How do you compute a baseball players batting average?

Number of hits divided by the official at bats. An error or fielder's choice counts against a batting average as an at bat, but it does not count as a hit. A sacrifice or walk does not count against a batter's at bat totals. These are plate appearances which are different. So, a batter could have 140 plate appearances, 100 at bats, and 30 hits. You would divide 30 by 100, which would give you a batting average of .300.