Contra Costa County is a suburban county in the San Francisco Bay Area of the U.S. state of California. As of the 2000 census, it had a population of 948,816. The county seat is Martinez. According to the U.S. Census Bureau, the county has a total area of 2,078 km² (802 mi²). 1,865 km² (720 mi²) of it is land and 213 km² (82 mi²) of it is water. The total area is 10.25% water.As of the census2 of 2000, there were 948,816 people, 344,129 households, and 242,266 families residing in the county