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.
During the 20th century, the mean center of population has shifted 324 miles (521 km) west and 101 miles (163 km) south. The southerly movement was much stronger during the second half of the century; 79 miles (127 km) of the 101 miles (163 km) happened between 1950 and 2000. It may not be a coincidence that it was after World War II that room air conditioning unit sales increased dramatically. According to the Air Conditioning and Refrigeration Institute  (http://www.ari.org/), 30,000 room air conditioners were sold in 1946, a number that increased to over 1,000,000 by 1953.
Mean center of population for the United States, 1790–2000 (U.S. Census Bureau)
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
The addition of Alaska and Hawaii to the union had the effect of moving the center about two miles farther south and about ten miles farther west for 1960.
In the first census, 1790, the mean population center was located a little under 8 miles (13 km) west, and slightly north, of Chestertown, Maryland, a few feet from a small branch of the Chesapeake Bay called Stavely Pond. Oddly enough, the spot is on the "Great Oak Mannour" property patented to Josiah Ffendall in the mid-1600s, one of Kent County's oldest and largest land grants. The patent was for 2000 acres (8 kmē). Fendall was an early governor of the colony. In 1800 it leaped across the Bay to a spot just east of today's Glenelg, Maryland.