Strikeouts Per 9 Innings (K/9) and Walks Per 9 Innings (BB/9) are rate statistics that measure how many strikeouts and walks a pitcher averages over nine innings. Of course, not many pitchers throw nine innings all at once anymore, but this is a way of standardizing the statistic so it’s on an easy-to-understand scale like many other pitcher statistics which are scaled to 27 outs. Alternatively, we also provide Strikeout Percentage (K%) and Walk Percentage (BB%) if you prefer a statistic which measures strikeouts or walk per batter faced. For pitchers, more strikeouts and fewer walks are the goal.
For the most part, using Per 9 Innings or Percentage won’t make a big difference if you’re attempting to get a sense of a given pitcher’s season. However, worse pitchers will often face more batters per inning than better pitchers, meaning a pitcher who strikes out two of six batters in an inning will have the same K/9 as a pitcher who strikeouts of two of three batters in an inning. Both would have 18.0 K/9 for that inning, but the first would have a 33 K% and the second would have a 66 K%. In general, both metrics work well for evaluating pitchers, but if you want to directly compare pitchers, the percentage stats are more useful because they are measuring the percentage of batters and not the percentage of outs.
Calculating both versions is a snap.
K% = Strikeouts / Batters Faced
BB% = Walks / Batters Faced
K/9 = Strikeouts*9 / Innings Pitched
BB/9 = Walks*9 / Innings Pitched
None of the statistics are park, league, or season-adjusted, so take note that a 25 K% describes a very different pitcher in 1985 and 2015. FanGraphs has both versions dating back to 1916, with K/9 and BB/9 going back even further.
Why Strikeout and Walk Rates:
Strikeout and walks rates are extremely important for evaluating pitchers. A plate appearance can essentially end in four ways: strikeouts, walks, home runs, and balls in play. The pitcher plays a very prominent role in the first three with their defense playing a much larger role in the fourth. Strikeouts are inherently good because they are automatic outs which don’t advance base runners. Walks, on the other hand, are free bases which help the opponent score.
We care about strikeout and walk rates for two primary reasons. First, pitchers have a lot of control over their strikeout and walk rates which means that they are a decent measure of pitcher performance and skills. Strikeouts and walks aren’t the only aspects of pitching, but they are two aspects of pitching which are mostly attributable to the pitcher rather than the pitcher and their team combined.
Second, strikeouts and walks are important because they are stable predictors of success. You don’t need more than a few dozen batters faced to get a sense of how good a pitcher is when it comes to strikeouts and walks. Obviously talent changes and opponents matters, but pitchers who collect strikeouts routinely prevent runs and pitchers who allow walks typically allow more runs and you can get a sense of where a pitcher stands pretty quickly when using K% and BB%.
As a rule of thumb, if you know a pitcher’s strikeout and walk rates, you know about half of what you need to know to truly understand them as a pitcher.
How To Use Strikeout and Walk Rates:
Using strikeout and walk rates is very simple. While these stats get blended into Wins Above Replacement (WAR) through Fielding Independent Pitching (FIP), evaluating pitchers using K% and BB% is very straightforward. Generally, you want to use the numbers in conjunction with each other. If a pitcher has tons of strikeouts, they can afford more walks. If they have fewer strikeouts, they better not walk a ton of batters.
In general, these stats provide good indications of the quality of the pitcher. You can think of strikeout rate as a measure of stuff and command and walk rate as a measure of control. Pitching is more complicated than that alone, but the idea is pretty simple: The larger the spread between strikeout and walk rate, the more effective the pitcher. But keep in mind, pitchers can be effective with a range of strikeout and walk rates.
Please note that these charts are meant as estimates, and that league-average strikeout and walk rates vary on a year-by-year basis. To see the league-average strikeout and walk rate for every year from 1901 to the present, check the FanGraphs leaderboards. Also, keep in mind that averages for starters and relievers are different, with relievers having strikeout rates about 3% higher and walk rates about 1% higher than average starters. Use this as a general guide.
Things To Remember:
Links for Further Reading: