County lookup quality depends on the source data. A county answer should not be guessed from a search snippet, an IP address, or a city name when an official boundary match is available. What County I Am In combines public geospatial services with clear fallbacks so users can understand where each answer comes from and when they should verify it with a government office.
US Census Bureau — Geographies API
For every US lookup we query the Census Bureau's Geographies endpoint, which performs a point-in-polygon check against the official TIGER/Line county boundary dataset. This returns the canonical county name and 5-digit FIPS GEOID. geocoding.geo.census.gov
This is the most important source for US county results because it answers the jurisdiction question directly: which county boundary contains this coordinate? The response can include county name, state code, county code, and GEOID, which helps avoid confusion between counties with the same name in different states.
OpenStreetMap — Nominatim
We use Nominatim for forward geocoding (address → coordinates), reverse geocoding (coordinates → city/place name), and as the primary data source for international lookups. nominatim.openstreetmap.org
OpenStreetMap is a global collaborative map. It is especially useful for turning a human-readable address into coordinates and for describing nearby place names. For US county jurisdiction, we still prefer Census boundary matching after coordinates are known because official county polygons are the strongest source for a county answer.
Zippopotam.us
We resolve US ZIP codes to representative coordinates via the free Zippopotam.us service, then run the coordinate lookup against the Census Bureau. api.zippopotam.us
Why ZIP lookup has limits
ZIP codes are postal routing identifiers, not county boundaries. A ZIP code can include more than one county, and a county can include many ZIP codes. ZIP lookup is helpful for a quick primary-county result, but address or coordinate lookup is the better choice for taxes, property records, courts, voting, permits, and any location near a county border.
How we think about accuracy
- Best: GPS, latitude/longitude, or exact address converted to coordinates and matched to Census county boundaries.
- Good: ZIP code converted to a representative coordinate for a quick primary-county lookup.
- Weak: City name, IP address, or vague place name without a precise coordinate.
Independent verification
For legal, emergency, tax, voting, property, or regulatory decisions, verify the result with the appropriate county office, state GIS portal, parcel map, election office, assessor, recorder, clerk, or emergency-service authority. Public boundary data is strong, but official decisions should always be confirmed with the agency responsible for that decision.