Phelps County is a county in the central portion of the U.S. state of Missouri. As of the 2020 United States Census, the population was 44,638. The largest city and county seat is Rolla. The county was organized on November 13, 1857, and was named for U.S. Representative and Governor of Missouri John Smith Phelps.

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.