Reapportionment of representation in the United States occurs every 10 years, sometime after the decennial census. The process divides representatives among a state's population to have legislative districts as close to equal size as possible.
As of September 2014, the reapportionment formula for Congress has been in use since 1930. A state is given seats in the House of Representatives based on its population divided by the geometric mean of the state's current number of representatives and the next level of representation. This ensures each state loses or gains seats because of population shifts in proportion to the other states. Once reapportionment occurs, new congressional district boundaries are created for states with more than one representative.