"Apportionment" is the process of dividing the 435 memberships, or seats, in the House of Representatives among the 50 states. The Census Bureau conducts the census at 10-year intervals. At the conclusion of each census, the results are used to calculate the number of House memberships to which each state is entitled. Apportionment is the initial use of the results of each census. While there are numerous other uses for census data, a second major use is for geographically defining state legislative districts (see Redistricting Data Program).

Drafted by the Constitutional Convention in 1787 and subsequently ratified by the states, the U.S. Constitution includes this passage from Article I, Section 2:

Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective numbers....The actual Enumeration shall be made within three Years after the first Meeting of the Congress of the United States, and within every subsequent Term of ten Years, in such Manner as they shall by Law direct.

Ratified in 1865, Amendment XIV, Section 2, to the Constitution gave additional guidance on conducting the census. The census link to tax collection from the states was rescinded by Amendment XVI (1913). Contemporary guidelines about the decennial census are found in Title 13, U.S. Code (1976).

As prescribed by the Constitution, the first decennial census was conducted in 1790. Thomas Jefferson, Secretary of State at the time, directed the enumeration. Since then, the census has been taken in each year ending in the zero digit. Thus, the 2020 Census is the second census of the 21st century.

The apportionment calculation is based upon the total resident population (citizens and noncitizens) of the 50 states. In the 2020 Census, the apportionment population also includes U.S. Armed Forces personnel and federal civilian employees stationed outside the United States (and their dependents living with them) that can be allocated back to a home state. These segments were also included in the apportionment population in the 1970, 1990, 2000, and 2010 censuses. The population of the District of Columbia is not included in the apportionment population. For more details about who is counted, see the answers to questions 2 through 7 of our Frequently Asked Questions.

The Constitution provides that each state will have a minimum of one member in the U.S. House of Representatives, and then the apportionment calculation divides the remaining 385 seats among the 50 states. Congress decides the method used to calculate the apportionment.

The method for calculating the apportionment has changed over time. The methods used through most of the 20th century and into the 21st century are 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. The method of equal proportions is calculated according to provisions of Title 2, U.S. Code.

For a technical description of how the method of equal proportions was used in developing the apportionment counts, see Computing Apportionment.

Title 13, U.S. Code requires that the apportionment population counts be delivered to the President within 9 months of the census date. In the 2020, 2010, and most 20th century censuses, the census date has been April 1, meaning that the statutory deadline for delivering the counts to the President is December 31 of the census year.

According to Title 2, U.S. Code, within one week of the opening of the next session of the Congress, the President must report to the Clerk of the House of Representatives the apportionment population counts for each state and the number of Representatives to which each state is entitled.

Also according to Title 2, U.S. Code, within 15 days, the Clerk of the House must inform each state governor of the number of representatives to which each state is entitled.