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, dropped/uncaught third strike, 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). It first became an official MLB statistic in 1984.
Traditionally, players with the best on-base percentages bat as leadoff hitter. 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 3000 plate appearances, is .482 by Ted Williams. The lowest is by Bill Bergen, who had an OBP of .194.
For small numbers of at-bats, it is possible (though unlikely) for a player's on-base percentage to be lower than his batting average (H/AB). This happens when a player has almost no walks or times hit by pitch, with a higher number of sacrifice flies (e.g. if a player has 2 hits in 6 at bats plus a sacrifice fly, his batting average would be .333, but his on-base percentage would be .286). The player who experienced this phenomenon with the most number of at-bats in a season was Ernie Bowman, who over 125 at-bats in 1963 had a batting average of .184 and an on-base percentage of .181.
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. Sacrifice 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.
All-time leaders 
bold is active player
Single-season leaders 
|1||Barry Bonds||.6094||San Francisco Giants||2004|
|2||Barry Bonds||.5817||San Francisco Giants||2002|
|3||Ted Williams||.5528||Boston Red Sox||1941|
|4||John McGraw||.5475||Baltimore Orioles||1899|
|5||Babe Ruth||.5445||New York Yankees||1923|
|6||Babe Ruth||.5319||New York Yankees||1920|
|7||Barry Bonds||.5291||San Francisco Giants||2003|
|8||Ted Williams||.5256||Boston Red Sox||1957|
|9||Billy Hamilton||.5209||Philadelphia Phillies||1894|
|10||Babe Ruth||.5156||New York Yankees||1926|
See also 
- "Career Leaders for On Base Percentage". Sports Reference, Inc. Retrieved 2011-06-25.
- "Single Season League Leaders for On-Base Percentage". Sports Reference, Inc. Retrieved 2011-06-25.