ZIP codes in a county, city, or state

Worked example: Travis County, Texas

The tool opens on Travis County, Texas, home of Austin. After the ZCTA file and county outlines load, it finds 47 ZCTAs whose Census internal point falls inside the county line, from 78617 near the airport to 78759 in north Austin. Their combined land area is about 863 square miles.

That total is smaller than the county itself because some ZCTAs that reach into Travis County have their internal point in Williamson, Hays, or Bastrop County, so they are listed there instead. Switch to City or town and pick Austin to see the other view: every ZCTA that overlaps the city by a meaningful share, whichever county its point is in.

How to use it

1. Choose County, City or town, or State above the inputs.
2. For a county, type its name, state abbreviation, or 5 digit FIPS code in County and pick it from the list. For a city, pick the State first, then the place in City, town, or village; the list is sorted by 2024 population. For a state, pick it from State.
3. The ZCTA and boundary files load once in your browser. The map outlines the county or state and drops a dot on each ZCTA internal point; in city mode an orange marker shows the place itself.
4. Read the count and combined land area on the right. The table below the map lists every code with its coordinates and land area, plus its county in city and state mode.
5. Press Copy as list for a comma separated list or Download CSV for a spreadsheet. The selection is kept in the page address, so the link reopens the same list.

How each mode decides what is inside

County tests the internal point of each of the 33,791 ZCTAs in the 2024 Gazetteer against the 2024 Census county outline. A ZCTA is listed when its point is inside. A point can sit in at most one county, so each code appears in one county list at most, and the county lists for a state add up without double counting. A few coastal codes whose point falls just outside the shoreline are in no county list.

State runs the same test against every county in the state and also reports which county holds each point. A few coastal points sit just outside the simplified county shoreline; for those the tool checks the state outline instead and leaves the county column empty. A couple dozen miss the state outline as well and are left out. Puerto Rico and the other territories are not in the state list.

City or town works differently, because the site does not ship city boundaries to your browser. It reads the ZCTA list stored with each place, built from the Census 2020 ZCTA to place relationship file. A ZCTA is listed when the overlap covers at least 10% of the ZCTA's land or at least 10% of the place's land. Small slivers along a city line are left out, and a ZCTA can appear for two neighboring cities.

ZIP codes versus ZCTAs

A ZIP code is a set of mail delivery routes, not an area with a fixed border. The Census Bureau draws ZIP Code Tabulation Areas by giving each census block the ZIP code used by most of its addresses. Most ZCTA codes match the ZIP code on an envelope, but codes for PO boxes, single large organizations, and military mail have no ZCTA, so they will never show up in these lists.

ZIP codes also cross county and city lines freely. A ZIP code with a mailing city of Austin can sit mostly in another county, and a suburb can share a ZIP code with its neighbor. That is why the county and city modes can give different answers for the same area.

Using the list

The copied list is comma separated, ready to paste into a CRM filter, an ad platform's location targeting, or a delivery zone form. The CSV adds the internal point coordinates, the land area in square miles, and the county FIPS code and name for each ZCTA.

For service areas that follow county lines, the county mode is the safer pick: each code is in or out, with no overlaps to resolve. For anything tied to a city name, such as a mailing to residents of one town, the city mode is closer, but check codes along the edge against the Census boundary files before relying on them.