The U.S. Census Bureau released the first county-to-county and county/minor civil division (MCD)-to-county/MCD migration flow estimates based on the American Community Survey (ACS) in April 2012. It used data collected from 2005 through 2009 that composed the first ACS 5-year dataset. The ACS program combines consecutive yearly datasets to increase the sample size and provide reliable estimates for smaller areas. Even though estimates are produced for 1-year and 3-year datasets, estimates are only published for geographic areas of at least 65,000 and 20,000 population, respectively. Only the 5-year datasets provide estimates for all counties and county equivalents in the United States.

The 2006-2010 ACS Migration Flow Files included additional files crossed by certain characteristics: age, sex, race and Hispanic or Latino origin. For the 2007-2011 release, the characteristics educational attainment, and household and individual income were used. Employment status, work status, and occupation were the characteristics used for the 2008-2012 release. That release was also the first time flows from Puerto Rico municipios to U.S. counties were included. The 2009-2013 release includes tables crossed by ability to speak English, place of birth, and years in the United States or Puerto Rico. It is also the first release that contains ACS metro area-to-metro area files.

The categories for the ability to speak English are divided among those who speak only English and those who speak another language. The latter group is subdivided into those who reported the ability to speak English "very well" and less than "very well."

The place of birth question on the ACS asks the state, U.S. Island Area, or foreign country of birth. Those born in the United States or Puerto Rico are classified on whether or not their state (including the District of Columbia and Puerto Rico) of birth is the same as either the current residence or residence 1 year ago. The place of birth categories for those born outside the United States or Puerto Rico are either countries or regions of the world. The requirement for a country to be listed individually was at least 100,000 people born in that country that either moved between counties or from abroad. The definitions of the regions are the same as those for residence 1 year ago listed in Appendix A.

The years in the United States or Puerto Rico characteristic are based upon the year of entry question. The problem for year of entry for multiple year ACS datasets is that the reference year of entry is set but the survey year is not. For the category entered in 2010 or later, those surveyed in 2013 includes anyone who came in the previous 3 years. Those surveyed in 2009 would include no one. For better comparability, year of entry was recoded to a period (years in the United States) rather than a date. The table below shows the relationship between the years in the U.S. categories and the values for year of entry.

The product package also contains flow data for minor civil divisions (MCD) for a few select states in which they are commonly used as the primary substate aggregate unit in addition to or instead of counties.

In addition, there are files that contain the migration flow, migration counterflow, net migration, and gross migration between each county pair or county/MCD pair. These files are only available without characteristics.