Clayton -- U.S. County in Iowa Population (2000): 18678 Housing Units (2000): 8619 Land area (2000): 778.807292 sq. miles (2017.101541 sq. km) Water area (2000): 14.003998 sq. miles (36.270186 sq. km) Total area (2000): 792.811290 sq. miles (2053.371727 sq. km) Located within: Iowa (IA), FIPS 19 Location: 42.852007 N, 91.320308 W Headwords: Clayton Clayton, IA Clayton County Clayton County, IA