How Does Congressional Apportionment Work? The 435 Seats and Formula

Congressional apportionment is the process, run every ten years, that divides the 435 seats in the U.S. House of Representatives among the 50 states based on population figures from the decennial census. Every state is guaranteed at least one seat by the Constitution. The remaining seats go to the states with the largest populations, distributed by a specific mathematical formula written into federal law. States that grew faster than the national average tend to gain seats; states that grew slower tend to lose them.¹

The 435-Seat Total Is Fixed

The House used to grow with the country. That changed in 1929, when the Permanent Apportionment Act capped the chamber at 435 voting members and set up an automatic reapportionment procedure after each census.²

Because the total does not move, apportionment is zero-sum. For one state to pick up a seat, another must give one up. That is why a difference of a few thousand residents, sometimes a few hundred, can decide whether a state’s delegation grows or shrinks for the next decade.

Who Gets Counted

The Census Bureau conducts the count in years ending in zero, with April 1 as the official census date. The Secretary of Commerce has nine months from that date to deliver the tabulated results to the President.³

The number used to allocate seats is the “apportionment population.” For each state, that means every resident, plus overseas federal civilian employees, military personnel, and their dependents who list that state as home. Crediting those individuals back to their home states keeps them from losing representation because of an overseas assignment.⁴

Residency is what counts, not citizenship. Noncitizens and undocumented immigrants living in a state are included. The 14th Amendment directs the count of “the whole number of persons in each state,” and the Census Bureau reads that as every person physically residing in the country on Census Day.⁵⁴

The Formula: The Huntington-Hill Method

Federal law requires seats to be distributed by the “method of equal proportions,” known as the Huntington-Hill method. Congress adopted it in 1941, and it has governed every apportionment since.⁶

The process starts by handing each of the 50 states the one seat the Constitution guarantees. That leaves 385 seats to hand out. The method then awards those seats one at a time, based on a priority ranking.

Each state’s priority value for its next possible seat equals its population divided by the square root of n multiplied by (n − 1), where n is the seat number being competed for. A state fighting for its second seat divides its population by roughly 1.414 (the square root of 2 × 1). A state fighting for its sixth divides by roughly 5.477 (the square root of 6 × 5). Whichever state has the highest priority value in a given round wins the next seat, its priority is recalculated for the following round, and the sequence repeats until all 435 seats are placed.⁷

The point of the geometric-mean divisor is to minimize the percentage difference in the number of people per representative between any two states. A straight population-to-seats division tends to favor larger states, because rounding hits smaller states harder in proportional terms. No formula can equalize district sizes across all 50 states with the House fixed at 435, but Huntington-Hill comes closer than the alternatives Congress considered.

What Happens After the Count

Once the Commerce Department transmits state population totals to the President, the statute sets a tight schedule.³

On the first day of the next congressional session, or within one week after, the President sends Congress a statement showing each state’s population and the number of seats it would receive under the method of equal proportions.
Within 15 calendar days of that statement, the Clerk of the House sends each governor a certificate showing how many representatives the state is entitled to.
State legislatures then redraw district lines to match the new seat count. Until a state completes redistricting, federal law provides interim arrangements, including electing any additional representatives at large.

Those steps set the seat totals. Drawing the actual district maps requires much more detailed data, and federal law requires the Census Bureau to deliver block-level redistricting data to each state by April 1 of the year following the census. For the 2030 count, that deadline falls on April 1, 2031.⁸⁷

Apportionment Also Sets Electoral College Votes

Each state’s Electoral College total equals its congressional delegation: House seats plus two senators. When a state gains or loses a House seat in apportionment, its electoral votes move by the same amount for the presidential elections that follow.⁹ A state that picks up two seats picks up two electoral votes; a state that loses one loses one. That is a large part of why the census figures land with so much political weight.

Territories and DC Are Not Part of Apportionment

The 435-seat allocation applies only to the 50 states. American Samoa, the District of Columbia, Guam, the Northern Mariana Islands, and the U.S. Virgin Islands each send a non-voting Delegate to the House. Puerto Rico sends a Resident Commissioner, who serves a four-year term rather than the usual two.

These members participate in debate, sponsor bills, serve on committees, and vote in committee. They cannot vote on final passage of legislation on the House floor. They may vote in the Committee of the Whole, but if their votes are decisive the House automatically holds a revote without them.¹⁰

The 2020 Apportionment in Practice

The most recent reapportionment followed the 2020 Census. Texas gained two seats. Colorado, Florida, Montana, North Carolina, and Oregon each gained one. Seven states each lost a seat: California, Illinois, Michigan, New York, Ohio, Pennsylvania, and West Virginia. Those results fix House delegation sizes and Electoral College votes through the 2030 count.¹¹

Some of those shifts were remarkably close. New York lost its 27th seat by fewer than 100 people in the final count. Census participation at the margin can directly determine whether a state keeps a seat.

Challenging a Count

Local and state governments that believe the Census Bureau miscounted their population can file through the Count Question Resolution (CQR) program. CQR reviews fall into two buckets. Boundary cases contest whether the Bureau used the correct legal boundaries for a jurisdiction. Housing count cases contest whether valid housing units and their residents were wrongly excluded or misassigned during processing.¹²

The program has real limits. CQR can only review data the Bureau already collected; it does not run a new count. Group quarters such as college dormitories, nursing homes, and correctional facilities are outside its scope. And even when the Bureau confirms an error, the corrected numbers go only to the affected government. They do not replace the official totals used for apportionment. For the 2020 cycle, the submission window closed on June 30, 2023.¹²

1
Library of Congress. Article I Section 2 – Constitution Annotated
2
Visit the Capitol. How Your State Gets Its Seats – Congressional Apportionment
3
Office of the Law Revision Counsel. 13 USC 141 – Population and Other Census Information
4
United States Census Bureau. Frequently Asked Questions – Congressional Apportionment
5
LII / Legal Information Institute. Apportionment Clause
6
Office of the Law Revision Counsel. 2 USC 2b – Number of Representatives From Each State in 78th and Subsequent Congresses
7
Office of the Law Revision Counsel. 2 USC 2a – Reapportionment of Representatives; Time and Manner
8
United States Census Bureau. Redistricting Data Program Management
9
National Archives. Distribution of Electoral Votes
10
Congress.gov. Parliamentary Rights of the Delegates and Resident Commissioner From Puerto Rico
11
United States Census Bureau. Table D1 – Number of Seats Gained and Lost by State, 2020 Census
12
United States Census Bureau. 2020 Census Count Question Resolution Operation