Census tract redistricting is the process of constructing legislative or administrative districts by aggregating census tracts—small, relatively permanent statistical subdivisions of a county or equivalent area defined by the national statistical agency. In the United States, census tracts are delineated by the U.S. Census Bureau and typically contain between 1,200 and 8,000 residents, with an optimum size of around 4,000. Because tracts come bundled with detailed demographic, racial, and socioeconomic data from the decennial census, they serve as the workhorse geographic unit for line-drawers seeking to balance population, comply with voting rights law, and document their reasoning.
In practice, mapmakers more often build districts from census blocks—the smallest census geography—because federal courts have generally required population equality measured at the block level for congressional districts. Tracts are nonetheless central to redistricting because they:
- Provide stable boundaries that change only modestly between decennial counts.
- Aggregate enough population to reveal community-of-interest patterns that single blocks obscure.
- Serve as the unit at which the American Community Survey publishes many estimates used in Voting Rights Act analyses.
Under U.S. law, redistricting using census data follows the decennial census mandated by Article I, Section 2 of the Constitution. The Reynolds v. Sims (1964) "one person, one vote" rule and Section 2 of the Voting Rights Act of 1965 both shape how tracts are combined. The Census Bureau's Redistricting Data Program (P.L. 94-171) delivers block- and tract-level population counts to states, typically within a year of Census Day.
Outside the U.S., analogous units include the United Kingdom's Output Areas used by the Office for National Statistics and Canada's dissemination areas used by Statistics Canada. Disputes over redistricting frequently center on whether tract-level splits crack or pack minority populations, fragment municipalities, or ignore communities of interest.
Example
After receiving P.L. 94-171 data in August 2021, the Michigan Independent Citizens Redistricting Commission used census tracts and blocks to redraw the state's congressional and legislative maps following the 2020 Census.
Frequently asked questions
A census block is the smallest geographic unit used by the U.S. Census Bureau, often bounded by streets; a census tract is a larger aggregation of blocks, typically containing 1,200–8,000 residents, designed to be relatively homogeneous and stable over time.
Keep learning