Official State of Iowa Website

Congressional Apportionment

What is It?

Apportionment is the process of dividing up the 435 memberships, or seats, in the House of Representatives among the 50 states according to population.

The Census Bureau has a dual responsibility in this connection:

  • It counts the population at 10-year intervals and
  • It uses the results of the count to calculate the number of House memberships each state is entitled to have.

Who's Counted?

The apportionment calculation is based upon the total resident population of the 50 states, excluding:

  • Individuals living on "foreign soil" within the United States, including the embassies, ministries, legations, consulates, and chancelleries of other nations
  • Citizens of other nations temporarily visiting or traveling (but not residing) in the United States
  • Residents of the District of Columbia or of any of the U.S. commonwealths and territories, jurisdictions whose residents are counted in the decennial censuses but do not have the same type of congressional representation as the Constitution provides to the residents of the 50 states.

How Is It Calculated?

The method of equal proportions has been used for calculating the apportionment since 1941.

  • Priority values are calculated by dividing the population of each state by the geometric mean of its current and next seats
  • Seats 51 through 435 are assigned by listing the states according to a priority value


Title 13, U.S. Code requires that the apportionment counts--the resident population totals for each state--be delivered to the President within 9 months of the census date.

  • In most recent censuses, the census date has been April 1, meaning that the Office of the President received the counts by December 31 of each census year.
  • Within a week of the opening of the next session of the Congress, the President must report to the Clerk of the House of Representatives the census counts for each state and the number of Representatives to which each jurisdiction is entitled.
  • Within 15 days, the Clerk of the House informs each state Governor of the number of Representatives to which each state is entitled.
  • The legislature in each state is responsible for geographically defining the boundaries of its congressional and other election districts--a process known as redistricting--and more detailed census results are used for these purposes.

More Information

For more information from the Census Bureau about apportionment, see: