What we know about unauthorized immigrants living in the U.S.

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The unauthorized immigrant population in the United States reached 10.5 million in 2021, according to new Pew Research Center estimates. That was a modest increase over 2019 but nearly identical to 2017.

A line chart showing that the number of unauthorized immigrants in the U.S. remained mostly stable from 2017 to 2021.

The number of unauthorized immigrants living in the U.S. in 2021 remained below its peak of 12.2 million in 2007. It was about the same size as in 2004 and lower than every year from 2005 to 2015.

The new estimates do not reflect changes that have occurred since apprehensions and expulsions of migrants along the U.S.-Mexico border started increasing in March 2021. Migrant encounters at the border have since reached historic highs.

Pew Research Center undertook this research to understand ongoing changes in the size and characteristics of the unauthorized immigrant population in the United States. The Center has published estimates of the U.S. unauthorized immigrant population for more than two decades. The estimates presented in this research are the Center’s latest, adding new and updated annual estimates for 2017 through 2021.

Center estimates of the unauthorized immigrant population use a “residual method.” It is similar to methods used by the U.S. Department of Homeland Security’s Office of Immigration Statistics and nongovernmental organizations, including the Center for Migration Studies and the Migration Policy Institute. Those organizations’ estimates are generally consistent with ours. Our estimates also align with official U.S. data sources, including birth records, school enrollment figures and tax data, as well as Mexican censuses and surveys.

Our “residual” method for estimating the nation’s unauthorized immigrant population includes these steps:

  1. Estimate the total number of immigrants living in the country in a particular year using data from U.S. censuses and government surveys such as the American Community Survey and the Current Population Survey.
  2. Estimate the number of immigrants living in the U.S. legally using official counts of immigrant and refugee admissions together with other demographic data (for example, death and out-migration rates).
  3. Subtract our estimate of lawful immigrants from our estimate of the total immigrant population. This provides an initial estimate of the unauthorized immigrant population.

Our final estimate of the U.S. unauthorized immigrant population, as well as estimates for lawful immigrants, includes an upward adjustment. We do this because censuses and surveys tend to miss some people. Undercounts for immigrants, especially unauthorized immigrants, tend to be higher than for other groups. (Our 1990 estimate comes from work by Robert Warren and John Robert Warren; details can be found here.)

The term “unauthorized immigrant” reflects standard and customary usage by many academic researchers and policy analysts. The U.S. Department of Homeland Security’s Office of Immigration Statistics also generally uses it. The term means the same thing as undocumented immigrants, illegal immigrants and illegal aliens.

For more details on how we produced our estimates, read the Methodology section of our November 2018 report on unauthorized immigrants.

Who are unauthorized immigrants?