Charlestown is located at 43°14'17" North, 72°25'29" West (43.238333, -72.425000).
According to the United States Census Bureau, the town has a total area of 0.88 mi2 (2.28 km2). 0.84 mi2 (2.18 km2) of it is land and 0.04 mi2 (0.10 km2) of it is covered by water. The total area is 4.39% water.
Charlestown is located in the Eastern Standard Time (EST) timezone, GMT-05:00. It observes Daylight Saving Time (DST).