The spreadsheet available for download below shows estimates of the twenty-five countries with the largest numbers of unauthorized residents in the US in 2013. Estimates are shown for unauthorized population sizes of 1,000 or more. All of the estimates are rounded to 1,000s. The sum of the numbers for the countries are not likely to agree with the US totals because estimates of fewer than 1,000 are not included in the table.
The table is the result of a project by the Center for Migration Studies (CMS) to estimate the size and characteristics of the unauthorized population in the United States at the national, state, and sub-state levels, and to make the information readily available to a wide cross-section of users. To derive the estimates, a series of statistical procedures were developed based on microdata collected by the US Census Bureau in the 2010 American Community Survey (ACS). 
The methodology involved three major steps: (1) applying a series of “logical edits”  to identify as many legal residents as possible based on responses in the survey; (2) deriving separate population controls, for 145 countries or areas, for unauthorized residents counted in the 2010 ACS; and (3) using those population controls to select individual respondents in the ACS to be classified as unauthorized residents. A final set of adjustments was made to account for undercount in the ACS.
Estimates of Unauthorized Residents in the United States from the Top 25 Countries, by State of Residence: 2013
 A detailed description of the project and the methodology is available at: Robert Warren, 2014 “Democratizing Data about Unauthorized Residents in the United States: Estimates and Public-Use Data, 2010 to 2013.” Journal on Migration and Human Security. DOI: http://dx.doi.org/10.14240/jmhs.v2i4.38.
 The ACS is an annual statistical survey covering approximately one percent of the total US population. The survey provides detailed social and economic data for all states, as well as all cities, counties, metropolitan areas, and population groups of 100,000 people or more.
 The term “logical edit” refers to the process of determining probable legal status by examining survey data. Respondents were assigned to the legal category if they worked in occupations that generally require legal status, were legal temporary migrants, were immediate relatives of US citizens, received public benefits, were from countries where most arrivals would be refugees, or were age 60 or older at entry.
Privacy of Respondents
A number of procedures were used by the US Census Bureau to ensure privacy in the ACS public use data. In addition, the estimates are based on an approximately one percent sample; they are shown in relatively broad categories; and estimates are not shown for countries that have fewer than 1,000 estimated unauthorized residents.
These estimates are based on sample data and are subject to sampling variability as well as other possible non-sampling errors. A number of statistical adjustments were made to the actual ACS PUMS sample data, including an adjustment for undercount of the unauthorized in the ACS. For these reasons, the estimates shown here are not comparable to the original ACS data. Because of the adjustment for undercount, in a few cases, the estimates of unauthorized residents may be near or even slightly higher than the number of noncitizens reported in the ACS. Finally, estimates of less than a few thousand should be used with caution.
See Methodology for more information on these estimates.