Get to Know: K/9

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:

Wikipedia: Strikeouts per 9 innings pitched
U.S.S. Mariner: Evaluating Pitcher Talent

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I think the rationale for K/BFP (at THT, we multiply BFP by 38, or the average number of batters per game in a given league in a given year) is stronger than that. Good pitchers are definitely underrated by K/9, because they face less batters per nine innings (that’s not just walks, but hits allowed, too). I guess the difference is “minimal,” but that depends on your expectations, right? Also, the differences are biased — good pitchers are underrated and bad pitchers are overrated — which makes K/9 less insightful and more misleading.