Baseball Glossary › Strikeouts per Nine Innings
Strikeouts per Nine Innings (K/9)
Strikeouts per nine innings (K/9) is the average number of strikeouts a pitcher records for every nine innings he pitches. Multiply strikeouts by 9 and divide by innings pitched. It measures swing-and-miss ability, and because relievers throw at full effort in short outings, their K/9 numbers usually run higher than starters'.
Formula
Worked example (illustrative numbers): a pitcher records 180 strikeouts in 162 innings.
K/9 = 9 x 180 / 162 = 1,620 / 162 = 10.0
Convert partial innings first: 162.1 IP is 162.333 innings
Athletics example: Dennis Eckersley
Dennis Eckersley struck out 2,401 batters in 3285.2 career innings. Convert the innings first: 3285.2 means 3285.667 true innings. See the full career line on the Dennis Eckersley player page.
K/9 = 2,401 x 9 / 3285.667 = 6.58
Career MLB totals from our player database (MLB Stats API).
What to Know
K/9 vs K%: K/9 is per inning, while strikeout rate is per batter faced. A pitcher who allows many baserunners faces more batters per inning and gets more chances to strike someone out, which can inflate K/9. Many analysts prefer K% for that reason.
FanGraphs' rough guide, which it notes is an estimate that varies by season, puts about 7.7 at average, 9.0 as great and 10.0 as excellent for K/9. Relievers typically run higher than starters.
Strikeouts remove the defense from the play, which is why K/9 is one ingredient in fielding independent pitching. Compare it with walks per nine innings to see both sides of a pitcher's command.
Frequently Asked Questions
What does K/9 mean in baseball?
K/9 means strikeouts per nine innings. It is the average number of batters a pitcher strikes out for every nine innings he pitches, a standard measure of strikeout ability.
How do you calculate K/9?
Multiply strikeouts by 9 and divide by innings pitched. For example, 180 strikeouts in 162 innings works out to a K/9 of 10.0. Convert innings like 162.1 to 162.333 first.
What is a good K/9?
FanGraphs' rule-of-thumb table treats about 7.7 as average, 9.0 as great and 10.0 or higher as excellent, while noting that league strikeout levels change by season.
What is the difference between K/9 and K%?
K/9 counts strikeouts per nine innings, while K% counts strikeouts per batter faced. K% is often preferred because K/9 can rise simply because a pitcher faces more batters per inning.
Why do relievers have a higher K/9 than starters?
Relievers pitch short outings and do not need to save energy, so they can throw at maximum effort to every hitter. That usually produces more strikeouts per inning than a starter pacing himself.
Why does K stand for strikeout?
K is the traditional scorekeeping symbol for a strikeout. In box scores and stat lines strikeouts may appear as SO or K, and K/9 uses the K shorthand.
Related Terms
Sources
- MLB.com Glossary: Strikeouts Per Nine Innings (K/9)
- FanGraphs Sabermetrics Library: Strikeout and Walk Rates
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