ISO Definition
ISO stands for Isolated Power. As you know, some baseball players are capable of hitting for far more power than others, and this statistic attempts to sort out those differences. When talking about ‘power’ in a hitting context, we are talking about extra-base hits – doubles, triples, and home runs.
Unlike some of the other advanced hitting statistics, such as wRC+, this stat is not attempting to tell the whole story of a batter’s ability in a single number and instead focuses exclusively on the aspect of power.
The Concept of ISO
ISO exists for the purpose of identifying players with the ability to hit for power. It’s really that simple.
In a baseball environment where there are plenty of complex modern statistics to learn, this one is fairly simple. If a player records a double, triple, or a home run in a given at-bat, he will be contributing to his ISO for the season.
Hitting for power is advantageous because extra-base hits generate more runs than singles. Obviously, home runs generate a minimum of one run each time, and more if there are runners on base. Doubles and triples are great for knocking in baserunners, and they also guarantee that a runner will be scoring position for the next batter.
Why is ISO Important?
It’s important to use ISO when you are trying to learn more about the profile of a hitter. Looking at other stats, such as OPS, might suggest that a player brings power to the plate, but you won’t immediately be able to separate the power component from the rest of their performance.
For instance, a batter with a .900 OPS is clearly performing well on the season, but how is he arriving at that number? Is he hitting for a high average and getting on base frequently without adding much power, or did he slug his way to that total? With ISO available, you can dig a little deeper to determine how a player is adding value at the plate. A power hitter is one who is likely to drive in a significant number of runs, potentially making him more valuable than a player who contributes primarily with singles.
Let’s illustrate the value of ISO with a quick example:
In the 2018 season, both Shin-Soo Choo and Joey Gallo finished the year with an .810 OPS. That’s a nice performance from each, placing them within the top-30 hitters in the American League in that category. So, they had nearly identical seasons as the plate, correct? Not really.
For Choo, it was all about getting on base. He used a .377 on base percentage to drive his OPS total, thanks to his patience at the plate and ability to draw walks. Joey Gallo, on the other hand, wound up with an .810 OPS thanks to his prodigious power. His OBP (on base percentage) was 55 points lower than Choo’s, but he made up for it with a slugging percentage just shy of .500. In total, Gallo hit 40 home runs, while Choo hit 21.
Both players added plenty of value at the plate, but they did it in very different ways. Gallo is a true slugger, and his lofty ISO represents his status as one of the most powerful hitters in the game. Choo, while a good hitter in his own right, is not on the same power level as Gallo, and that can be seen by viewing their respective ISO scores.
How is ISO Calculated?
There are multiple ways to calculate ISO, but each will leave you with the same result.
If you have access to a player’s batting line for a given season, the quickest way to calculate ISO is as follows: ISO = SLG – AVG
That’s it. The difference between a player’s slugging percentage and his batting average is his Isolated Power.
If you’d like, you can also go about calculating ISO in a couple of slightly more complicated ways: ISO = ((2B)+(2*3B)+3*HR))/AB
Or
ISO = Extra bases / AB
What is a Good ISO?
If you are used to slugging percentage as an indication of power, you might initially be surprised to see such low numbers for this stat. However, ISO will always be much lower than traditional slugging percentage, because as shown above it’s the difference between slugging and batting average.
Players who are managing an ISO of over .250 are some of the most powerful in the game. While hitters in the .200 – .250 range are not quite elite power hitters, but still contribute plenty of extra bases over the course of a season.
The goal of ISO is to identify the players with the best power in the game. As you will see, this stat succeeds at doing just that, as the players listed below are known to be dangerous power hitters.
Top Ten ISO Scores in MLB, 2018 Season (min. 3.1 PA per game)
| Player | ISO |
| Mike Trout | .316 |
| Khris Davis | .302 |
| J.D. Martinez | .299 |
| Mookie Betts | .294 |
| Joey Gallo | .292 |
| Jose Ramirez | .282 |
| Trevor Story | .276 |
| Christian Yelich | .272 |
| Matt Carpenter | .266 |
| Nolan Arenado | .264 |
Top Ten ISO Scores in MLB, 1900 – 2018 (min. 3.1 PA per game)
| Player | ISO | Season |
| Barry Bonds | .536 | 2001 |
| Babe Ruth | .473 | 1920 |
| Babe Ruth | .469 | 1921 |
| Mark McGwire | .454 | 1998 |
| Barry Bonds | .450 | 2004 |
| Barry Bonds | .429 | 2002 |
| Mark McGwire | .418 | 1996 |
| Mark McGwire | .418 | 1999 |
| Babe Ruth | .417 | 1927 |
| Sammy Sosa | .409 | 2001 |
What are the Problems with ISO?
The limitation of ISO is that it is only looking at what a batter does in terms of power production. That doesn’t make it a bad stat by any means, it simply limits how much information it can provide to you. It’s up to the user of the stat to wisely look at other metrics when trying to gain a complete picture of a player’s offensive performance. If you only want to judge the power of one player as compared to others, ISO is the perfect stat. If you want a bigger picture, you’ll need other inputs.
The other limitation of this stat is the lack of park or league adjustments. Since park factors are not included in the calculation, you’ll need to keep them in mind when viewing various ISO numbers. A player who gets to play in a hitter-friendly home ballpark like Coors Field will have an ISO advantage over a batter playing his home games in a pitcher-friendly location such as Petco Park.