Eureka is located at 38°30'7" North, 90°37'38" West (38.502222, -90.627500).
According to the United States Census Bureau, the city has a total area of 10.11 mi2 (26.19 km2). 10.05 mi2 (26.03 km2) of it is land and 0.06 mi2 (0.16 km2) of it is covered by water. The total area is 0.60% water.
Eureka is located in the Central Standard Time (CST) timezone, GMT-06:00. It observes Daylight Saving Time (DST).