Methodology and data sources
Where our data comes from, how the lookup works, and where its limits are.
Updated · Data: U.S. Census Bureau
Data sources
| Data | Source | License |
|---|---|---|
| County boundaries, land area, neighbors | Census TIGER/Line 2024 | Public domain |
| Population, home value, property taxes paid | ACS 2019-2023 5-year | Public domain |
| City residents by county | ACS place-in-county estimates (summary level 155) | Public domain |
| Cities, towns and their location | Census Gazetteer 2024 | Public domain |
| ZIP code areas by county | Census 2020 ZCTA-county relationship file | Public domain |
| County seats, official websites | Wikidata | CC0 |
| USPS ZIP place names; location of PO box and single-site ZIPs | GeoNames postal codes | CC BY 4.0 |
| County time zones | timezone-boundary-builder (IANA 2026e) x TIGER counties | ODbL |
How the lookup works
- Your browser gets your position (with your permission), or reads the ZIP, city or coordinates you type.
- It downloads the county lines for the states around that point. These files are identical for every visitor.
- It tests which county polygon contains the point (point-in-polygon), then measures the distance to the nearest other county.
Accuracy and limits
- County lines are simplified to about 55 m so the files load fast. Within 500 m of a line we always show a warning.
- GPS-enabled phones are typically accurate to about 4.9 m (16 ft) under open sky, and worse near buildings and indoors (GPS.gov).
- Area ZIPs use Census ZIP Code Tabulation Areas, which approximate USPS ZIP codes. ZIPs without a ZCTA (many PO box and single-site ZIPs) are placed at their GeoNames location and matched to Census county lines.
- County boundaries come from Census TIGER/Line, published for statistical data collection and tabulation purposes only; they are not a determination of jurisdictional authority or rights of ownership and are not legal land descriptions.
- ACS figures are survey estimates with margins of error; small places and counties have larger margins.
- Property-tax ratios are our calculation (median taxes paid / median home value), not official tax rates.
- County seats come from Wikidata; where a county lists more than one, we show all.
Updates
We rebuild the site when the Census Bureau publishes new files: TIGER/Line boundaries each year, ACS 5-year estimates each December. Each page shows when its content last changed, and every source line shows the date of the data files.
Corrections
If something looks wrong, tell us with the place and what you expected. We check the source files, fix errors in the next build and reply to you.
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Use your location, a city or a ZIP code. The check runs on your device; your location is never sent or stored.