What County Is a ZIP Code In? ZIP to County Lookup
Type a ZIP code to see its county or counties, from Census ZCTA-to-county data. ZIPs that cross county lines list every county.
How this is calculated
This is a lookup, not a calculation. The page loads the part of the Census Bureau's 2020 ZCTA-to-county relationship file for your ZIP's first three digits and finds the ZIP Code Tabulation Area you typed.
- The ZIP must be five digits. Anything else shows an error.
- The relationship file lists each county that overlaps the ZCTA, with the land area of the overlap. The page sorts the counties by that land area, largest first, and drops counties that overlap only in water.
- The result shows the county with the most overlapping land. If other counties remain, they appear in the "Also overlaps" line.
- If the ZIP is not in the file as an area, the page says so and shows no county.
Order is by land area only. The page does not show percentages, because the file's columns do not say anything about people or addresses, and a ZIP area's largest county by land is not always the county where most of its addresses are.
County names follow the 2020 vintage of the file. Where the 2024 county boundaries on the county page differ, as in Connecticut, the two pages can use different names for the same ground.
Why a ZIP can cross county lines
ZIP codes are built for mail routes, and mail routes pay no attention to county lines. A ZIP area can therefore span two or more counties, and in some places more than one state. The Census ZCTA is an area drawn to approximate that, so the same is true of it. That is why this page can answer with several counties instead of one. In the file this page uses, 10,180 of the 33,791 ZCTAs overlap more than one county by land, and 137 of them reach into more than one state. (We counted these from the file, ignoring overlaps that are water only.)
It is also why a ZIP code alone is not enough for some questions. If a tax, fee or court rule depends on the county, use the street address, or use your device position on the county page.
ZIP code or ZCTA?
The lookup is by ZCTA. A ZCTA approximates a USPS ZIP area but is not the same thing, and the Census Bureau says it should not be used to identify the official USPS ZIP for mail. Its FAQ gives the example of 30361, which has no ZCTA of its own and is folded into 30309. A ZIP with no ZCTA returns "not found as an area" here, not a guess. See ZIP code vs ZCTA for the longer explanation.
If you need your own ZIP and not a typed one, use the ZIP page, or the home page for the whole card.
Where the figures come from
- Census Bureau, Relationship Files. Checked 9 November 2026.
- Census Bureau ZCTA FAQ (archived copy, 2012). Checked 9 November 2026.