Congressional Districts and FIPS: CD Codes Explained

Congressional district codes use a different format than the numeric FIPS codes for states and counties. Rather than a purely numeric identifier, CD codes combine a state abbreviation with a zero-padded 2-digit district number — for example, WA-07 for Washington's 7th Congressional District. Some agencies use a numeric encoding (state FIPS + 2-digit district number), but the abbreviation-number format is the most human-readable and widely used in public data.

Congressional districts are redrawn after each decennial census through a process called redistricting. The 116th Congress districts (based on 2010 Census) differed from the 118th Congress districts (based on 2020 Census) in nearly every state. This means CD codes in datasets from before and after the 2020 Census redistricting are not directly comparable — WA-10 may cover a very different geographic area in a 2024 dataset than it did in a 2018 dataset.

In federal data, congressional districts appear in Census demographic profiles (ACS publishes estimates at the CD level), FEC campaign finance data, and the Census Bureau's Voting and Registration surveys. The Census publishes CD-level demographic data using the numeric encoding (state FIPS + district number), while the House of Representatives and most political data use the abbreviation-number format. Our CD pages, like WA-07, show both and link to the state's full congressional delegation.

One important geographic note: congressional districts don't follow county boundaries. A single CD can span parts of multiple counties, and a single county can contain parts of multiple CDs. King County, WA is split among several CDs. For analysis that requires linking CD data to county-level federal statistics, you'll need a CD-to-county crosswalk. The Census Bureau publishes these relationships through their geographic relationship files.