Clay County is a county in the southeastern portion of the U.S. state of Illinois. As of the 2020 United States Census, the population was 13,288. Since 1842, its county seat has been Louisville, in the center of the county's area.

The mean center of the United States population is determined by the United States Census Bureau from the results of each national census. The Bureau defines it as follows:The concept of the center of population as used by the U.S. Census Bureau is that of a balance point. The center of population is the point at which an imaginary, weightless, rigid, and flat surface representation of the 50 states and the District of Columbia would balance if weights of identical size were placed on it so that each weight represented the location of one person. More specifically, this calculation is called the mean center of population.