K% and BB%: rates for the pitcher's soul
Why do analysts quote K% instead of the K/9 you grew up with?
K/9 counts strikeouts per nine innings — but innings are a moving target. A pitcher who allows tons of baserunners faces more hitters per inning, giving himself more strikeout chances per nine. Bad pitchers get a small statistical subsidy.
Per batter, not per inning
K% asks the honest question: of the batters you faced, how many did you strike out? BB% is the same for walks. For hitters, the identical stats describe plate discipline: K% under 15% is elite contact, BB% over 12% is an elite eye.
K-BB% — just subtract — is one of the fastest-stabilizing, most predictive pitching numbers there is. Strikeouts and walks involve no defense, no park, no sequencing luck. When a pitcher's ERA and K-BB% disagree, bet on K-BB% telling you the truth about next month.
The formula (optional — skippable)
K% = K ÷ Batters Faced BB% = BB ÷ BF K-BB% = K% − BB%
How this stat lies to you
- For hitters, a high K% looks worse than it is when paired with big power — some strikeouts are the cost of doing damage.
- K% says nothing about what happens on contact; a 28% K pitcher can still get crushed.
- Comparing across eras is treacherous — league K% has drifted up for decades.
Check yourself
1. Why is K/9 subtly misleading?
2. A pitcher has a 4.80 ERA but an elite 24% K-BB%. What's the likely story?