How Many Illegal Aliens Live in the United States?
By Matt O'Brien, Spencer Raley and Casey Ryan| September 2019 | View the full PDF version of the How Many Illegal Aliens Live in the United States? issue brief
As of 2019, FAIR estimates that there are approximately 14.3 million illegal aliens residing within the United States. This number is notably higher than FAIR’s previous estimate of 12.5 million in 2017.
Based on estimates derived from FAIR’s most recent cost study, illegal immigration is likely imposing a net fiscal burden of at least $131.9 billion annually on U.S. taxpayers.[i]
We have revised our estimate based on updated information that has recently become available due to growing interest in the immigration issue. Increasing numbers of illegal aliens can be attributed to a number of factors, including:
- Unsecured borders.
- An explosion in sanctuary jurisdictions throughout the United States, where local law enforcement authorities refuse to cooperate with U.S. Immigration and Customs Enforcement (ICE).
- The availability of jobs. Illegal aliens know they will find work because America has yet to put in place mandatory E-Verify for all employers.
- The increasing number of social welfare programs, and other benefits, given to illegal aliens by states and local governments – including in-state college tuition and driver’s licenses.
- Easily exploited asylum laws, flawed detention policies and a growing immigration court backlog that, for years, have allowed illegal aliens to obtain release from ICE custody and disappear into the interior of the United States.
- The on-going promise of amnesty by members of Congress and powerful special interests.
Unless the federal government takes meaningful action to eliminate the incentives that fuel illegal immigration, the total number of illegal aliens residing in the United States could surge to over 21 million by 2025, at a cost of nearly $200 billion, annually.[ii]
Difficulty in Estimating the Illegal Alien Population
Estimating the size, distribution, and characteristics of the illegal alien population is an inexact science. The methods used by those claiming to have calculated a definitive figure should be closely scrutinized because there is no reliable source of information on illegal aliens. The Department of Homeland Security (DHS) only counts foreign nationals who enter and leave the United States in a lawful manner. The fact is, we do not know how many people cross the border unlawfully and evade immigration authorities. After all, the primary aim of illegal aliens is to avoid detection by U.S. Customs and Border Protection (CBP) and ICE.
Most current estimates of the total number of illegal aliens are based on U.S. Census Bureau data or survey data collected by private research organizations. All of these studies presume that nearly all unlawful migrants respond to demographic questionnaires and that they provide accurate information. As people whose very presence in the United States is an ongoing violation of law, many illegal aliens believe that revealing any information about their situation may lead to their arrest by immigration authorities. Therefore, when asked about how, when and why they entered the United States, illegal aliens have a strong motive to either lie or refuse to respond at all.
Demographic surveys typically accept respondents’ answers at face value. They do not verify that respondents are answering demographic questions honestly. For example, the Census Bureau’s weighted results from the American Community Survey (ACS) can vary by millions, depending on what information is requested and how questions are phrased.[iii]
In short, estimating the number of illegal aliens in the United States is an attempt to quantify an unknown, akin to counting the number of raindrops that did not fall during a storm. And the accepted techniques for doing so generally lead to estimates that are acknowledged to be lower than the actual illegal alien population.
How we Reached our Estimate
To determine FAIR’s estimate of the total number of illegal aliens in the United States, we first calculated an approximate total number of all foreign-born residents currently presumed to be living here. In order to do this, we first analyzed the latest relevant information available from the Census Bureau’s 2017 “American Community Survey” (ACS). Then, based on recent immigration trends, and using standard methods of statistical analysis, we projected the 2017 numbers to 2019.[iv]
Next, to determine a base number of illegal aliens suspected to be present in the United States, we subtracted the total number of lawfully-present migrants in the country from the total foreign-born population.
The number obtained after subtracting lawfully-present migrants from the total number of all foreign-born residents is our base illegal alien estimate.[v] However, as previously noted, due to the fact that many illegal aliens do not respond to surveys about their immigration status, or provide false information, this number is presumed to be a significant undercount. In the past, most reputable research organizations have considered a raw calculation based on ACS data to be anywhere from 15-35 percent lower than the actual suspected total illegal alien population. However, most private and academic research organizations are pro-mass migration, and they deliberately minimize the limitations inherent in their methods in order to create the false impression that the illegal alien population in the United States is smaller than it really is.
So FAIR corrected for this undercount, once again using standard methods of statistical analysis based on historically-accepted ACS undercount assumptions. We then estimated the net number of illegal aliens suspected to have entered the country since 2017. And, combining these two numbers, we reached our estimate of 14.3 million.[vi]
This total excludes an estimated 4.8 million American-born children of illegal aliens. These children of illegal aliens are U.S. citizens under the current interpretation of the Fourteenth Amendment. However, but for their parents’ unlawful presence, these children would not have been born in the U.S. And, while they are not counted as illegal aliens, they are a significant part of illegal immigration’s fiscal impact on the U.S. taxpayer.
Who is an 'Illegal Alien'?
An “illegal alien” is anyone who entered the United States without authorization or anyone who unlawfully remains in the United States once their authorized period of stay has expired.
It is important to define exactly who is and is not an illegal alien because many organizations deliberately misclassify some illegal aliens in an effort to suggest that the population is smaller than it is in reality. Many mainstream organizations incorrectly classify unaccompanied alien minors (UAMs), recipients of Deferred Action for Childhood Arrivals (DACA), and those with Temporary Protected Status (TPS) as being lawfully present in the United States.
That classification is inaccurate. Individuals who have been paroled into the United States, who have been granted any form of deferred action, or who have received TPS have not been granted “lawful status.” Rather, federal immigration authorities have acknowledged their unlawful presence and have decided to defer their removal from the U.S. solely for the administrative convenience of the government. These types of limited relief from removal are subject to revocation or rescission in a wide variety of circumstances. FAIR offers a detailed explanation of who should be considered an illegal alien in a study titled “Why ‘Illegal Alien’ is the Correct Term.”[vii]
What’s Wrong With Estimates from Other Organizations?
FAIR’s estimate of the illegal alien population is higher than the figures released by many private and academic research organizations. It is also lower than some recent estimates produced by well-known universities.
What accounts for these differences? There are several key factors:
Ideology: Many pro-mass migration organizations do not want Americans to know just how big our illegal alien problem really is. Therefore, in order to minimize the problem, they engage in all manner of mathematical gymnastics to produce illegal alien population estimates that they see as “tolerable” to the bulk of American society. The most common technique involves classifying broad classes of illegal aliens – for instance DACA recipients – as “lawfully present” and refusing to include them in illegal alien counts. (Conversely, some pro-mass immigration groups publish stratospheric estimates hoping to convince the public that mass amnesty is inevitable because it would be impossible to remove such a large number of illegal aliens from the United States.)
Bad Science: Immigration is currently a hot issue. As a result, there are many researchers, both inside and outside academic institutions, who want the notoriety that comes with commenting on something that is regularly discussed on the evening news. The problem is that many of these researchers have no experience with immigration law and policy. Indeed, many have limited experience analyzing socio-political events.
Take a recent study by researchers from the Massachusetts Institute of Technology/Yale University, for example. This report was led by a Ph.D. in business management, and used modeling techniques that are suitable for analyzing problems in the hard sciences (e.g., weather phenomena) but of questionable utility for examining complex phenomena like illegal mass migration.[viii]
Flawed Assumptions: As noted already, it was once a point of consensus that relying on ACS data alone would result in an undercount of the estimated illegal alien population by as much as 15-35 percent. However, many estimates now assume that the ACS undercount is less than 5 percent. The explanation offered for this assumption is that the latest ACS algorithms have become more effective, therefore the ACS produces a better estimate of the illegal alien population. However, very little has been offered to back up this theory, other than ideological pronouncements about the benefits of mass migration.[ix]
In addition, some organizations further reduce the pool of potential illegal aliens by making unsupported assumptions about who to include in their counts and who to exclude. Some examples:
- Immigrants who access welfare must be in the country lawfully. (This is untrue. Illegal aliens regularly access all kinds of benefits to which they are not entitled under the law.)
- Anyone who entered the country prior to 1980 is not an illegal alien. (This is wildly inaccurate. The U.S. government has recognized an illegal migration problem at least as far back as 1904 when the United States Department of Commerce and Labor dispatched mounted watchmen to interrupt the flow of illegal Chinese migrants.)[x]
- Anyone who holds a professional license or certification will not be in the country without authorization. (This has not been true for quite some time. Increasing numbers of states are allowing immigrants, including illegal aliens, to apply for professional and occupational licenses. In fact, some states are now allowing illegal aliens to practice law.)[xi]
FAIR cannot, and does not, claim that its estimate of the illegal alien population is more definitive than any other. However, based on FAIR’s 40 years of experience in dealing with issues of immigration law and policy, we do assert that our estimate is more reasonable, reliable and accurate than most others.
Where do Illegal Aliens Live in the United States?
Unsurprisingly, illegal aliens tend to live near the United States’ border with Mexico and in states that offer incentives for breaking American immigration law. The ten states with the largest estimated illegal alien populations account for just under three-fourths (73.7%) of the national illegal alien total.
However, this is not to suggest that states which hold a comparatively small share of the illegal alien population are unaffected by its negative effects. In fact, as FAIR has pointed out in other studies, illegal immigration often hits these states the hardest.[xii]
The following graphic estimates how many illegal aliens reside in each state, as well an estimate for the total number of illegal aliens and their children:
Footnotes and endnotes
[i] Based on the 2017 net cost to taxpayers per illegal alien, as calculated by the authors.
Matt O’Brien and Spencer Raley, “The Fiscal Burden of Illegal Immigration on U.S. Taxpayers,” Federation for American Immigration Reform, September 27, 2017, https://www.fairus.org/issue/publications-resources/fiscal-burden-illegal-immigration-united-states-taxpayers.
[ii] This projection is based on trends exhibited by data within the Census Bureau’s American Community Survey. As is the case with all projections, any change in policy by states and/or the federal government could cause this estimate to ultimately become too aggressive or too conservative.
Census Bureau, “American Community Survey,” Accessed August 2018, https://www.census.gov/programs-surveys/acs.
[iii] Basing the total number of foreign-born residents in the United States on responses to the ACS question requesting citizenship information yields a weighted total that is more than two million individuals lower than when the estimate is based on survey questions regarding how long respondents have lived in the country.
[iv] FAIR examined the total number of ACS respondents who identified as foreign-born when asked how many years they have resided in the United States and then subtracted the total number of U.S. citizens born abroad. As noted previously, this is a more accurate way to determine the foreign-born population than examining responses to the citizenship questions asked in the ACS.
[v] To estimate the total number of lawful migrants in the United States, we examined both data from the ACS and the comprehensive “Yearbook of Immigration Statistics” published annually by the DHS.
U.S. Department of Homeland Security, “Yearbook of Immigration Statistics,” Accessed August 2018,
[vi] To calculate the total net number of new illegal aliens to the U.S. since 2017, we analyzed data from the DHS, including: The percentage of migrants who ultimately failed to show up for their immigration hearings and other recent in absentia removal orders; the total number of UAMs who entered the country since 2017 against the total number removed; the total number of illegal crossings missed by CBP against the number of confirmed movements out of the country at both the southern and northern borders, as well as along the coasts; and those who have overstayed their visas since 2017 and have yet to depart the country.
[vii] Matt O’Brien, Spencer Raley, and Casey Ryan, “Why ‘Illegal Alien’ is the Correct Term,” Federation for American Immigration Reform, July 2018, https://www.fairus.org/sites/default/files/2018-07/IssueBrief_Illegal-Alien-is-the-Correct-Term.pdf.
[viii] The primary problem with the Yale/MIT study is that the modeling program they developed produced extremely inconsistent results that varied by as much as 15 million or more. With so many complicated and unknown factors that encompass the question of how many illegal aliens are residing in the United States, it is unwise to base a conclusion on predictive modeling – just as it is unwise to try and predict the weather before the most crucial variables are observable.
Mohammad Fazel-Zarandi, Jonathan Feinstein, and Edward Kaplan, “The number of undocumented immigrants in the United States: Estimates based on demographic modeling with data from 1990 to 2016,” PLoS ONE, September 21, 2018, https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0201193.
Massachusetts Institute of Technology, Mohammad Fazel-Zarandi – Biography, Accessed September 2019, https://mitmgmtfaculty.mit.edu/mzarandi/about/.
[ix] Some organizations, such as the Pew Research Center, believe that the ACS is more reliable at estimating the total number of illegal aliens than previous programs deployed by the Census Bureau. However, FAIR believes there is little evidence to back this up.
Jeffrey Passel, “Measuring illegal immigration: How Pew Research Center counts unauthorized immigrants in the U.S.,” Pew Research Center, July 12, 2019, https://www.pewresearch.org/fact-tank/2019/07/12/how-pew-research-center-counts-unauthorized-immigrants-in-us/.
[x] U.S. Customs and Border Protection, “Border Patrol History,” Accessed September 2019, https://www.cbp.gov/border-security/along-us-borders/history.
[xi] “Professional and Occupational Licenses for Immigrants,” Catholic Legal Immigration Network, Accessed September 2019, https://cliniclegal.org/resources/professional-licenses-undocumented-immigrants.
Dan Cadman, “Illegal Aliens Practicing Law,” Center for Immigration Studies, July 19, 2017, https://cis.org/Cadman/Illegal-Aliens-Practicing-Law.
[xii] Matt O’Brien and Spencer Raley, “The Fiscal Burden of Illegal Immigration on U.S. Taxpayers,” Federation for American Immigration Reform, September 27, 2017, https://www.fairus.org/issue/publications-resources/fiscal-burden-illegal-immigration-united-states-taxpayers.