Everything on this site that describes a ZIP code comes from one documented source, and every tool states what it can and cannot tell you. This page is the methodology note.
Postal reference data
City, state, place names, county associations and coordinates are derived from the GeoNames postal code dataset, downloaded on September 26, 2026, covering the United States, Puerto Rico, Guam, American Samoa, the Northern Mariana Islands and the U.S. Virgin Islands. The normalized union contains 41,706 distinct ZIP codes. GeoNames data is published under a Creative Commons Attribution license; the original notice is preserved with our source files.
Population, income and area data
Where a ZIP code has a matching ZIP Code Tabulation Area (ZCTA), its page shows land area from the Census Bureau's 2023 Gazetteer file and, where published, population, median household income, median home value and housing units from the 2023 American Community Survey (ACS) 5-Year Estimates. Both are official U.S. Census Bureau products, retrieved directly from census.gov. ACS figures are survey estimates, not a full count, and carry a margin of error that widens for small areas; see ZIP code demographics and population for how to read them.
What the fields mean
- Recorded place names are the postal names in the dataset. They are not legal municipal boundaries, and one ZIP may carry several names.
- County association is the county supplied with the record. A ZIP that crosses county lines may list only one county. For complete relationships use the HUD-USPS crosswalk files.
- Estimated coordinate is a representative point, not a centroid of a boundary. Distances between points are straight-line estimates computed with the haversine formula.
What we do not do
- We do not verify delivery addresses or return ZIP+4 extensions; the USPS ZIP Code Lookup does.
- We do not draw ZIP boundaries or claim that reference points prove two ZIPs touch.
- Population, income, home value and land area come only from the Census Bureau's own published figures for the matching ZCTA, never estimated or interpolated by us; see ZIP code demographics and population.
- We do not invent records for codes missing from the dataset. Their pages say so and point to USPS.
Coverage and refresh
A code missing from this dataset is not necessarily invalid: new assignments, PO Box-only codes and unique ZIPs may be absent. The reference snapshot date appears in the footer of every page and will change when the dataset is refreshed. Authoritative sources consulted while building the site: USPS, the Census Bureau's ZCTA guidance and the HUD-USPS crosswalks.