South Flat is located at (43.930394, -108.020176)
According to the United States Census Bureau, in 2000 the CDP has a total area of 22.2 square miles (57.4 km2), of which, 21.7 square miles (56.2 km2) of it is land and 0.5 square miles (1.3 km2) of it (2.25%) is water.