ZIP codes and ZCTAs: what's the difference
A ZIP code is a U.S. Postal Service delivery route, not an area. A ZIP Code Tabulation Area (ZCTA) is an area the Census Bureau draws to approximate where a ZIP code delivers, so that it can publish statistics for it. Census demographics, including every ZIP code page on this site, describe the ZCTA.
How ZCTAs are made
The Census Bureau builds ZCTAs from census blocks: each block is assigned the ZIP code used by most of the addresses in it, and blocks with the same code are joined into one area. ZCTAs are redrawn once a decade, with each decennial census; the current set dates from the 2020 Census (about 33,800 ZCTAs). The 5-digit ZCTA code is the ZIP code it was built from.
Where ZIP codes and ZCTAs differ
- Not every ZIP code has a ZCTA. ZIP codes for PO boxes only, single buildings or large organisations, and military mail, deliver to too few residential addresses of their own and usually have none.
- Boundaries differ. Because a block goes to its most common ZIP code, a ZCTA can include some addresses with a neighbouring ZIP code and leave out some of its own.
- ZIP codes change; ZCTAs do not until the next census. A ZIP code created or redrawn after 2020 is not reflected in the 2020 ZCTAs.
- A ZCTA can cross county and state lines. Olympus shows every county a ZCTA overlaps, with its share of the ZCTA's land, from the Census 2020 relationship file, and lists each ZCTA under the state of its primary county.
How Olympus matches a code
The US Geographic & Demographics API and these pages resolve a 5-digit code only to the ZCTA with exactly the same code. No ZIP-to-ZCTA crosswalk is applied, and a ZIP code without a ZCTA returns no data rather than a guess. Every response states that it describes a ZCTA.
The figures are American Community Survey 5-year estimates: they describe a five-year period, not today, and come with a 90% margin of error that can be wide for small ZCTAs.