St. Peters is located at 38°46'44" North, 90°36'11" West (38.778893, -90.603131).
According to the United States Census Bureau, the city has a total area of 21.18 mi2 (54.87 km2). 21.18 mi2 (54.87 km2) of it is land and 0.00 mi2 (0.00 km2) of it is covered by water. The total area is 0.00% water.
St. Peters is located in the Central Standard Time (CST) timezone, GMT-06:00. It observes Daylight Saving Time (DST).