The Census Bureau uses census tracts and census blocks to organize census data. A census tract is composed of census block groups. A census block group is composed of census blocks.

The Census Bureau uses demographic data to define census tracts, so that the inhabitants of each tract have a common economic status and population profile. The Census Bureau does not use demographic data to define census blocks.

A census block group contains an average of 39 census block. Each census block group has a maximum of 3,000 inhabitants. A census tract can contain between one and nine census block groups.

Census tracts have between 1,500 and 8,000 inhabitants. The Census Bureau uses physical features to define the area of a census tract, such as roads, rivers, and railroads. The shape and size of a census tract can change over time as the population changes. Census tracts do not cross state or county boundaries.