One of the three towns in Nassau County, Hempstead occupies the southwest part of the county. According to the U.S. Census Bureau, it has a total area of 191.3 square miles (495.5 km2). 120.0 square miles (310.7 km2) of it is land and 71.4 square miles (184.8 km2) of it (37.3%) is water.
There are 22 incorporated villages completely or partially in the town. With a combined population of 759,757 at the 2010 Census, Hempstead is by far the largest town in New York. In fact, if it were to be incorporated as a city it would be the second largest in New York behind only New York City, and the 16th largest in the country.