In baseball statistics, onbase percentage (OBP) (sometimes referred to as onbase 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 timesonbase (TOB) or plate appearances in calculating OBP). OBP is added to slugging average to determine onbase plus slugging (OPS). It first became an official MLB statistic in 1984.
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Traditionally, the best leadoff hitters in the game have high onbase percentages. The league average for onbase percentage has varied considerably over time; in the modern era it is around .340, whereas it was typically only .300 in the deadball era. Onbase 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.
Though extremely unlikely, it is possible for a player's onbase 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 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 onbase percentage would be .286). An example of this phenomenon would be the HallofFame pitcher Phil Niekro's hitting statistics from 1982. In 87 AB he never walked, but had four sacrifice hits, leading to a Batting Average of .195 and an OBP of .193.
Onbase percentage is calculated using this formula:
where
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 onbase percentage, are not included in the calculation for onbase 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.^{[1]}
bold is active player
#  Player  OBP^{[3]}  Team  Year(s) 
1  Barry Bonds  .609  San Francisco Giants  2004 
2  Barry Bonds  .582  San Francisco Giants  2002 
3  Ted Williams  .551  Boston Red Sox  1941 
4  Babe Ruth  .542  New York Yankees  1923 
5  Barry Bonds  .529  San Francisco Giants  2003 
6  Babe Ruth  .528  New York Yankees  1920 
7  Ted Williams  .526  Boston Red Sox  1957 
8  Billy Hamilton  .517  Philadelphia Phillies  1894 
9  Barry Bonds  .515  San Francisco Giants  2001 
10  Ted Williams  .512  Boston Red Sox  1954 

