ISO, isolated power, subtracts batting average from slugging percentage. What's left is extra bases per at-bat, a number that doesn't care about park factors, a lucky week, or a fence that happens to sit 15 feet closer than it should.
What exactly is ISO?
ISO is slugging percentage minus batting average. It tells you how many extra bases a player racks up beyond a normal single. The formula is simple, but it catches something home run totals miss entirely: every extra-base hit, not just the ones that clear the fence. A guy who rips 40 doubles gets the same credit in ISO as a guy who hits 25 homers, because both are producing power that batting average alone hides.
Take Freddie Freeman. He's never been a huge home run guy next to true sluggers, but his gap power keeps his ISO respectable year after year because he's constantly doubling into the corners. Put him next to a player who hits .300 but almost never gets past first base, and ISO separates them instantly, even though their batting averages might look identical on a stat sheet.
Why do home run totals lie about power?
Home run totals depend heavily on park dimensions, weather, and plain sequencing luck, not just how hard a guy hits the ball. A player in a bandbox like Cincinnati's Great American Ball Park can pad his home run count on fly balls that would die on the track anywhere else. A legitimately powerful hitter stuck in a pitcher's park like Oracle Park in San Francisco gets shortchanged every season.
According to FanGraphs park factors, some ballparks have historically boosted home run rates by 10-15% or more compared to a neutral field, while others suppress them by a similar margin. That swing alone can be the difference between 25 and 35 home runs for the same swing, the same exit velocity, the same guy.
ISO doesn't erase park effects, but it smooths them out better than home run counting does because it credits doubles and triples too. A player getting robbed of homers by a deep fence is often still doubling off that same wall, so his ISO holds up even in a year his home run total dips.
How does ISO compare across different types of hitters?
ISO separates true power hitters from empty-average slap hitters even when their basic stats look similar on paper. A .320 hitter with a .350 slugging percentage has an ISO of just .030, next to nothing, meaning almost every hit he gets is a single. A hitter batting .260 with a .520 slugging percentage has a .260 ISO, more than eight times higher. The second guy is doing far more damage every time he makes contact, even though his batting average looks worse at a glance.
Giancarlo Stanton is a good real-world example of an elite ISO guy. Even in seasons where injuries limited his overall numbers, his per-at-bat power, tracked through advanced metrics on Baseball-Reference, stayed near the top of the league because every ball he squares up travels a long way. That's the kind of thing ISO catches that a simple home run count buries.
If you're trying to figure out who's actually strong at the plate versus who just happens to play in a friendly park, ISO gives you a cleaner read. It's one reason serious power evaluation has moved past counting stats, similar to how raw shooting percentage can mislead you in basketball without more context.
What counts as a good ISO number?
A good ISO in modern MLB sits around .150 to .200, with elite power hitters climbing above .250. League average ISO tends to hover in the .140 to .160 range depending on the year and the overall run-scoring environment. Anything under .100 usually signals a hitter with almost no power, even if his batting average or on-base numbers look fine otherwise.
Players like Aaron Judge have posted ISO figures well above .300 in peak seasons, a level that's genuinely rare and puts them in a different tier than nearly everyone else in the league. Most everyday players who aren't considered power threats land somewhere between .120 and .160, a real but unspectacular gap between doubles and homers on one side and pure singles on the other.
Once you know these benchmarks, ISO becomes a fast way to size up a hitter you don't know much about. Pull up a stat line, do the quick subtraction, and you know right away whether you're looking at a legitimate power bat or someone compiling empty batting average.
Why should you track ISO instead of just home runs?
ISO rewards every extra base a hitter earns, not just the ones that leave the yard, so it gives you a fuller picture of raw power. Home run counting is easy and satisfying, but it's incomplete. Two hitters can finish a season with 20 home runs each while one of them also racked up 40 doubles and the other barely doubled at all. ISO catches that gap instantly. Home run totals never will.
This matters even more when you're comparing players across different roles, ballparks, or eras. A metric like ISO travels better across contexts than a counting stat does, the same way a rate stat like PER gives you more signal than raw box score totals in basketball. If you want to know who's going to keep producing power rather than who got lucky with a short porch in right field, ISO is the number to watch.
FAQ
Is ISO the same thing as slugging percentage?
No, ISO is slugging percentage minus batting average, which isolates only the extra-base value and removes credit for ordinary singles that inflate slugging percentage on their own.
Can a low batting average hitter still have a strong ISO?
Yes, plenty of power hitters run relatively low batting averages while posting excellent ISO numbers because nearly every hit they get goes for extra bases instead of a single.
Does ISO account for walks or on-base ability?
No, ISO only measures power on batted balls that result in hits, so it says nothing about a player's patience, walk rate, or overall on-base skill.
Knowing a stat like ISO cold is exactly the kind of edge that separates casual fans from sharp predictors. Download GAGE and start putting that knowledge to work on real player predictions.