Warren -- U.S. County in Illinois Population (2000): 18735 Housing Units (2000): 7787 Land area (2000): 542.520783 sq. miles (1405.122319 sq. km) Water area (2000): 0.646574 sq. miles (1.674618 sq. km) Total area (2000): 543.167357 sq. miles (1406.796937 sq. km) Located within: Illinois (IL), FIPS 17 Location: 40.876975 N, 90.622695 W Headwords: Warren Warren, IL Warren County Warren County, IL