An often used number (in 2006-2007) is 12 million illegal immigrants, but this is only an estimate.
According to the Bureau of Transportation statistics there are over 200,000,000 legal border crossings from Mexico each year, ~80% by automobile.
The number of illegal immigrants emigrating [leaving] the U.S. is estimated at about 240,000 per year (~20% of illegal population)
The “residual method” is widely used to estimate the illegal immigrant population, where the reported census number of self proclaimed foreign born people in the U.S. census is subtracted from the known number of legal immigrants to obtain the unauthorized immigrant (residual) population. This methodology is used by the US Department of Homeland Security, the Pew Hispanic Center, the US Census Bureau and others. Since unauthorized immigrants have many reasons for not answering the census correctly and no penalties for answering the census incorrectly, a direct subtraction has a well known source of undercount error and has to be corrected. All known users of this methodology correct the foreign born population (~ 35-50 million) by 10-40% (3-12 million) to account for this undercount effect. Critics claim this correction is in error--whatever size correction is used.
Using the residual methodology with a minimal 10% foreign born undercount correction (reason for correction size unstated) for the 2000 Census and a 700,000 net illegal immigrant increase/year assumption and data from the March 2004 Current Population Survey [CPS Survey] (U.S. Census Bureau and Department of Labor) Pew estimated 10.3 million unauthorized immigrants in 2004. Assuming the same rate of growth Pew projected this population reached at least 11 million as of March 2005.
The Pew Hispanic Center estimates that in the 1980’s the net advance of the undocumented population was at the 130,000 per year increasing to 450,000 per year from 1990-94, and further increasing to 750,000 per year from 1995-1999 and staying at 700-850,000+ per year since about 2000. Illegal Mexican immigration amounts to about 500,000 per year of this influx since about 1999. According to the same Pew Hispanic Center study as of March 2005, the undocumented population had reached 11 million or more including more than 6.5 million illegal Mexicans (~60% of all illegal immigrants). Assuming the same rate of growth as in recent years gives about 12,000,000 illegal immigrants in the United States as of January 2006, increasing at 700,000–850,000 per year with illegal Mexicans amounting to about 60%+ (7+ million) of the total by 2006. By September 2006 the illegal population is thought to be about 13 million. About one-sixth of the undocumented population — about 2.0 million people — is under 18 years of age..
After 2000 the estimation of the growth of the illegal population becomes more difficult because of a lack of good information. The rate of growth of the illegal population is estimated with the Consumer Price Survey data  which suffers from the same under counting problems of the Census plus the problem of a much smaller statistical sample used of only 10,000–20,000. Its accuracy may well be suspect for lack of a truly representative "random" sample and well known non-random distribution of the illegal immigrant populations. Using these techniques Pew comes up with from 12+ million illegal aliens in January 2006 with an estimated growth rate of 700,000 to 850,000 net illegal aliens per year. This is the so called "consensus" number used by most reporters. The unstated cumulative error in total illegal aliens by 2006 could easily be an additional 8 million illegal aliens or more and the error in the growth rate since 2000 could also be very large but again is unstated by Pew and others. There is a high probability of the illegal alien population's size in 2006 being significantly larger than the 12 million predicted as all additional information points to a significant increase (300+ %) in the advance rate of illegal aliens after 2000 not a reduction as initially predicted by Pew.
Investigative journalists Donald L. Barlett and James B. Steele estimates in Time magazine in its September 12, 2004 issue, that the number of illegal aliens entering into the U.S. that year will total 3 million/year — enough to fill 22,000 Boeing 737-700 airliners, or 60 flights every day for a year.
In 2006 legal immigrants to the United States now number approximately 1,000,000 legal immigrants per year of which about 600,000 are Change of Status immigrants who already are in the U.S. Legal immigrants to the United States are now at their highest level ever at over 35,000,000. Net advance by illegal aliens has also soared from about 130,000 per year in the 1970s, to 300,000+ per year in the 1980s to over 500,000 per year in the 1990s to over 700,000 per year in the 2000s. Total entrance by illegal aliens may be as high as 1,500,000 per year [in 2006] with a net of at least 700,000 more illegal aliens arriving each year to join the 12,000,000 to 20,000,000 that are already here. (Pew Hispanic Data Estimates )
|Mexican Remittances and Illegal Population Growth|
|Year|| * Remittances|
| Illegal **|
| Pew ***|
|Non-Mex.||4.0 to 6.0|
|Total illegal aliens||20.0 to 22.0 million|
| Assumes the amount of remittances from the U.S. is proportional to the number of Mexicans living in the U.S.|
As can be seen the Pew and Bear Stern numbers are in basic agreement to about 2000.
Total remittances in 2005 were about 20 billion.
* Banco de Mexico
** Bear Stern’s investigators
*** Pew data
Bear Sterns' investigators came up with another way to attack this very difficult problem. They made the assumption that the amount of remittances (money sent back to Mexico) is directly proportional to the number of Mexican immigrants in the United States. Other data used for their estimates are the increases of households and school enrollment in Mexican immigrant communities. They conclude that the number of illegal aliens in the United States may well be twice the official number put out by the U.S. Census of 9 million and may be 20 million people or higher. Information from The Mexican Central Bank and (U.S. Federal Reserve Banks) details the remittances and shows their growth . According to that data, remittances stayed fairly stable until 2000 when a steady and dramatic increase began. The change in remittances between 1997 and 1999 is most likely a problem in accounting — the three year average is still about 450 thousand/year consistent with other data. The agreement with the Pew estimate is reasonably good up to 2001 where there is a significant difference — just where the Pew and Census data becomes harder to extrapolate because of lack of good data. Using this technique Bear Sterns investigators come up with a possible illegal population of 20 million or greater. (See figure for calculation) Other data confirming their estimates are the dramatic increases of households and school enrollment in Mexican immigrant communities (read their report for more details). Border Arrest data do not show this dramatic increase in apprehensions.
According to a Pew Hispanic Center report, Mexicans make up 57 percent of the undocumented immigrants. Another 24 percent are from other Latin American countries. Approximately 9 percent are from Asia, 6 percent from Europe and Canada, with the remaining 4 percent from the rest of the world
|Country of Origin (January 2006)|
|Latin & Central Amer.||3,000,000||24%|
|Europe + Canada||720,000||6%|
|Rest of World||480,000||4%|
The number of Mexican legal immigrants and Mexican illegal aliens in the United States has grown quite rapidly over the past 35 years, increasing almost 15-fold from about 760,000 in the 1970 Census to more than 11 million in 2004—an average annual growth rate of more than 8 percent, maintained over more than 3 decades. This remarkable growth has been largely driven by the encroachment of illegal aliens. On average the net Mexican population living in the United States has grown by at least a half million people a year over the past decade. About 80 to 85 percent of the immigration from Mexico and Central America in recent years has been illegal. (Pew report Figure 4 and page 2)