In baseball statistics, strikeouts per 9 innings pitched (K/9, SO/9, or SO/9IP) is the mean of strikeouts (or Ks) by a pitcher per nine innings pitched. It is determined by multiplying the number of strikeouts by nine and dividing by the number of innings pitched. To qualify, a pitcher must have pitched 1000 innings, which generally limits the list to starters. A separate list is maintained for relievers with 300 innings pitched or 200 appearances.
The all-time leader in this statistic is retired pitcher Randy Johnson (10.61). The only two other players who have averaged over 10 are Kerry Wood (10.32) and Pedro MartÃnez (10.04). Since Wood's retirement in May 2012, Tim Lincecum (9.7636) took over as the active leader.
Among qualifying relievers, Rob Dibble (12.17) is the all-time leader in strikeouts per nine innings. Active leader David Robertson (12.03), Craig Kimbrel (14.7) and Aroldis Chapman (15.3) are the only other qualifying relievers averaging more than 12.
One effect of K/9 is that it may reward or "inflate" the numbers for pitchers with high batting averages on balls in play (BABIP). Two pitchers who strike out identical percentages of hitters, but have varying BABIPs, will have different K/9 rates since one pitcher will pitch fewer innings to face the same number of batters.