Cass -- U.S. County in Illinois Population (2000): 13695 Housing Units (2000): 5784 Land area (2000): 375.900754 sq. miles (973.578443 sq. km) Water area (2000): 7.830552 sq. miles (20.281035 sq. km) Total area (2000): 383.731306 sq. miles (993.859478 sq. km) Located within: Illinois (IL), FIPS 17 Location: 39.975926 N, 90.269918 W Headwords: Cass Cass, IL Cass County Cass County, IL