According to the U.S. Census Bureau, the county has a total area of 509 square miles (1,320 km2), of which 504 square miles (1,310 km2) is land and 4.2 square miles (11 km2) (0.8%) is water. Fairfield County sits just on the edge of Ohio's Appalachian region. While the once-glaciated northern portion of the county is fairly flat, as one travels south along U.S. 33 one can easily recognize the foothills of a mountainous region beginning around the village of Carroll.