Address to County Lookup

• What this tool does
• Anatomy of a US county
• How to use the tool
• Common lookup examples
• Use cases & worked examples
• Methodology
• How this compares to alternatives
• Limitations & accuracy notes
• Glossary
• Frequently asked questions

What this tool does

Address to county lookup answers a deceptively common question: "what county is this address in?" Counties are the unit of local government, court jurisdiction, public health districts, property tax assessment, school districts, and most state-level reporting — but address forms almost never include the county field, so people end up Googling for it.

This tool replaces that Google search with an instant, browser-side lookup. Type an address (or ZIP, or city, or click anywhere on the map), and the tool runs a point-in-polygon test against the official US Census Bureau county boundary file to find the matching county. It then enriches the answer with population, area, density, median income, county seat, and a link to the full county directory for that state.

Because the entire county boundary database loads into your browser once, every subsequent lookup is local and instant. There's no API key, no rate limit, and no sign-up — unlike commercial address-verification tools that charge per lookup.

Anatomy of a US county

The United States has 3,143 county-level subdivisions. Most are called "counties", but there are several historical and regional variants — all functionally equivalent in federal data.

All six types share the same federal 5-digit FIPS code structure — first 2 digits state, last 3 digits county-within-state — so they can be joined to any federal dataset (Census, BLS, HUD, EPA, IRS, CDC) without special handling. This tool returns the FIPS code in every result.

How to use this tool

Common lookup examples

Famous landmarks and their counties — useful as a sanity check for what to expect when you enter an address.

Who needs to look up a county for an address

1. Tax filing and property records

Property tax, sales tax, transfer tax, and recording fees are set at the county level. Title companies, real estate agents, and tax professionals look up counties constantly to route filings to the right office. The county seat tells you which courthouse holds the records.

2. Healthcare and public health reporting

CMS, state health departments, and disease registries report at the county level. Hospitals match patient addresses to counties for service-area analysis, certificate-of-need filings, and Medicaid reporting. The county is also the unit for vital records — birth and death certificates are filed by county.

3. Court jurisdiction and legal filings

Civil suits, criminal matters, probate, and family court are handled by county courts. Lawyers verify the county for venue and to look up local rules. Process servers use the county to plan service of process.

4. Sales tax and payroll tax

Many states layer county-level sales taxes on top of the state base rate. Payroll providers need the county for local income tax (Indiana, Maryland, several Ohio counties). E-commerce platforms use county lookups to apply the right tax rate for delivery addresses.

5. Voting and political analysis

Counties are the unit for election returns. Campaigns, journalists, and political scientists look up county for any address to join voter rolls to county-level results, demographics, and historical voting patterns.

6. School district and education

In some states, school districts are organized by county (Florida, Maryland, Nevada). Families relocating use county lookups to find their assigned school board and ratings. Education researchers use them to join school data with demographics.

7. Insurance and risk modeling

Many insurance carriers price by county (auto, homeowners, flood). FEMA flood maps, wildfire risk databases, and earthquake hazard zones are joined at the county level. Underwriters look up counties to apply the right rate territory.

8. Government program eligibility

Federal programs (USDA rural development, HUD CDBG, EPA brownfields, SBA disaster loans) often use county-based eligibility criteria. Applicants and grant writers look up counties to confirm a project site qualifies before filing.

Methodology

Boundary data

County polygons come from the US Census Bureau TIGER/Line shapefiles, distributed in topojson format via the us-atlas project. The 10 m resolution variant is used — high enough fidelity for accurate point-in-polygon at the meter level but small enough to load instantly in a browser (~1 MB compressed).

Address geocoding

Addresses are converted to lat/lng coordinates using the Photon geocoder, which is built on OpenStreetMap data. Photon supports typo-tolerant autocomplete and works for street addresses, ZIPs, cities, and landmarks. Reverse geocoding (turning the lat/lng back into a display address for confirmation) uses Nominatim.

Point-in-polygon test

Once we have a lat/lng, the tool runs a ray-casting point-in-polygon test against each county feature. Ray-casting works by drawing a horizontal ray from the point and counting how many times it crosses the polygon's edges — odd = inside, even = outside. Holes (lakes inside counties) are handled by inverting the result for inner rings.

for each ring in polygon:
  for each edge (i, j) in ring:
    if (yi > lat) ≠ (yj > lat) and
       lng < (xj−xi) · (lat−yi) / (yj−yi) + xi:
      flip inside flag
return inside

The scan iterates ~3,140 features, but each test is microseconds. End-to-end lookup runs in 5–20 ms on modern hardware — too fast to perceive a delay.

Demographic enrichment

Once a county is matched, the tool joins to counties.json — a pre-built file containing population, area, density, median income, median age, college rate, poverty rate, county seat, and counts of cities and ZIPs for every US county. The base data comes from the US Census ACS aggregated by SimpleMaps.

How this compares to alternatives

County lookup is offered by several APIs and commercial tools. Here's an honest side-by-side.

Limitations & accuracy notes

• Geocoding is not 100% accurate. The Photon/OpenStreetMap geocoder may return a slightly different address than typed, especially in rural areas. The county lookup uses the geocoded coordinate, not the typed text — so verify the displayed address matches your intent.
• Boundary precision is ~10 m. County polygons are simplified from the original Census shapefiles to ~10 m tolerance. For addresses within ~10 m of a county line, the result could fall in either polygon. Always verify with the county assessor for legal or tax purposes near a border.
• One county per point. The tool returns the first county whose polygon contains the point. In the rare case where a polygon is multi-part and boundaries overlap (this should not happen in Census data, but theoretically could), other matches are not reported.
• US only. The boundary file covers only the 50 states + DC. Puerto Rico, Guam, and other US territories are not included.
• Demographics lag. Population, income, and other ACS metrics are 1–3 years out of date. Boundary data refreshes only after each decennial Census (most recent: 2020).
• Not a substitute for authoritative records. Use county assessor records, the FCC API, or Census Geocoder for legal, tax, or compliance work that requires the official answer in writing.

Glossary

County

A primary subdivision of a US state, used for local government, court jurisdictions, public health districts, and tax assessment. The US has 3,143 county-level units (counties + parishes in Louisiana + boroughs in Alaska + independent cities).

FIPS code

A 5-digit Federal Information Processing Standard identifier — first 2 digits are the state FIPS, last 3 are the county FIPS within that state. Example: Los Angeles County, CA = 06037 (06 = California, 037 = LA County).

Parish

Louisiana's name for a county. Functionally identical, historical legacy of French Catholic ecclesiastical districts. Louisiana has 64 parishes.

Borough

Alaska's name for a county-equivalent. Alaska has 19 organized boroughs plus the unorganized borough (which Census splits into census areas).

Independent city

A city that is not part of any county and reports its own county-equivalent FIPS. Most are in Virginia (39 of them); other examples: Baltimore MD, St. Louis MO, Carson City NV.

Reverse geocoding

The process of converting a latitude/longitude into a human-readable address. This tool uses Nominatim (OpenStreetMap) for the address label after the county lookup.

Point-in-polygon

A computational geometry operation that determines whether a point lies inside a polygon. Used here with ray-casting against the Census county boundary file.

TIGER/Line

Topologically Integrated Geographic Encoding and Referencing — the US Census Bureau's authoritative geographic data product, including county, state, and ZCTA boundaries.

Related tools and resources

For a GPS-only county finder (no address typing needed), use What County Am I In?. If you also need the city or ZIP for the same address, What City Am I In? and What ZIP Code Am I In? handle those. To browse every county in a given state, see the per-state county directories:

Frequently asked questions

County boundary polygons from the US Census Bureau TIGER/Line shapefiles, distributed via us-atlas (10 m resolution). County demographics aggregated from the SimpleMaps US ZIP Codes Database via the US Census American Community Survey. Address autocomplete via Photon and reverse geocoding via Nominatim. Map rendering via MapLibre GL JS with OpenFreeMap tiles. Point-in-polygon implemented as inline ray-casting (no library dependency).