According to the United States Census Bureau, Williamstown has a total area of , of which is land and , or 3.13%, is water. Williamstown Lake, a reservoir on the South Fork of Grassy Creek, is in the northeast part of the city.
According to the United States Census Bureau, Williamstown has a total area of , of which is land and , or 3.13%, is water. Williamstown Lake, a reservoir on the South Fork of Grassy Creek is in the northeast part of the city.