Apportionment is the process of dividing the 435 memberships, or seats, in the U.S. House of Representatives among the 50 states. At the conclusion of each decennial census, the results are used to calculate the number of seats to which each state is entitled. Each of the 50 states is entitled to a minimum of one seat in the U.S. House of Representatives.

The apportionment calculation is based upon the total resident population (citizens and noncitizens) of the 50 states. In the 2010 Census, the apportionment population also includes U.S. armed forces personnel and federal civilian employees living outside the United States (and their dependents living with them) that can be allocated back to a home state. The populations of the District of Columbia and Puerto Rico are not included in the apportionment population because they do not have voting seats in the U.S. House of Representatives.

The calculation methods used through most of the 20th century have been based upon the use of a mathematically determined priority listing of states. Adopted by Congress in 1941 and used each census thereafter, the method of equal proportions also results in a listing of the states according to a priority value—calculated by dividing the population of each state by the geometric mean of its current and next seats—that assigns seats 51 through 435. For more information, see Computing Apportionment.