On-base Percentage

$OBP = ( H + BB + HBP )/( AB + BB + HBP + SF )$

$(H)"number of hits"$

$(BB)"number of bases on balls"$

$(HBP)"number of hits by pitch"$

$(AB)"number of at bats"$

$(SF)"number of sacrificed flies"$

The Math / Science

The On-base Percentage is a measure of how often a batter reaches base for any reason other than a fielding error, fielder's choice, dropped/uncaught third strike, fielder's obstruction, or catcher's interference (the later two are ignored as either times-on-base or plate appearances in calculating OBP). The formula for On-base Percentage is :

$OBP=(H+BB+HBP)/(AB+BB+HBP+SF)$

where:

OBP = On-base Percentage
H = number of hits (times reached base because of a batted, fair ball without error by the defense)
BB = number of bases on balls (also called a walk, hitter not swinging at four pitches called out of the strike zone and awarded first base)
HBP = number of hits by pitch (times touched a pitch and awarded first base as a result)
AB = number of at bats (Plate appearances, not including bases on balls, being hit by pitch, sacrifices, interference, or obstruction)
SF = number of sacrificed flies (Fly ball hit to the outfield which although caught for an out, allow a baserunner to advance)

OBP is added to the to determine (OPS).

Baseball Stats Calculators

Baseball Base Separation:

References

Wikipedia (https://en.wikipedia.org/wiki/On-base_percentage and https://en.wikipedia.org/wiki/Baseball_statistics)