Counties (or Statistically Equivalent Entities) are legal divisions within states. These are not uniform and have changed over time. For example, the Census Bureau still uses the counties for Connecticut even though they have been dissolved as legal entities.

Please click on the title to see exceptions and historical issues for specific states.

Size Range: 1200-8000 people
Optimal Size: 4000 people
Boundaries: Mostly related to existing boundaries
Numbers: Up to 4 digits with an optional 2 digit suffix after a decimal (ex. 1457.02, 23)
Codes: 6 digits, the last 2 representing the digits after the decimal (ex. 145702, 002300)

Size Range: 600-3000 people
Boundaries: Within a tract, made up of census blocks, never cross major boundaries.
Codes: 0-9 within a tract; 0 used to represent water

Boundaries: small areas bounded by features like streets, streams, property lines, and school districts.
Numbers: 4 digit numbers from 0000-9999; first number indicates block group

Optimal Size: the 435 areas are supposed to be as close in size as they can be.
Codes: 01-53; 00 indicates a single district for a state; 98 indicates a nonvoting delegate.

Before using Census data based on Zip Code Tabulation Areas, please read the documentation provided on the U.S. Census Bureau website. Remember that Zip Codes assigned by the USPS change over time, and the ZCTAs don't always reflect that.

From the U.S. Census website: "The Census Bureau defines ZCTAs by allocating each block that contains addresses to a single ZCTA, usually to the ZCTA that reflects the most frequently occurring ZIP Code for the addresses within that tabulation block. Blocks that do not contain addresses but are completely surrounded by a single ZCTA (enclaves) are assigned to the surrounding ZCTA; those surrounded by multiple ZCTAs will be added to a single ZCTA based on limited buffering performed between multiple ZCTAs."

Map showing the regions and divisions that the U.S. Census Bureau uses to discuss different parts of the United States.

Diagram showing the relationship between the different geographies described in the U.S. Census. This document was last updated in 2010.

Geographic codes and identifiers used to relate demographic and geographic data.

An Open Geospatial Textbook

By David DiBiase with contributions by James L. Sloan II, Ryan Baxter, Wesley Stroh, Beth Fletcher King, and many students