Sports Math: On-Base Percentage

Calculate On-Base Percentage

Major League Baseball hitters are commonly judged based on their batting average, homeruns, and runs batted in (RBI). Those are statistics familiar to most fans, although they only tell a very small part of the story. Another important statistic is On Base Percentage (OBP). It shows how often a batter reaches base safely, whereas batting average only considers hits.

On Base Percentage is calculated by adding hits, walks, and hit-by-pitches and dividing by the sum of at bats, walks, hit by pitches, and sacrifice flies:

$$ OBP = \frac{H+BB+HBP}{AB+BB+HBP+SF} $$

Batters typically have an on base percentage that is roughly .050 points higher than their batting averages. More aggressive hitters will have a OBP closer to their batting average because they walk less. Those batters who are frequently walked can have on base percentages well over .500.

Here's a quick example: Ichiro Suzuki had a record 262 hits in 2004. He also walked 49 times and was hit by 4 pitches. The sum is 262 + 49 + 4 = 315. He had 704 at bats, 49 walks, 4 hit by pitches, and 3 sacrifice flies on the year. That sum is 704+49+4+3=760. Dividing 315 by 760 gives the on base percentage of .414. That's not too bad, but it's not much higher than his batting average, which was an impressive .372. By comparison, Jose Bautista had a respectable batting average of .286 in 2014, but still reached base at a very strong .403 clip, helped by 104 walks.