So What Are K/9, BB/9, and K/BB Anyway?

Continuing with the series on explaining useful statistics as requested in the excellent For All the Dummies post, this entry looks at three key pitching statistics that both scouting and statistical analysts consider very important:  strikeout rate (K/9), walk rate (BB/9), and strikeout-to-walk ratio (K/BB, or alternatively K:BB).  Again, the purpose here is provide a brief introduction and (likely oversimplified) explanation of these stats, not to start any debates or provide any in-depth analysis.

So what are K/9, BB/9, and K/BB?  K/9 is strikeouts per nine innings.  The formula is simple and the same as ERA except using strikeouts instead of earned runs:  K/IP x 9.  Some analysts prefer to use strikeouts per game pitched (K/G) or strikeout percentage (K%), which provide slightly sharper accuracy as they are based on strikeouts per batters faced instead of innings pitched.  But given the minor differences, K/9’s simpler formula, and its wider usage, this post will use K/9. 


BB/9 is bases on balls (walks) per nine innings.  The formula, as you can probably guess, is BB/IP x 9.  As with strikeouts, BB/G and BB% are alternatives. 


K/BB is simply the ratio between a pitcher’s strikeouts and walks.  K/BB is the formula too (total strikeouts divided by total walks). 


Why do K/9 and BB/9 matter?  K/9 and BB/9 are what are known as rate stats (just like ERA, FIP, batting average, OBP, etc.).  Rate stats are excellent for evaluating and comparing players because they neutralize playing time (the number of innings pitched varies widely from pitcher to pitcher) and focus on the player’s level of performance.  Counting stats, like raw totals of strikeouts and walks, usually need some context to provide meaning. 


For example, consider that in 2008, Brett Tomko (41 Ks) recorded one more strikeout than Robinson Tejeda (40 Ks).  This gives an initial (and misleading) impression that Tomko and Tejeda were roughly equal at striking hitters out.  But Tejeda (39.1 IP) pitched 20 fewer innings than Tomko (60.2 IP).  Using the K/9 rate stat immediately conveys the superiority of Tejeda (9.38 K/9) over Tomko (5.93 K/9) in this aspect of the game.  Because K/9 rate provides this immediate context, it is much better than just listing the raw number of innings pitched and strikeouts.  And, frankly, most readers (maybe even without realizing it) will probably end up trying to do the math in their head to compare the number of strikeouts per inning. 


The same goes for BB/9. 


The other great thing about strikeout and walk rates is that based on evaluating years and years of data, strikeout and walk rates correlate very strongly from year-to-year and are two of the most repeatable skills in baseball.  In other words, past strikeout and walk rates are very good indicators of future strikeout and walk rates.


So what are good K/9 and BB/9 rates?  As a preliminary note, what is considered good and bad strikeout and walk rates will depend on whether the pitcher is a starter or a reliever.  On average, starters have lower K/9 rates and BB/9 rates than relievers.  One of the main reasons is that starters generally do not throw as hard as relievers because starters must pace themselves, while relievers usually can throw at maximum effort given that they will only face a handful of batters.  Throwing with less velocity and force generally will decrease strikeouts but improve control, leading to fewer walks. 


The major league averages for the last three seasons are:



Overall:  6.8 K/9, 3.4 BB/9

Starters:  6.5 K/9, 3.1 BB/9

Relievers:  7.5 K/9, 3.9 BB/9




Overall:  6.7 K/9, 3.3 BB/9

Starters:  6.3 K/9, 3.1 BB/9

Relievers:  7.4 K/9, 3.7 BB/9



Overall:  6.6 K/9, 3.3 BB/9

Starters:  6.2 K/9, 3.1 BB/9

Relievers:  7.3 K/9, 3.7 BB/9


Thus, in general, the average pitcher should be expected to post rates around 6.7 K/9 and 3.3 BB/9, with the average starter closer to 6.3 K/9 and 3.1 BB/9 and the average reliever closer to 7.4 K/9 and 3.7 BB/9.


Among the Royals regulars last year, Greinke (8.14 K/9) and Meche (7.83) led the starters, while Tejeda (9.38) and Soria (8.82) led the relievers in strikeout rate.  Davies (5.65), Bannister (5.57), and Hochevar (5.02) were all below average.


As for walk rate, Greinke (2.49 BB/9) and Bannister (2.86) led the full-time starters, swingman Tomko (1.93 BB/9) led the regulars overall, while Peralta (2.39) and Soria (2.54) led the relievers.  Meche was average (3.12), and Hochevar (3.28) and Davies (3.42) were moderately below average. 


What about K/BB?  As the major league averages listed above suggest, the average is right around 2.0 K/BB.  In other words, the average pitcher should post twice as many strikeouts as walks.  K/BB is useful because it is a good indicator of a pitcher’s command.  A low BB/9 rate may show that a pitcher can throw strikes, but a good K/BB shows that a pitcher can throw quality strikes. 


While not conclusive, good pitchers tend to post K/BB above 2.0 while bad pitchers tend to post ratios below 2.0.  Last year, Soria (3.47 K/BB) and Greinke (3.27) led the Royals, while Duckworth (1.05) and Gobble (1.17) were the worst among the regulars.  Among the other starters, Meche was solid (2.51), Bannister was just about average (1.95), while Davies (1.65) and Hochevar (1.53) fell below average.  In the majors overall, the four pitchers who posted K/BB rates above 5.0 were four of the best pitchers in baseball – Halladay (5.28), Haren (5.15), Beckett (5.06), and Lee (5.00), while the bottom four were generally lousy – Greg Smith (1.28), Barry Zito (1.18), Kenny Rogers (1.15), and Daniel Cabrera (1.06). 


So all you really need to look at is K/BB, right?  While K and BB rates are the most important components in evaluating pitchers, a pitcher can still struggle if his K/BB is above 2.0, but it usually takes a disaster involving batted balls (allowing lots of home runs or line drives) to do so.  For example, last year, Tomko (3.08 K/BB), Farnsworth (2.77 K/BB), and Peralta (2.71 K/BB) all posted good K/BB ratios, but struggled because they posted horrendous home run rates – Tomko (1.63 HR/9), Farnsworth (2.24 HR/9), Peralta (2.56 K/9).  Home run rates are a topic for another day, but for context, an average home run rate is somewhere around 1.0 to 1.1 HR/9, and among AL pitchers with 50+ IP, Peralta finished dead last, Farnsworth was second to last, and Tomko was fifth to last.


On the flip side, a pitcher can still contribute with a poor K/BB if he can, among other things, limit home runs – despite below average K/BB rates, Davies (0.80 HR/9) and Hochevar (0.84 HR/9) both posted decent home run rates last year (although Hochevar is more likely to repeat that result than Davies for reasons that will become apparent when home run per fly ball rate is discussed). 


In summary, the advantages provided by high strikeout or low walk rates should be obvious.  Last year, Greinke had one of the best strikeout rates (8.14 K/9) among AL starters – 21.5% of all batters that Greinke faced made an out before even putting the ball in play – which is approximately 5% better than the average starter.  In contrast, Horacio Ramirez (3.13 K/9) struck out only 8.5% of the batters he faced, one of the worst rates in all of baseball.  Forcing an out without giving the hitter an opportunity to get a hit or even advance runners with an out is the best possible outcome for pitchers, as it essentially eliminates any chance of the other team scoring as a result of that at-bat. 


Again, the same goes for BB/9 – the fewer walks surrendered, the fewer base runners, and the fewer opportunities to allow runs. 

This FanPost was written by a member of the Royals Review community. It does not necessarily reflect the views of the editors and writers of this site.

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