K% and BB%

Strikeout rate (K%) and walk rate (BB%) measure how often a hitter walks or strikes out on a per plate appearance basis. They’re measured in percentage form, so it’s easy to compare between players and years, but you should be cautious because league average has shifted over time.

High walk rates are good for batters because it means they’re reaching base often, while low walk rates are only acceptable if a player has exceptional power or contact skills to make up for it. Strikeout rates are a bit tougher to pin down — while making an out is bad, striking out isn’t necessarily worse than any other sort of out. If a player is still getting hits, walking, and reaching base at a high rate, then they can still be a valuable offensive piece with a high strikeout rate. As always, it’s important to consider strikeout or walk rate in the context of the overall set of skills.

Calculation:

K% and BB% are two of the easiest statistics to calculate. You simply divide the strikeouts or walks by the total number of plate appearances:

K% = K / PA

BB% = BB / PA

Please remember that this is a raw statistic and no corrections are made for league average, park effect, or games situation. You may be familiar with K/9 or BB/9 with respect to pitchers, but since batters are much more typically judged on a per PA scale, there’s not much use for a per game strikeout or walk metric for hitters.

Why K% and BB%:

Generally speaking, K% and BB% tell you something about a player’s plate discipline and contact skills. If a player routinely draws walks, they are likely very good at distinguishing between balls and strikes, which will not only lead to more times on base via a walk, but will also likely lead them to make better contact because they will be more likely to swing at pitches which they can drive. Not only does BB% provide a summary of how often a batter has walked, but it also provides some information about their underlying approach.

K% is the same in that it provides you with a summary of how frequently a batter strikes out, but it also gives you a sense of the batter’s ability to make contact. Since strikeouts are almost always automatic outs, you know that a strikeout, unlike a ball in play, is always a negative outcome, so a batter who strikes out frequently is someone who is failing to provide value during those PA. Additionally, a large number of strikeouts is also an indication that a batter has a difficult time making contact or recognizing pitches.

Both statistics provide you with a basic summary of some raw information that is often relevant, but also help you make quick inferences about the type of player you’re observing. Ideally, you would like to be a high walk, low strikeout player, but it’s not uncommon to observe players with high walk rates and high strikeouts rates (they often hit for power), or players with low walk and low strikeout rates (they’re often higher BABIP guys). Typically, a low walk, high strikeout player won’t do very well in MLB unless they have massive power or amazing defensive ability.

How To Use K% and BB%:

In one sense, using K% and BB% is extremely easy. You know that both stats are a percentage of total PA, so if you’re just looking for frequency, you read them like any other percentage. If you want to use them to make inferences about players, you need to be attuned to sample size and league averages.

With respect to sample size, it’s important to know that K% and BB% tend to “stabilize” in a relatively low number of PA. You don’t need more than about 60 PA for K% or 120 PA for BB% before the numbers start to become meaningful, which means that it’s very unlikely that even a half season of K% or BB% are simply functions of random variation. More directly, a player’s K% and BB% are skills that you can estimate from a reasonably small amount of data. This means that if you have a good sample of PA, you can feel good about the validity of the information you’re using.

Additionally, it’s important to consider the league context. Strikeout and walk rates vary across eras, so while a 20% strikeout rate is nothing in 2014, it would have horrified players in the 1970s, for example. As the nature of the game changes, your expectations about these things have to change as well. Specifically, during the 2010s, the strikezone has expanded league-wide, leading to far more strikeouts than ever before. Is it right to say that hitters are getting worse? Not really, because there are contextual factors at play.

Generally speaking, we use K% and BB% to get a sense of what kind of an approach someone has. If you have a lot of Ks and limited BBs, you’re label as a hacker. If you have a ton of BBs, you’re patient. No one or two stats ever tell the whole story, but these two are among the most consistent and safe bets. It’s important to understand that while a given BB% might be considered poor, it is possible to have success with that poor rate if you have other valuable skills.

Context:

Please note that this chart is meant as an estimate, 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.

Rating	K%	BB%
Excellent	10.0%	15.0%
Great	12.5%	12.5%
Above Average	16.0%	10.0%
Average	20.0%	8.0%
Below Average	22.0%	7.0%
Poor	25.0%	5.5%
Awful	27.5%	4.0%

Things to Remember:

● Power hitters tend to have high strikeout and walk rates, since they may swing and miss often, yet are pitched around by pitchers. Contact hitters are the opposite; they tend to have low strikeout and walk rates.

● The more a player strikes out, the tougher it is for them to maintain a high batting average since they are putting fewer balls in play.

● In the past, FanGraphs carried K% as K/AB. In 2011, K% was changed to K/PA.

● K% and BB% are not park, league, or context adjusted.

● Strikeout rate is currently on the rise, so you might need to update your conception of a higher K%.

Links for Further Reading:

How Many Strikeouts is Too Many? – Hardball Times