Divide the total number of hits allowed, bases on balls allowed and batters hit by pitch by the total of the opposing batters' at-bats, bases on balls allowed, batters hit by pitch and sacrifice flies allowed. For example, if Greg Maddux allowed opposing batters 220 hits in 991 at-bats, allowed 27 walks, hit 7 batters with pitches and allowed 6 sacrifice flies, his opponents' on-base percentage would be .246 ((220+27+7)/(991+27+7+6)).