In baseball statistics, on base percentage (OBP) (sometimes referred to as on base average [OBA], as the statistic is rarely presented as a true percentage) is a measure of how often a batter reaches base for any reason other than a fielding error, fielder's choice, fielder's obstruction, or catcher's interference (the latter two are ignored as either times-on-base [TOB] or plate appearances in calculating OBP). OBP is added to slugging average to determine on-base plus slugging (OPS).
The league average for on base percentage has varied considerably over time; in the modern era it is around .340, whereas it was typically only .300 in the dead-ball era. On base percentage can also vary quite considerably from player to player. The record for the highest career OBP by a hitter, based on over 5000 plate appearances, is .481 by Ted Williams. The lowest is .265 by Hobe Ferris, although this was during the dead-ball era. If the threshold is dropped to 3,000 at-bats, the career low belongs to Bill Bergen, who had an OBP of .194.
Though extremely unlikely, it is possible for a player's on base percentage to be lower than his batting average (H/AB). However very few players in major league history fall into this category, with the majority of them having under 100 ABs, as it requires having almost no walks or times hit by pitch, with a relatively higher number of sacrifice flies (e.g. if a player has 2 hits in 5 at bats with a sacrifice fly, his batting average would be .400, but his on base percentage would be .333).
On base percentage is calculated using this formula:
NOTE: Sacrifice flies were not counted as an official statistic until 1954. Before that time, all sacrifices were counted as sacrifice hits (SH), which included both sacrifice flies and bunts. Bunts (sacrifice hits since 1954), which would lower a batter's on base percentage, are not included in the calculation for on base percentage, as bunting is an offensive strategy – often dictated by the manager – the use of which does not necessarily reflect on the batter's ability and should not be used to penalize him. For calculations of OBP before 1954, or where sacrifice flies are not explicitly listed, the number of sacrifice flies should be assumed to be zero.