K/9 (strikeouts per 9 innings): The average of how many batters a pitcher strikes out per 9 innings pitched.
Calculated as: (SO * 9) / IP
Why you should care: K/9 is a perfectly suitable way to evaluate a player’s ability to strike batters out.
Current Baselines (2002-2007): The average K/9 for starting pitchers is 6.17 and 7.21 for relievers. For starting pitchers the top and bottom 20th percentile are a K/9 above 7.56 and below 4.89. Relievers top and bottom 20th percentiles are a K/9 above 8.94 and below 5.54.
Variations: Some people prefer to use strikeouts per batter faced (K% or K/G) to express a player’s ability to strike batters out. The difference is minimal and the argument for using K% is that K/9 excludes walked batters and K% does not, suggesting that K/9 may either overstate or understate a pitcher’s overall effectiveness (not pure strikeout ability).
Links and Resources:
David Appelman is the creator of FanGraphs.