Formulas for common baseball statistics

Offensive statistics

Batting average (BA)
BA = (Hits)/(At Bats)
At bats = Total times at bat - Walks - Hit by Pitch - Sacrifice

Slugging percentage (SP)
SP = (Total Bases)/(At Bats)
SP gives extra credit for extra base hits. Some writers feel that SP is a better measure of a hitter's value than Batting average.

On base percentage (OBP)
(Hits + Base on Balls + Hit by Pitch)/(At Bats + Base on Balls + Hit by Pitch)

Pitching statistics

Earned Run Average (ERA) Measure average number of earned runs a pitcher allows per 9 inning game
ERA = (Total Earned Runs x 9)/Innings Pitched
Example Pitcher gives up 5 earned runs in 21 innings
ERA = (5 * 9)/21 =2.143