ZIP code to county lookup – every county in a ZIP code

About one US ZIP code in four crosses a county line, and most ZIP code lookups hide that: they give one county, usually the one with the post office or the one at the centre of the ZIP code, and a sales tax rate, a voter registration or an insurance quote built on that answer can be wrong for a good share of the people who live there. This lookup answers the full question. Type a ZIP code and the tool draws its ZIP Code Tabulation Area, the Census Bureau's outline of the ZIP code, over the county lines on a map, cuts it along every line and lists each county it covers with the share of the ZIP area, the area in square miles and an estimate of how many of the ZIP code's 2020 census residents live in that part. The largest part comes first, which is the county most databases give. Click a county to zoom to it, copy the list or download it as CSV. A whole column of ZIP codes, pasted from a spreadsheet or opened as a CSV file, becomes a ZIP-to-county crosswalk with one row per ZIP code and county, the kind of table you would otherwise piece together from Census relationship files. For a single address, click the house on the map instead and you get the county of that exact point.

How to use it

1. Type a five-digit ZIP code (ZIP+4 is cut to five digits) and press Go.
2. Read the counties in the list, largest share first, and see the ZIP area outlined in red over the county lines.
3. Click a county in the list to zoom to it, or download the list as CSV.
4. For many ZIP codes, open "Look up a list or a spreadsheet", paste them and download the ZIP–county crosswalk CSV.
5. For a single address, open "Find the county of an address point instead" and click the house on the map.

How accurate is it?

How the shares are measured

The ZIP code area is the Census Bureau's 2020 ZIP Code Tabulation Area (ZCTA) at 1:500,000, and the counties are the 2023 cartographic county outlines at the same scale, both public domain. The tool intersects the two in your browser with the polygon-clipping library and measures every part on the WGS84 ellipsoid with GeographicLib, so the square miles are true areas, not areas on a flat web map. To keep the files small, the ZIP outlines here are simplified by about 130 m, while the county outlines are not. Where a ZIP code follows a county line, the simplified edge wobbles across the line and leaves thin strips in the county next door. A part whose mean width is under about 250 m is treated as such a strip and left out, and so is a part under 0.5 % of the ZIP area; the shares are of what remains and add up to 100 %. For a real part, the share is normally within one or two percentage points of the exact figure.

Checked against another source

We compared the result with the county GeoNames gives for 3,365 US postal codes (every tenth ZIP code). The GeoNames county was among the counties the tool found for 99.6 % of them, and it was the largest part for 95.9 %. Nearly all of the remaining 4 % are ZIP codes whose post office or mailing address lies in a different county from most of the ZIP area, such as 30350 in Sandy Springs, which GeoNames puts in DeKalb County although the ZIP area lies in Fulton County.

The people in each county are an estimate

The number of residents comes from the 2020 census count for the whole ZIP Code Tabulation Area (table P1). The tool splits it by area, as if people were spread evenly over the ZIP code, and rounds the parts so they add up to the total. Real ZIP codes have a town on one side of the line and farmland on the other, so the split can be far off. The HUD-USPS crosswalk files, which count residential addresses, are better for this; use the share of area here as a guide to where the ZIP code lies, not as a head count.

Good to know

A ZIP code is a set of mail routes, not an area

The Postal Service draws ZIP codes as delivery routes and changes them when it needs to. The Census Bureau turns them into areas (ZCTAs) from the addresses in each census block, so the outline is a good picture of where a ZIP code's homes are, but some addresses near the edge belong to the neighbouring ZIP code, and ZIP codes created after 2020 are missing. ZIP codes that serve only post office boxes, a single company or a military base have no area at all; the tool then shows the county at the ZIP code's point.

The county of an address is a different question

Knowing that a ZIP code is 70 % in one county does not tell you which county a particular house is in. For property tax, voting, court papers or a permit, find the address on the map and click the house, which gives the county of that point and warns you within 300 m of a county line, or check the county's own parcel map.

County names change

The county outlines are the 2023 edition, so Connecticut shows its nine planning regions instead of its eight old counties, and Alaska's Valdez-Cordova appears as the Chugach and Copper River census areas. Data that uses older county codes will not match those rows.