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Welcome, RotoBallers, to our overview of plate discipline. This article is a deeper dive into plate discipline, and is part of our ongoing series "Using Sabermetrics for Fantasy Baseball."
In this article, we'll look at how to predict K% and BB% using advanced plate discipline stats such as swinging strike percentage (SwStr%) and chase rate (O-Swing%) in plain terminology anyone can understand.
You can find our entire sabermetrics glossary, which includes links to many other sabermetric stats as part of this series. Each stat deep dive will be released over the next few days. Stay tuned!
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How to Interpret Plate Discipline? Sabermetrics Glossary
No matter how high a particular player's BABIP may be, his average will be mediocre at best if he strikes out too much. This is why fantasy managers have known for years that players like Christopher Morel are potential drains on a squad's batting average. Furthermore, players who whiff a lot tend to continue doing so -- it is a very sticky trait.
In 2024, the league average K% was 22.6 percent, meaning that roughly one in five MLB PAs ended in a K. Players who strike out less frequently tend to hit for higher averages, while more strikeouts hurt batting averages. Of course, a player may put up a fluke K% just as easily as a fluke BABIP.
FanGraphs is once again the best place to look for plate discipline data, located toward the bottom of each player's page right before fielding. Let's learn how analyzing stats related to plate discipline can help improve the performance of your fantasy baseball team entering the 2025 season.
Using SwStr% to Support K% Changes
Sabermetrics may be used to determine whether a given player "deserved" his K% over a particular period. The first number to check is the swinging strike percentage or SwStr%. This metric simply tracks what percentage of pitches a batter swings and misses at. The league average was 11.1 percent in 2024, with higher numbers indicating a proneness to K.
If a player improves his strikeout rate without a corresponding improvement in SwStr%, the improvement may not stick moving forward. Likewise, a career-worst strikeout rate backed by a normal SwStr% will likely regress in the player's favor.
Notably, Baseball Savant's Whiff% is not the same thing as SwStr%. Whiff% measures how often a batter swings and misses on all swings, while SwStr% uses all pitches seen instead. Whiff% figures are, therefore, much higher than SwStr%. Since SwStr% is used much more frequently in fantasy analysis, the rest of this article will focus on it.
Further detail is offered by chase rate or O-Swing%, a measure of how often a batter swings at pitches outside the strike zone. Batters usually want to hit "their pitch," which they never get to see if they pop up a fastball over their head early in the count. In 2024, the league averaged an O-Swing% of 31.8 percent. Numbers significantly higher than this indicate an increased likelihood of chasing a bad pitch.
This stat is also used to examine a player's walk rate, or BB%, in much the same manner as SwStr% is used to double-check K%. A strong walk rate when a player is still chasing too many pitches is not based on any repeatable skill, and will likely normalize moving forward. Likewise, a lower walk rate paired with a career-average O-Swing% indicates that the walks should return.
Fantasy managers should always care about walks even if their format does not directly reward them. Every BB is a chance to steal a base or score a run, and players who know the zone tend to hit for higher averages to boot! The league average BB% in 2024 was 8.2 percent.
Evaluating Players Through Plate Discipline
Juan Soto is the undisputed plate discipline king, and his metrics are a marvel. His 18.1 percent BB% bested his 16.7 percent K% in 2024, something virtually unheard of in today's game. Digging deeper, Soto has the peripherals to back up those numbers. He only chased pitches outside the strike zone 18.4 percent of the time last year. He also rarely missed the ball with a SwStr% of 7.4 percent.
You would think Soto's outstanding plate discipline was an outlier, but it's par for the course:
It is safe to conclude that Soto will continue to demonstrate outstanding plate discipline.
Aggression or passivity at the plate can confound this analysis slightly. On average, major league hitters swung 47.8 percent of the time in 2024. Soto's outright refusal to swing at borderline pitches (36.9 percent Swing% last year) leads to more Ks than his raw SwStr% numbers would suggest since he sometimes takes Strike 3. Still, it's hard to find much fault in Soto's approach.
Morel is the polar opposite of Soto. He hit .196/.288/.346 with 21 HR last season, demonstrating a good eye with a 10 percent BB% backed by a 25.3 percent chase rate. His 47 Swing% was also essentially considered league average. However, he doomed himself to fantasy irrelevance with a 14.8 percent SwStr% that was nearly two points better than his career 16.5 percent rate. That's a lot of swing-and-miss.
Other plate discipline metrics exist, such as Z-Swing%, O-Contact%, and Z-Contact%, but SwStr% is usually a good enough proxy for fantasy purposes. One exception to this rule is a change in SwStr% rooted exclusively in pitches outside of the zone. Sometimes, missing those pitches can be better than hitting them.
Conclusion
To conclude, both K% and BB% are useful for fantasy purposes but fail to tell the whole story. SwStr%, or how often a batter swings and misses, is a better indicator of a player's future strikeout rate than K% alone. O-Swing%, or how often a batter chases pitches outside of the zone, performs similarly concerning BB%.
If you want to learn how to use more metrics to determine fantasy performance, stay tuned to our series on making sabermetrics more accessible to fantasy managers!
