal Service introduced the ZIP code in 1963 to sort and deliver a growing volume of mail. The first digit stands for a broad part of the country, from 0 in the east to 9 in the west. The next two digits narrow it to a region, and the last two identify a post office.
A ZIP code is a set of delivery routes, not an area with fixed borders. Post offices assign ZIP codes to the routes they deliver.
Sources: USPS Postal Facts: decoding the ZIP Code; U.S. Census Bureau: ZIP Code Tabulation Areas (ZCTAs)
ZCTAs: the Census Bureau's version
To map ZIP codes and report figures for them, the Census Bureau builds ZIP Code Tabulation Areas, or ZCTAs. It takes each census block that has addresses and assigns the whole block to the ZIP code that appears most often among those addresses. The blocks given to one ZIP code together make its ZCTA.
A ZCTA is an approximation, not the official ZIP code. The Census Bureau says not to use ZCTAs to find the ZIP code for mail, and the Postal Service changes ZIP codes from time to time.
Sources: U.S. Census Bureau: ZIP Code Tabulation Areas (ZCTAs); U.S. Census Bureau: TIGER/Line technical documentation, chapter 5 (ZCTAs)
ZIP codes with no area on the map
Some ZIP codes serve only post office boxes. Others are unique ZIP codes that belong to one organization. The lowest ZIP code, 00501, belongs to the Internal Revenue Service in Holtsville, New York.
Because ZCTAs are built mainly from home addresses, ZIP codes like these may have no ZCTA at all. On this site, a ZIP code with no ZCTA takes its area codes from the nearest ZIP code area in the same state.
Sources: USPS: postal ZIP Code boundaries; U.S. Census Bureau: ZIP Code Tabulation Areas (ZCTAs); U.S. Census Bureau: TIGER/Line technical documentation, chapter 5 (ZCTAs)
What ZIP+4 adds
In 1983 the Postal Service added four more digits, written after a hyphen. ZIP+4 narrows a five-digit ZIP code down to a specific street or a single delivery point.
ZIP+4 is for addressing mail rather than for finding a place, so the pages on this site use the five-digit code.
Sources: USPS Postal Facts: decoding the ZIP Code; USPS: postal ZIP Code boundaries
Which one to use
- To address mail, use the Postal Service ZIP code, with ZIP+4 if you have it.
- To look up population or other Census Bureau figures by ZIP code, use the ZCTA.
- To see which area codes are local to a place, look up its town or ZIP code here.
Browse towns and ZIP codes by state
Sources: U.S. Census Bureau Data Gem: what is a ZCTA?; U.S. Census Bureau: ZIP Code Tabulation Areas (ZCTAs)