Fitchburg is located at 42°57'38" North, 89°28'9" West (42.960833, -89.469444).
According to the United States Census Bureau, the city has a total area of 34.86 mi2 (90.29 km2). 34.82 mi2 (90.18 km2) of it is land and 0.04 mi2 (0.11 km2) of it is covered by water. The total area is 0.12% water.
Fitchburg is located in the Central Standard Time (CST) timezone, GMT-06:00. It observes Daylight Saving Time (DST).