Brentwood is located at 37°55'54" North, 121°41'40" West (37.931944, -121.694722).
According to the United States Census Bureau, the city has a total area of 11.66 mi2 (30.20 km2). 11.64 mi2 (30.16 km2) of it is land and 0.01 mi2 (0.04 km2) of it is covered by water. The total area is 0.12% water.
Brentwood is located in the Pacific Standard Time (PST) timezone, GMT-08:00. It observes Daylight Saving Time (DST).