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

Print This Post

David Appelman is the creator of FanGraphs.

6 Responses to “Get to Know: K/9”

You can follow any responses to this entry through the RSS 2.0 feed.
  1. studes says:

    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.

    Vote -1 Vote +1

  2. I don’t think that makes K/9 is any less insightful as long as you know what you’re using. In all honesty they are really insanely similar with a r-squared of .96. The biggest difference you’re ever really looking at is not much more than 1 K per 9/G.

    But, with that said, the players who have their K/9 most “undervalued” compared to K/G were typically those who had pretty good or great seasons. And likewise players who were most “overvalued” were players with poor seasons.

    With K/G there’s a slight possibility (and I do mean ever so slight) you may be missing out on a potential breakout pitcher since it undervalues pitchers with high K/9 that walked a lot of batters and gave up a lot of hits. It’s tough to say K/9 undervalues players that already have K/G up in the 9+ range which is already quite high.

    I don’t think either of these stats should be used in isolation, but if you had to use one in isolation I probably would use K/G. Assuming I had the rest of the stats on hand like BB/9 (or whatever flavor of that you want) along with HR/FB etc… I’d probably stick with K/9.

    Vote -1 Vote +1

  3. studes says:

    Not to drag it out, but why exactly do you prefer K/IP over K/BFP?

    Vote -1 Vote +1

  4. I think it isolates strikeout potential better than K/BF does.

    Vote -1 Vote +1

  5. I also agree that K/IP is the best approach to reading the numbers in gauging potential for pitchers. You definitely have a keeper if your closer gets over 8.

    Vote -1 Vote +1