The Constitutional basis for conducting the decennial census of population is to reapportion the U.S. House of Representatives. Apportionment is the process of distributing the 435 memberships, or seats, in the U.S. House of Representatives among the 50 states. With the exception of the 1920 Census, an apportionment has been made by the Congress on the basis of each decennial census from 1790 to 2020.

The apportionment population for 2020 consists of the resident population of the 50 states plus overseas federal employees (military and civilian) and their dependents living with them, who were included in their home states. The resident population counts for the District of Columbia, Puerto Rico, and the U.S. Island Areas are not included in apportionment calculations because they do not have any voting seats in the U.S. House of Representatives. The 2020 Census apportionment population was 331,108,434.

This report examines trends in congressional apportionment and discusses the apportionment population—what it is, who is included, and what method is used to calculate it. The report is part of a series that analyzes population and housing data collected in the 2020 Census.