Clay -- U.S. County in Mississippi Population (2000): 21979 Housing Units (2000): 8810 Land area (2000): 408.559856 sq. miles (1058.165124 sq. km) Water area (2000): 7.460400 sq. miles (19.322346 sq. km) Total area (2000): 416.020256 sq. miles (1077.487470 sq. km) Located within: Mississippi (MS), FIPS 28 Location: 33.629550 N, 88.722699 W Headwords: Clay Clay, MS Clay County Clay County, MS