Somersworth is located at 43°15'41" North, 70°51'56" West (43.261389, -70.865833).
According to the United States Census Bureau, the city has a total area of 10.01 mi2 (25.93 km2). 9.78 mi2 (25.33 km2) of it is land and 0.23 mi2 (0.59 km2) of it is covered by water. The total area is 2.29% water.
Somersworth is located in the Eastern Standard Time (EST) timezone, GMT-05:00. It observes Daylight Saving Time (DST).