After the US Census is held every ten years, the number of Congressional representatives assigned to each state is calculated in a process called apportionment. Since each state gets a number of electoral votes equal to its Congressional representation, the total number of electors has changed over the history of the United States as the size of the House of Representatives grew (or decreased, as it did after the 1840 census).
There have been 538 electoral votes since 1964. This accounts for each state having two senators (50 x 2=100), the 435 voting members in the House, and three electoral votes for Washington, DC, granted by the 23rd Amendment in 1961 [100 Senators + 435 Representatives + 3 DC votes = 538 electoral votes]. See State Elector Power for a chart of how many electoral votes are currently allotted to each state.
Determining the Number of Representatives
Article I, Section II of the Constitution says:
“Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers… The Number of Representatives shall not exceed one for every thirty Thousand, but each State shall have at Least one Representative.”
Starting in 1790, Congress enacted a law to determine the number of voting representatives after each census. In 1911, Congress fixed the number at 433, with the understanding that two more would be added when Arizona and New Mexico became states (Public Law 62-5 enacted on August 8, 1911, in effect since 1913). The 1929 the Permanent Apportionment Act established a permanent maximum number of voting representatives of 435.
There are currently 441 members in the House of Representatives: the 435 voting members representing 50 states; five non-voting delegates from DC, Guam, the US Virgin Islands, the Commonwealth of the Northern Mariana Islands, and American Samoa; and one non-voting Resident Commissioner representing Puerto Rico.
The population numbers established by the 2020 Census will be used in the automatic reapportionment and take effect for the 118th Congress of 2023-2025.
Deciding Which Apportionment Method to Use
There has been debate over the method of how congressional representatives should be allocated since the beginning of the United States. The Census Bureau notes that “It is impossible to attain absolute mathematical equality… when there must be a whole number of representatives per state.”
Essentially, since dividing state populations into the 435 available seats doesn’t result in a whole number, the government had to devise a mathematical method for dealing with fractions. If the math shows that a state should have 3.453 seats based on its population, for example, should that state receive 3 or 4 seats in the House?
There have been five basic apportionment methods used since 1790, according to the Census Bureau. Four of the methods had various equations for rounding fractional remainders, such as rounding everything down to the nearest whole number (Jefferson method) or rounding up at .5 and above (Webster method). Only the Hamilton-Vinton method (used from 1850-1900) does not involve rounding; rather, it assigns seats based on whole numbers then ranks the fractions from largest to smallest to allocate remaining seats.
The Method Equal Proportions (aka Huntington-Hill) has been in place since Congress adopted it in 1941. Under this method, each state automatically receives one representative as required in the Constitution. The remaining 385 seats are distributed using formulas that set a “priority value” for each state based in part on state populations from the most recent census.
Methods of Apportionment Used Since 1790
The chart below shows the method of apportionment used following each census and the resulting total number of voting seats in the US House of Representatives. In some years, the actual method used diverged slightly from the listed method, but these methods were the general guidelines, according to the Census Bureau.
Seats were sometimes added in between the apportionment periods when new states joined the Union. For example, nine Representatives were added to the 1840 apportionment between 1845 and 1850 for California, Florida, Iowa, Texas, and Wisconsin, bringing the total from 223 to 232.
| Year | Apportionment Method | # of House Seats |
| 1787 | Set by Constitution | 65 |
| 1790 census | Jefferson | 105 |
| 1800 census | Jefferson | 142 |
| 1810 census | Jefferson | 181 |
| 1820 census | Jefferson | 213 |
| 1830 census | Jefferson | 240 |
| 1840 census | Webster | 223 |
| 1850 census | Hamilton-Vinton | 233 |
| 1860 census | Hamilton-Vinton | 241 |
| 1870 census | Hamilton-Vinton | 283 |
| 1880 census | Hamilton-Vinton | 325 |
| 1890 census | Hamilton-Vinton | 356 |
| 1900 census | Hamilton-Vinton | 386 |
| 1910 census | Webster-based (no name) | 433 |
| 1920 census | No reapportionment held | — |
| 1930 census | Webster-based (no name) | 435 |
| 1940 census | Method of Equal Proportions | 435 |
Sources:
1. Congressional Research Service, “Apportionment and Redistricting Process for the U.S. House of Representatives,” fas.org, Oct. 10, 2019
2. Royce Crocker, “The House of Representatives Apportionment Formula: An Analysis of Proposals for Change and Their Impact on States,” fas.org, Aug. 26, 2010
3. National Archives and Records Administration, “Distribution of Electoral Votes,” archives.gov, Mar. 6, 2020
4. United States House of Representatives, “Determining Apportionment,” house.gov (accessed Dec. 7, 2020)
5. United States House of Representatives, “The House Explained,” house.gov (accessed Dec. 7, 2020)
6. US Capitol, “How Your State Gets Its Seats,” visitthecapitol.gov (accessed Dec. 16, 2020)
7. US Census Bureau, “About Congressional Apportionment,” census.gov, Mar. 30, 2020
8. US Census Bureau, “Congressional Apportionment: Historical Perspective,” census.gov, Mar. 30, 2020