County Lookup: What County Is a ZIP Code In?

Find the county a ZIP code is in — or, because a ZIP is a postal route rather than a county, the counties it spans — plus the single county for any street address. ZIP mapping uses the federal HUD-USPS crosswalk; addresses use official U.S. Census boundaries.

Street address
City or place
ZIP code

ZIP code or street address:

The right source for a postal question

A ZIP code is a U.S. Postal Service construct — a set of mail-delivery routes, not a shape on a map. So “what county is this ZIP in?” is a question about where the mail goes, and the authoritative answer is the HUD-USPS ZIP-County crosswalk: a federal dataset built from USPS delivery-point records that maps every ZIP to the counties its addresses fall in. For a specific street address we use the Census Bureau’s TIGER boundaries — the legal source of record — via the Census geocoder. Both are official U.S. government data, not the crowd-sourced maps most lookup tools rely on.

How to look up a county

Enter a ZIP code or a street address. Type a 5-digit ZIP code to see every county it touches, or a full street address to get the one county that address is in.
We match it to the authoritative source. A ZIP is matched against the federal HUD-USPS ZIP-County crosswalk (built from USPS delivery data); an address is matched against the Census Bureau’s official TIGER county boundaries.
See the county — or counties — and their share. A ZIP shows each county it reaches with the share of its residential addresses there; an address shows its single county with population, income, and unemployment.

Why one ZIP code can span two or more counties

The Postal Service draws ZIP codes to move mail efficiently along carrier routes — following roads and delivery density, not county lines. Where a carrier route crosses a county boundary, the ZIP crosses with it, so the same five digits can deliver to addresses in two, three, or more counties. In the current HUD-USPS crosswalk, 11,372 of 39,484 ZIP codes touch more than one county. That is why a ZIP-based lookup returns a set of counties with each one’s share, while an address-based lookup can return the single, exact county.

ZIP code vs. ZCTA — why most tools get this wrong

There are two different things people call a “ZIP,” and confusing them produces wrong answers. The postal ZIP is the USPS delivery construct above. A ZCTA (ZIP Code Tabulation Area) is something else: a polygon the Census Bureau draws to approximate a ZIP, by assigning each census block to the ZIP most of its addresses use. ZCTAs are handy for statistics, but they are a re-drawing — and they don’t always match where the mail actually goes.

ZIP 10505 (Katonah, New York) is the textbook case. HUD’s postal crosswalk puts most of its addresses — about 60% — in Putnam County, and the remaining 40% in Westchester County. But the Census ZCTA for 10505 is drawn entirely inside Westchester. So a tool built on ZCTA boundaries doesn’t just miss a sliver — it names the wrong primary county, reporting 10505 as Westchester when the majority of its addresses are actually in Putnam. This lookup uses the postal (HUD-USPS) mapping, so it returns both counties in the order the mail actually favors — the more correct answer to the question you asked.

Common questions

Can one ZIP code be in more than one county?

Yes — thousands of ZIP codes span two or more counties, because ZIPs follow postal delivery routes rather than county boundaries. Enter the ZIP above to see each county it reaches and the share of its addresses there.

What county is ZIP 10505 in?

ZIP 10505 (Katonah, New York) spans two counties: about 60% of its addresses are in Putnam County and 40% in Westchester County. Many tools show only Westchester because they use the Census ZCTA, which is drawn inside Westchester alone — but the postal reality is that most of 10505 is in Putnam.

Is a ZIP code the same as a county?

No. A ZIP code is a postal delivery route owned by USPS; a county is a unit of government with its own courts, elections, property records, and taxes. A ZIP can sit inside one county, cross several, and occasionally cross a state line — which is why the county has to be looked up, not assumed.

What is a county FIPS code?

Every U.S. county has a 5-digit FIPS/GEOID — a 2-digit state code plus a 3-digit county code. Putnam County, New York is 36079; Westchester County is 36119. Each county in the results above shows its FIPS code.

Sources

HUD-USPS ZIP-County Crosswalk, 2026 Q1(U.S. Department of Housing and Urban Development & U.S. Postal Service) — ZIP-to-county mapping and residential-address ratios.
U.S. Census Bureau — TIGER/Line county boundaries and the Census geocoder, for the address-to-county lookup.