Blog Home

Archive

Calculating Times On-Base and On-Base Percentage

On-base percentage (OBP) and On-base percentage plus slugging (OPS) are two statistics commonly used to track for hitter performance. OBP indicates how often a player gets on base and OPS shows the true value of a batter by combining on-base ability with a power of the slugging percentages. Both stats are based on the times on-base (OB) stat.

These stats should be easy to calculate, right? Times on-base is the total number of times a player reaches base and OBP is the percentage of those times divided by plate appearances?

Not quite. As with many things in baseball, the answer isn’t quite that easy. But once understood, these stats become even more powerful and meaningful. The OB stat doesn’t count every time a batter safely reaches base. It only counts times where the batter "productively" reaches base – hits (H), walks (BB) and hit-by-pitches (HBP). Not counted are fielder’s choice (FC), times reached on error (ROE) and dropped third strikes. So, the batter only gets credit for a time on base if they reach safely on their own accord. If the batter causes another runner to be tagged or forced out (FC) or they reach on error of a fielder (ROE), they don’t get credit. FC especially makes sense here – why award the batter’s statistics if the end result of their action is an out for the team without moving a runner forward? ROE implies that the fielder, in normal circumstances, would have recorded an out without the error, so it is also logical that this does not count for times on base as well.

Since only H, HBP and BB are counted towards the OB stat, the On-base percentage is also a little more difficult to calculate than simply dividing OB/PA. The formula looks like this:

(H + HBP + BB) / (AB + BB + HBP + SF)

In the divisor, at bats (AB) will already take care of hits, so only BB and HBP need added back in. Also note that sacrifice flies (SF) are also added in. To be more specific, note that in modern baseball stats, Major League Baseball differentiates between sacrifice flies and sacrifice bunts (SAC). Sacrifice bunts are seen as a strategic move, therefore the batter's stats are not punished by them. However, sacrifice flies are not considered strategic, and are counted against the batter.

For reference, the OBP statistic is defined as item 12.21(f) in the MLB rule book :

On-base percentage, divide the sum of hits, bases on balls and times hit by pitch by the sum of at-bats, bases on balls, times hit by pitch and sacrifice flies.