On-Base Percentage Calculator & Formula

A player's on-base percentage or on-base average, OBP, is an indicator of how often a player reaches base per plate appearance. It does not count events such as fielding errors, fielder's choice, dropped third strikes, fielder's obstruction, catcher's interference, or sacrifice hits.

Formula for OBP

On-base percentage is not actually a percent but a number between 0 and 1. The equation is

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

It is interesting to compare this number to a player's batting average, BA, given by the simpler formula

BA = H/AB

Rarely a player may have a lower OBP than BA. The mathematical condition for OBP to be less than BA is

(AB - H)(BB + HBP) < (H)(SF)

In other words, a player must have almost no walks or hits by pitch and a relatively high number of sacrifice flies. This occurred in 1982 with pitcher Philip Niekro. He had a total of 87 at-bats, 17 hits, 0 bases on balls, 0 hits by pitch, and 1 sacrifice fly. His OBP and BA were

OBP = (17 + 0 + 0)/(87 + 0 + 0 + 1) = 17/88 = 0.1932

BA = 17/87 = 0.1954

Sabermetricians often add the OBP and SLG (slugging percentage) to create a unified statistic called on-base plus slugging, OPS.

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