What county am I in?

Worked example: the Colorado capitol

The point 39.739, -104.985 is inside exactly one county polygon: Denver County, Colorado. Its FIPS code 08031 splits into state code 08 (Colorado) and county code 031.

The readout then joins that code to the Census tables. The county had 729,019 residents in the Vintage 2024 estimate, 95th of 3,144 nationally, on 153.1 square miles of land. That is 4,763 people per square mile, 20th densest in the country. SAIPE puts its 2023 median household income at $93,572.

How point-in-polygon lookup works

Each county is stored as one or more polygons of longitude and latitude vertices, from the Census 2024 cartographic boundary file simplified for the web. To test a point, the tool casts an imaginary ray from it and counts how many polygon edges the ray crosses. An odd count means the point is inside. Holes, such as an independent city inside a county, are tested the same way and flip the answer back to outside.

The boundary file, about 1.6 MB, downloads once and the test runs in your browser. Your coordinates are not sent to this site. If you type an address, only that text goes to the Photon geocoder to get coordinates.

Why GPS near a county line can pick the neighbor

Phone GPS fixes wander, more so indoors or between tall buildings. The browser reports its own accuracy estimate, and a fix can drift across a boundary that runs down a street or river. The boundary file is also generalized for web use, so its lines can sit some distance from the legal boundary.

If you are close to a line, click your exact spot on the map or enter a full street address. For tax, voting, or legal purposes, confirm with the county assessor or election office.

Parishes, boroughs, and independent cities

Of the 3,144 county-equivalents in the 2024 data, 2,999 are called counties. Louisiana has 64 parishes. Alaska has 30 boroughs and census areas. There are 41 independent cities, most in Virginia, plus Baltimore, St. Louis, and Carson City. Washington, DC counts as one. Connecticut is covered by 9 planning regions.

The tool treats all of them the same way and uses the right word in the result.

Connecticut and Alaska

The outlines are the Census 2024 cartographic boundary file. It draws Connecticut's 9 planning regions with codes 09110 to 09190, the county-equivalents that current Census tables use. A point in Hartford returns Capitol Planning Region (09110) with its 2024 population.

Alaska's Chugach (02063) and Copper River (02066) census areas, split from Valdez-Cordova in 2019, are in the same file, so points there return the current unit.

The outlines are simplified for the web, so a point within a few hundred meters of a county line can land on the wrong side. If the answer matters, such as for a property tax or voter question, check the county's own GIS viewer or your tax bill for the final word.