K% and BB% Definitions
K% and BB% represent strikeout percentage and walk percentage, respectively. These are important statistics for both pitchers and hitters. Obviously, hitters would like to maximize BB% while minimizing K%, and pitchers would like to do just the opposite.
While walks and strikeouts have been tracked since the earliest days of baseball, these stats have received renewed attention in recent years. Walk rates and strikeout rates are used in many advanced metrics, so it’s important to understand them as a foundational piece of more complicated statistical analysis.
The Concept of K% and BB%
The concept of tracking walk and strikeout rate is that it is easier to evaluate players on a rate, or per plate appearance basis, than it is to evaluate them on raw totals.
For instance, if you were told that a particular batter had struck out 50 times on the season, that number would have little meaning without more context. If the player had only batted 100 times, that strikeout total would be a disaster. On the other hand, it would be quite impressive if the batter had come to the plate 600 times.
By using rates rather than raw numbers, you can immediately learn something about the player by viewing this statistic.
Why are K% and BB% Important?
There are a couple of things which make strikeout rate and walk rate so important. For starters, the outcome of these two types of events is assured, since the defense can’t do anything about either a walk or a strikeout.
A strikeout is so valuable to a pitcher because it’s an automatic out, unless the ball gets away from the catcher. On the flip side, a walk is so good for a batter because it’s an automatic way to reach base. Even a hard-hit line drive could turn into an out if it goes right to a fielder, but that won’t happen with a walk.
These are also important stats because they normalize quite quickly and tend to hold steady from season to season. In other words, a player’s ‘true talent’ is established rather early on when it comes to drawing walks and avoiding strikeouts (or avoiding walks and recording strikeouts, for a pitcher). Walks and strikeouts can be used within so many other advanced stats in large part because they tend to be predictive of the actual ability a player brings to the field.
How are K% and BB% Calculated?
The calculations here are extremely simple—all you need to know is the total number of walks or strikeouts for a given player, along with the total number of plate appearances. Then you just divide the total of strikeouts or walks by the total number of plate appearances, and you have the rate. This number can be expressed as a decimal value, such as 0.15, or as a percentage, such as 15%.
While the math here is simple, let’s go through two quick examples just to make sure everything is clear. First, let’s calculate both K% and BB% for one of the best pitchers in MLB, Jacob deGrom. deGrom had an incredible 2018 season, so it’s not surprising that he was among the leaders in these categories. In 2018, deGrom faced a total of 835 batters. Of those, 46 walked, and 269 struck out.
BB% = 46/835 = 5.5%
K% = 269/835 = 32.2%
On the other side of the game, J.D. Martinez of the Boston Red Sox also had an excellent 2018 season. Of his 649 plate appearances, 69 ended in walks and 146 ended in strikeouts. Again, the math is easy.
BB% = 69/649 = 10.6%
K% = 146/649 = 22.5%
What is a Good K% and BB%?
Evaluating a good K% or BB% can be tricky, as there are context elements to keep in mind. First, the era in which the player competed has to be taken into account. Modern baseball features far higher walk and strikeout rates than were seen in generations gone by. Batters who strike out at a high rate are more accepted today, as long as they produce in other ways.
Second, the type of player needs to be considered when looking at these two rate stats. A power hitter will naturally strike out at a high rate, since those big, powerful swings come with a lot of misses. At the same time, a power hitter should walk a lot, as pitchers will be wary of making a mistake right down the middle. There are also pitchers to focus more on generating weak contact than actually recording strikeouts, so this concept works both ways.
To highlight this concept, let’s look at a few lists. One quick note: in baseball statistics, ‘qualifying’ players are those who meet a specific playing time threshold. For pitchers, that is one inning pitched for each game the team plays. For hitters, it is 3.1 plate appearances per game.
The top five qualifying batters from the 2018 in terms of lowest K%.
The top five qualifying batters from the 2018 season in terms of highest BB%.
There are some good players on that list, to be sure, but none of them would be classified as a pure power hitter. Michael Brantley hit 17 homers during the 2018 season, which is a nice total, but certainly isn’t among the league leaders.
Let’s now look at some pitchers who stay in the zone regularly:
The top five qualifying pitchers from 2018 in terms of avoiding walks.
The top five qualifying pitchers from 2018 in strikeout percentage.
The name that stands out here is Justin Verlander. Known as one of the best strikeout pitchers in baseball, Verlander is also one of the best at avoiding walks. With such a powerful combination working in his favor, it’s no wonder that Verlander is among the best in the game year after year.
What are the Problems with K% and BB%?
It’s hard to say that there are any problems with these two statistics, as they are simply reflective of what’s happening on the field. There are no complex calculations to call into question, and no degree of human judgement is required (outside of the umpire calling balls and strikes, of course).
If there is one thing to mention regarding these stats it is that the league-wide environment changes over time, so comparing walk and strikeout rates across generations is virtually useless. Yes, modern hitters strike out more than hitters in the past, so does that mean they are worse? Not necessarily. Pitchers throw harder now than ever before, and the game is more focused on big, power swings than making contact. It’s important to only use these stats to compare players to their peers, not to players of past generations.
Along the same lines, it’s best to avoid using K% and BB% alone to make any player value judgements. These are important stats, and they are a component used in many advanced metrics, but K% and BB% alone don’t tell a complete picture.