Data sources and methodology

Everything on this site is built from free public data. Here is what each number means and where it comes from.

ZIP codes and city names

The list of 41,154 ZIP codes in the 50 states, Washington, D.C. and Puerto Rico, with each one's preferred city name, comes from the GeoNames postal code dataset (CC BY 4.0). Other territories and military (APO/FPO) ZIP codes are not included yet. Common alternate spellings of city names (St./Saint, Ft./Fort, Mt./Mount) lead to the same city page.

ZIP code areas and centers

The U.S. Postal Service treats ZIP codes as delivery routes and does not publish ZIP code boundaries. The Census Bureau builds ZIP Code Tabulation Areas (ZCTAs) that approximate them from census blocks. 33,774 ZIP codes on this site have a matching ZCTA; for those, the ZIP code's center is the ZCTA's internal point and land and water area come from the Census Bureau's 2024 Gazetteer files.

ZIP codes without a ZCTA are usually PO Box ZIP codes or ZIP codes assigned to a single organization (a university, a large company, a government agency). They are marked "PO Box" and use the post office location from GeoNames.

ZIP code boundaries

Boundary outlines are the Census Bureau's 2020 cartographic boundary ZCTAs (1:500,000), simplified to about 30 meters so they load quickly. Small islands are dropped. "Click to see which ZIP code" uses these outlines, so near a border it reflects the ZCTA, not the exact USPS route.

Time zones

Each ZIP code's time zone is the zone containing its center, using the timezone-boundary-builder project's boundaries (from OpenStreetMap). A few ZIP codes straddle a time zone line.

Counties

Each ZIP code's main county is the county containing the largest share of its ZCTA land area, from the Census Bureau's 2020 ZCTA-to-county relationship file. ZIP codes that also reach into other counties are listed as "partly in" those counties. Connecticut uses its eight traditional counties, which is how that relationship file and most people still refer to them.

Metro and micro areas (MSAs/CBSAs)

Counties are assigned to core-based statistical areas using the Office of Management and Budget's July 2023 delineation (the 2020 delineation for Connecticut's traditional counties). A ZIP code belongs to the metro or micro area of its main county.

Population, income, rent, age and homeownership

From the Census Bureau's American Community Survey (ACS) 2020–2024 5-year estimates for ZCTAs: total population (B01003), median household income (B19013), median gross rent (B25064), median age (B01002), owner-occupied share of occupied housing units (B25003) and median owner-reported home value (B25077), median gross rent by number of bedrooms (B25031), the share of adults 25 and older with a bachelor's degree or higher (B15003), and average one-way commute time, computed as aggregate travel time (B08013) divided by workers who commute (B08303). ACS figures are survey estimates with margins of error, which are larger for small ZIP codes. City, county and metro totals add up the ZIP codes assigned to them; "average median income" is the population-weighted average of ZIP code medians, not a true median.

Typical home value

Where available, the typical home value is the Zillow Home Value Index (ZHVI) for all homes, smoothed and seasonally adjusted, for September 2026, with its 12-month change. Zillow covers about 26,000 ZIP codes; elsewhere we show the ACS median owner-reported value.

Distances and radius searches

Distances are great-circle ("as the crow flies") miles between ZIP code centers, using a mean Earth radius of 3,958.8 miles. A radius search includes a ZIP code when its center falls inside the circle, so a ZIP code that only partly overlaps the circle may be left out.

Addresses

Address lookups use the free U.S. Census Bureau Geocoder, which matches addresses against Census TIGER address ranges. Addresses are not stored.

Maps

Base maps are from OpenFreeMap using OpenStreetMap data.

Updates

Data is rebuilt when sources update: Zillow monthly, ACS each December, and ZIP code lists periodically. Spot an error? Email hello@zipcodemap.org.

This site is not affiliated with the U.S. Postal Service. For mailing, always confirm a ZIP code with USPS.