How county detection actually works

Step 1: Get coordinates

When you click "Use My Current Location," your browser's Geolocation API requests permission and returns a latitude and longitude. On phones this typically uses GPS, Wi-Fi, and cell triangulation; on desktops it's usually Wi-Fi based. For ZIP lookups we convert the ZIP to a representative coordinate via Zippopotam.us. For addresses we geocode the address using OpenStreetMap Nominatim.

The site does not guess your county from your IP address because IP geolocation is often too broad for county boundaries. A mobile carrier, VPN, office network, or internet provider can route traffic through a different city or county. Permission-based browser geolocation is a stronger signal when you need the county you are physically standing in right now.

Step 2: Point-in-polygon against county boundaries

For US locations, we send your coordinates to the US Census Bureau's Geographies API, which runs a point-in-polygon check against the official TIGER/Line county boundary dataset — the same dataset federal, state, and local governments rely on. The result includes the county name, state FIPS code, and a full 5-digit county GEOID (FIPS code).

“Point-in-polygon” means the lookup takes one coordinate point and tests which official boundary shape contains it. Because county boundaries are polygons, the answer can be precise even when the nearest city, ZIP code, or mailing address is confusing. This is especially useful near county borders, state lines, rural roads, lakes, parks, and unincorporated areas.

Step 3: Enrich with reverse geocoding

We also query OpenStreetMap Nominatim to add city, ZIP, country, and a human-readable display name. International coordinates fall back to Nominatim's administrative-boundary data.

Step 4: Return the county, state, and FIPS code

A strong county result should include more than a name. County names repeat across the country, so we show the state and county FIPS code when available. The FIPS code is the standardized numeric ID used by federal data systems and helps separate places such as Washington County, Oregon from Washington County, Pennsylvania.

How accurate is it?

County boundaries are stable, well-defined polygons — so as long as your GPS coordinate is accurate, the county result is essentially exact. The main source of error is GPS accuracy itself (typically 5–50 meters on a phone, sometimes worse indoors).

Why county-border locations need exact inputs

If you are near a border, a small GPS error can place the coordinate on the wrong side of the line. The same issue can happen with rural routes, highway exits, campsites, farms, and new developments that are not well represented in address databases. If the result affects taxes, courts, emergency services, or official records, compare the lookup with the relevant county office or parcel map.

What about privacy?

We don't store your location. Coordinates are sent only to the public Census and OpenStreetMap endpoints required to answer your lookup, and the response is rendered in your browser. No accounts, no tracking pixels.

Best lookup method by search intent

• “What county am I in right now?” Use current location / GPS.
• “What county is this house in?” Use the full street address.
• “What county is this ZIP in?” Use ZIP lookup for the primary county, then verify with address if needed.
• “What county are these coordinates in?” Use latitude and longitude lookup.

Frequently asked questions