In baseball statistics, strikeouts per nine 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 1,000 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 through 2019 was Yu Darvish (11.12). The only other players who had averaged over 10.50 strikeouts (through 2019) were Chris Sale (11.08), Randy Johnson (10.61), Stephen Strasburg (10.60), and Max Scherzer (10.58). The top four in 2019 were Gerrit Cole (13.82), Max Scherzer (12.69), Robbie Ray (12.13), and Justin Verlander (12.11).
Among qualifying relievers, Rob Dibble (12.2) was the all-time leader in strikeouts per nine innings through 2019 with 200 appearances, followed by David Robertson (11.9) and Brad Lidge (11.9).
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 may have the same K/9 rates despite striking out a different percentage of batters since one pitcher will pitch to more batters to obtain the same cumulative number of strikeouts. For example, a pitcher who strikes out one batter in an inning, but also gives up a walk or a hit, strikes out a lower percentage of batters than a pitcher who strikes out one batter in an inning without allowing a baserunner, but both have the same K/9.