According to the U.S. Census Bureau, the county has a total area of 804 square miles (2,080 km²), of which 716 square miles (1,850 km²) is land and 88 square miles (230 km²) (11%) is water.^[16]

Contra Costa County's physical geography is dominated by the bayside alluvial plain, the Oakland Hills–Berkeley Hills, several inland valleys, and Mount Diablo, an isolated 3,849-foot (1,173 m) upthrust peak at the north end of the Diablo Range of hills. The summit of Mount Diablo is the origin of the Mount Diablo Meridian and Base Line, on which the surveys of much of California and western Nevada are based.

The Hayward Fault Zone runs through the western portion of the county, from Kensington to Richmond. The Calaveras Fault runs in the south-central portion of the county, from Alamo to San Ramon. The Concord Fault runs through part of Concord and Pacheco, and the Clayton-Marsh Creek-Greenville Fault runs from Clayton at its north end to near Livermore. These strike-slip faults and the Diablo thrust fault near Danville have all considered capable of significantly destructive earthquakes and many lesser related faults are present in the area that cross critical infrastructures such as water, natural gas, and petroleum products pipelines, roads, highways, railroads, and BART rail transit.