What is ISO (Isolated Power)? Definition, Formula, and Example
ISO (Isolated Power) measures a hitter's raw extra-base power by subtracting batting average from slugging percentage, isolating only doubles, triples, and home runs.
Plain-English Definition of ISO
Isolated Power (ISO) is the per-at-bat rate at which a hitter produces extra bases. It strips singles out of slugging percentage so you see only the doubles, triples, and home runs. A hitter with a high ISO does damage when he connects; a hitter with a low ISO puts the ball in play but mostly for singles. It is the cleanest single-number answer to "how much power does this guy actually hit for?"
How ISO Is Calculated
ISO equals slugging percentage minus batting average:
ISO = SLG − AVG
Or, derived directly from raw components:
ISO = (2B + 2·3B + 3·HR) / AB
Each extra base counts once: a double adds 1 to the ISO-weighted total, a triple adds 2, a home run adds 3. Singles add nothing. The denominator is at-bats — not plate appearances — so walks, HBPs, and sacrifices do not dilute the number.
Rough grading by modern MLB standards:
- .300+ — elite, MVP-tier power
- .250 — All-Star power
- .200 — solid everyday regular
- .140 — league average
- .100 or lower — well below average, slap hitter
Worked Example: Judge vs. Arraez
Aaron Judge, 2024: .322 AVG, .701 SLG → .379 ISO, which led all qualified MLB hitters. Breaking it apart: in 559 at-bats he hit 36 doubles, 1 triple, and 58 home runs — 211 ISO-weighted extra bases translating to a .378 mark before rounding. That is effectively a doubles rate of 6% plus a home-run rate of 10% every time he came to the plate.
Contrast Luis Arraez the same season: .314 AVG, .346 SLG → .032 ISO. Nearly identical batting average to Judge, but Arraez's extra bases came from 25 doubles, 0 triples, and 4 homers across 615 at-bats. He is, statistically, a singles machine.
Why ISO Matters
ISO is the stat front offices and fantasy managers reach for when they want to separate contact hitters from power hitters. Two players can both hit .280, but the one with a .240 ISO is a cleanup bat and the one with a .080 ISO is a leadoff slap hitter — their roles, contracts, and roster value are not comparable. ISO also predicts park-adjusted run production better than batting average does, because extra bases drive runs at a much higher rate than singles. In DFS, ISO paired with barrel rate is the fastest way to surface hitters with home-run upside on any given night.
Limitations and Misconceptions
ISO is not park-neutral: Coors Field inflates it and Oracle Park suppresses it, so always read it alongside park factors. It also says nothing about how often a hitter reaches base — a .300 ISO hitter with a .280 OBP is wasting his thump. Do not confuse ISO with slugging percentage; SLG still counts singles, and a .500 SLG hitter who also hits .300 has a .200 ISO, not .500. Finally, ISO is descriptive, not inherently predictive — for year-over-year stability look underneath it at barrel rate and hard-hit rate, which drive it.
Related Terms
In Legends Deck
ISO feeds the Power rating on every Legends Deck hitter card. We blend ISO with Statcast barrel rate and average exit velocity so that card power reflects both outcomes and the quality-of-contact that produced them — a .280 ISO hitter whose underlying barrel rate also sits in the 90th percentile grades higher than one whose ISO came from a cluster of short-porch homers. When you pull a Power 95 out of a pack, you know it is backed by the same math front offices use, not by reputation.