Isolated Power (ISO) is a measure of a hitter’s raw power and tells you how often a player hits for extra bases. We know that not all hits are created equally and ISO provides you with a quick tool for determining the degree to which a given hitter provides extra base hits as opposed to singles. While batting average and slugging percentage each offer part of the answer, they aren’t very good at distinguishing players without being considered together, even if you know a player’s walk rate as well.
For example, a four singles and zero home runs in 10 at bats is a .400 batting average and .400 slugging percentage. One home run and zero singles in 10 at bats is a .100 batting average and .400 slugging percentage. The first player’s ISO is .000 and the second player’s ISO is .300, which tells you that the second player hits for extra bases more often. ISO doesn’t replaced a metric like OPS or wOBA, it simply helps you determine the type of player at which you’re looking.
Since ISO is a very simple statistic, you can calculate it three different ways, depending on what information you have in front of you:
ISO = SLG – AVG
ISO = ((2B) + (2*3B) + (3*HR)) / AB
ISO = Extra Bases / At-Bats
ISO is not park or league adjusted, so you should treat it like batting average, on-base percentage, or slugging percentage when making comparisons. However, this makes it easy to calculate for small or large samples of data because the formula never changes.
ISO is useful because two players with identical batting averages can be having very different seasons and two players with the same slugging percentages can be having very different seasons, even if you hold walks, plate appearances, park effects, and luck constant. A .300 average with very few extra base hits is quite different from a .300 average with 40 home runs. The same is true of a .500 slugging percentage that is driven by many singles versus one driven by lots of doubles and home runs.
ISO tells you the average number of extra bases a player gets per at bat and this is a piece of information you want to know. You want to know what share of a player’s hits and singles and what share are extra base hits. ISO doesn’t tell you anything you can’t learn from AVG and SLG together, but it saves you a step.
How to Use ISO:
Using is ISO is very simple. It tells you the number of extra bases the player averages per at bat and signals to you the degree to which a particular hitter is a power hitter. Around .140 is league average and hitters in the .200+ range are typically the premier sluggers.
However, you need to be careful because not all .200 ISO are created equally. A .250 average and .450 slugging and a .350 average and .550 slugging both return ISOs of .200 because both batters have the same rate of extra base hits per at bat, even though the latter hitter is clearly the better player overall.
It’s also important to know that ISO is not park or league adjusted so batters will do better in hitter’s parks and when the league’s run environment is higher. Additionally, ISO is not a measure of value in that a triple is not worth 50% more than a double. If you want something that weighs each hit properly, you want wOBA.
Additionally, ISO takes some time to become predictive and a 40 AB sample of a .350 ISO doesn’t tell you much about how that hitter will perform in the future. It takes about 550 PA or so before ISO becomes predictive of future ISO, but it obviously does a fine job offering retrospective information right away.
Please note that the following chart is meant as an estimate, and that league-average ISO varies on a year-by-year basis. To see the league-average ISO for every year from 1901 to the present, check the FanGraphs leaderboards.
Things to Remember:
● It takes a long time for a player’s ISO to have predictive power going forward; a sample size of 550 plate appearances is recommended to draw any conclusions. In other words, if a player has a .550 ISO two weeks into the season, it’s way too early to expect that to continue.
● ISO is not park or league adjusted which means a higher ISO in a pitcher’s park is more impressive than the same ISO in a hitter’s haven, with same being true of league wide run environments.
● ISO is descriptive in that it counts extra bases, but it does not properly weigh their importance in a value sense, like wOBA does.
● ISO is context neutral, meaning it counts all doubles equally regardless of the number of base runners, outs, or the score.
Links for Further Reading: