Jefferson -- U.S. County in Kansas Population (2000): 18426 Housing Units (2000): 7491 Land area (2000): 536.112380 sq. miles (1388.524630 sq. km) Water area (2000): 20.831478 sq. miles (53.953277 sq. km) Total area (2000): 556.943858 sq. miles (1442.477907 sq. km) Located within: Kansas (KS), FIPS 20 Location: 39.234439 N, 95.391016 W Headwords: Jefferson Jefferson, KS Jefferson County Jefferson County, KS