The mean center of U.S. population is determined by the United States Census Bureau after tabulating the results of each census. The Bureau defines it to be:
- the point at which an imaginary, flat, weightless, and rigid map of the United States would balance perfectly if weights of identical value were placed on it so that each weight represented the location of one person on the date of the census.
The following counties included the mean center of U.S. population during the 20th century:
- 1900 Bartholomew County, Indiana
- 1910 Monroe County, Indiana
- 1920 Owen County, Indiana
- 1930 Greene County, Indiana
- 1940 Sullivan County, Indiana
- 1950 Clay County, Illinois
- 1960 Clinton County, Illinois
- 1970 St. Clair County, Illinois
- 1980 Jefferson County, Missouri
- 1990 Crawford County, Missouri
- 2000 Phelps County, Missouri