Strikeout Rate (K/9) and Walk Rate (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 for most fans. For pitchers, more strikeouts are obviously better, while walks are bad. Good pitchers normally strike out at least twice as many batters as they walk.
Some people prefer to measure pitcher strikeout and walk rates by using K% and BB%, since those account for the fact that some pitchers face more batters per inning than other pitchers (therefore, getting more chance to strike them out or walk them). Both K% and BB% are calculated using plate appearances in the denominator, not at bats.
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.
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