High Quality Content by WIKIPEDIA articles! In baseball statistics, on-base percentage (OBP) is a measure of how often a batter reaches base for any reason other than a fielding error, fielder`s choice, dropped or uncaught third strike, fielder`s obstruction, or catcher`s interference. OBP is calculated in Major League Baseball (MLB) by dividing the sum of hits, walks, and times hit by a pitch by the sum of at-bats, walks, times hit by pitch and sacrifice flies. A hitter with a .400 on-base percentage is considered to be great and rare; only 59 players in MLB history with at least 3,000 career plate appearances (PA) have maintained such an OBP. Left fielder Ted Williams, who played 19 seasons for the Boston Red Sox, has the highest career on-base percentage, .4817, in MLB history. Williams led the American League (AL) in on-base percentage in twelve seasons, the most such seasons for any player in the major leagues.Barry Bonds led the National League (NL) in ten seasons, a NL record. Williams also posted the then-highest single-season on-base percentage of .5528 in 1941, a record that stood for 61 years until Bonds broke it with a .5817 OBP in 2002. Bonds broke his own record in 2004, setting the current single-season mark of .6094.