Redistricting 101

States typically redraw their congressional districts every 10 years, following the completion of the U.S. census. Typically, census data is published at the end of year in which a count takes place. The most recent U.S. census was carried out in 2020. However, due to the Covid-19 pandemic, the 2020 census results were delayed until April 2021. 

The next census will take place in 2030, and states will begin the redistricting process again in 2031. 

States will occasionally update their districts in between the processes that occur every decade. For example, Alabama and Louisiana used new lines in 2024 following court orders, and lawmakers in North Carolina and New York made adjustments to their map. Ohio plans changes in 2025.

Population changes in the census count affect both the number of congressional districts a state has as well as the size and shape of those districts. The U.S. House of Representatives has a fixed number of 435 total seats, which are divided among the states to create congressional districts with roughly even populations. 
 

Reapportionment – the redrawing of congressional district boundaries to reflect population changes – is a zero-sum game. For one state to gain representation, another must lose. Senate representation is unaffected by population changes as each state has two members.

1. After each census count, the Census Bureau provides states with initial population numbers.
2. The bureau provides states with block-by-block population counts. These census blocks are areas bound by visible features, such as roads and railroads, and invisible boundaries, such as property lines and county limits.
3. State mapmakers use the census blocks to redraw congressional districts of equal population size to account for population fluctuations. 

As of April 1, 2020, the total U.S. population for apportionment was 331,108,434. Although it can vary, the average House district now represents 761,169 people – up from 710,767 in 2010. 

States must comply with the federal Voting Rights Act and ensure that their newly drawn congressional boundaries don't adversely impact voter representation based on race or ethnicity. Depending on the state, additional rules or criteria may also apply

States create their own processes for redrawing congressional districts. In most states, the state legislature controls the redistricting process. However, some states – such as Washington, California, and Arizona – use independent or bipartisan commissions. 

When it comes to drawing new congressional boundaries, partisanship reigns supreme: 28 states put their legislative bodies in charge of drawing new congressional maps after the 2020 census. At that time, Republicans controlled the legislatures and therefore had final authority over district maps in 20 of those states, whereas Democrats controlled boundarymaking in just eight states. This political advantage likely played a role in the Republican party netting several congressional seats in subsequent elections because Republicans focused on putting vulnerable incumbents into politically safe districts.