Isolated Power and the Power Metrics Explained

By 4Baseball Staff · November 5, 2026 · 10 min read

How to calculate isolated power (ISO), how it differs from slugging, how home run rate and Statcast barrels measure power, and what each number misses, with league and career figures from the historical record.

Isolated power, usually written ISO, answers a narrow question: how many extra bases does a hitter produce per at-bat? It is calculated by subtracting batting average from slugging percentage, and the result strips out singles, which are the part of a hitter's output that has the least to do with strength. A hitter who bats .300 and slugs .400 has an ISO of .100. A hitter who bats .250 and slugs .450 has an ISO of .200 and is, in a specific sense, twice as powerful, even though he gets fewer hits.

This article covers the formula, what it counts and ignores, how the league-wide figure has moved over a century, and which hitters lead the career list in the historical record. It then moves on to the other ways people measure power: home run rate, and the batted-ball measures that Statcast added after 2015, including exit velocity, launch angle and barrels. The goal is to leave you able to read a stat line and decide which of these numbers answers the question you are actually asking.

The formula, with a worked example

Slugging percentage is total bases divided by at-bats, where a single is worth one base, a double two, a triple three and a home run four. Batting average is hits divided by at-bats. Subtract the second from the first and the singles cancel, because every single appears once in each number. What remains is the extra bases: doubles counted once, triples twice and home runs three times.

The formula, written out, is ISO = SLG minus AVG, which equals (doubles + 2 x triples + 3 x home runs) divided by at-bats. Here is a hypothetical comparison, with invented hitters to keep the arithmetic clear.

Contact Carl collects 25 more hits than Power Pete, and he gets there with 120 singles. Pete has 74 singles and 30 home runs. Carl's slugging percentage is .396 and Pete's is .474. The ISO values, .096 and .224, show the gap more starkly because they remove the singles from both rows. If you were asked which hitter hits the ball hard and far, ISO answers in one number. If you were asked which hitter is more valuable, you would need to know a lot more: walks, defense, position and baserunning.

What ISO counts, and what it leaves out

ISO counts extra-base hits and nothing else. That includes doubles and triples, which are as often products of speed and gap-hitting as of raw strength. A fast hitter with a good eye for the gap can post a respectable ISO without being a home run hitter. It also means that a ballpark with a deep outfield and spacious gaps can inflate doubles and triples in a way that has little to do with muscle.

ISO ignores walks and hit-by-pitches, because it divides by at-bats, which exclude them. A hitter who walks 120 times a year has an ISO that tells you nothing about that skill. It ignores strikeouts, except in that a strikeout is an at-bat with no hit. It also does not adjust for the league or the park. A .200 ISO in 1968, when the league's was .104, is a different accomplishment from a .200 ISO in 2019, when the league's was .183. Everything in this paragraph follows from the formula; it is a feature of the stat, not a flaw.

The sample matters as well. Over a few dozen at-bats a hitter's ISO swings wildly with a couple of home runs, so a power number from a hot month deserves less trust than one from a full season. Think of ISO as a measure that needs a few hundred plate appearances before the noise settles.

How league-wide ISO has moved

The table below uses the combined American and National League totals in the historical record, with ISO computed as extra bases divided by at-bats. The Negro leagues are not included in these rows.

Two things stand out. First, the climb between 1919 and 1930 is the live-ball era; the article on how the live-ball era changed baseball tells the history. Second, the league batting average in 2025 was .245, lower than in any year in the table except 1968 and 1908, while the league ISO was far higher than in 1968. In modern baseball the average hitter makes less contact and hits for more power than the average hitter of 1930 or 1987. The two numbers moved in opposite directions, and ISO is the stat that makes that visible. The at-bats-per-home-run column shows the same shift: 302 in 1908, 167 in 1919, 55 in 1930, 25 in 2019.

The career leaders in ISO

Using the historical record through 2025 and a minimum of 3,000 career at-bats, the top of the ISO list reads as a roll call of power hitters. The minimum eliminates players with short careers.

Ruth's .348 is the best by a wide margin, and notice that it was achieved with a .342 batting average. The hitters at the top are not all home run hitters who struck out a lot. Ruth, Gehrig and Williams all hit for average and for power. McGwire is the other kind: a .263 hitter whose ISO is higher than his batting average.

The historical record includes Negro leagues statistics, and some players from those leagues, such as Mule Suttles, with an ISO of .277 in the database, have enough at-bats to appear on a longer version of this list. Coverage of those seasons is partial, so a Negro leagues player's totals in the historical understate his career, and the rate statistics are less certain than the totals. For that reason, the list above should be read as the best that the database can tell us rather than a final ranking. The active players on it, including Judge and Ohtani, are counted through the 2025 season only.

The other side: high-average hitters with little power

ISO is most useful for showing what a hitter is not. Among players with at least 2,000 career hits in the database, the lowest career ISO belongs to Maury Wills at .050, with 2,134 hits and 20 home runs. Patsy Donovan, Larry Bowa and Juan Pierre are near him at the bottom. None of them was a bad player, and Wills stole 490 bases for the Dodgers. Their value came from average, speed and defense, none of which ISO measures.

That is the right way to use the stat. It is not a grade of overall quality. It tells you the shape of a player's offense. A .090 ISO hitter and a .250 ISO hitter might have the same OPS, and one would be a table-setter and the other a middle-of-the-order bat. If you want one number that combines everything, a park-adjusted measure such as wRC+ is the better tool. If you want to know why a hitter's batting average is high or low, BABIP is the stat to consult.

Home run rate and its variants

The simplest power statistic is the home run itself. A raw total is influenced by playing time, so rate versions are more useful. The common ones are home runs per at-bat or its reciprocal, at-bats per home run; home runs per plate appearance, which counts walks in the denominator and so makes a patient hitter's rate look slightly lower; and home runs per fly ball, which asks what share of a hitter's fly balls leave the park.

Home runs per fly ball is a clue to luck and park. If a hitter's home runs per fly ball jumps for a season, the cause may be strength, a change in the ball, a favorable park or a few lucky gusts of wind. A common check is whether the rate is far above the hitter's norm, because an outlier tends to fall back toward it. The league-wide version is useful as well. The at-bats-per-home-run column in the table above shows how much the sport changed: 302 at-bats per home run in 1908 and 25 in 2019.

Statcast and the batted-ball measures

Statcast, which has tracked batted balls in every major-league park since 2015, added measurements that earlier decades could not make. The central ones are these.

• Exit velocity: the speed of the ball as it leaves the bat, in miles per hour. Harder-hit balls fall for hits more often and travel farther.
• Launch angle: the vertical angle at which the ball leaves the bat. Ground balls have low or negative angles, and fly balls high ones.
• Barrel: a batted ball with the combination of exit velocity and launch angle that, historically, has produced a very high batting average and slugging percentage. The definition begins at 98 miles per hour with a launch angle of roughly 26 to 30 degrees, and the range of acceptable angles widens as the speed rises.
• Hard-hit rate: the share of batted balls hit at 95 miles per hour or more.
• Sweet spot: a launch angle between 8 and 32 degrees, the range in which line drives and productive fly balls live.
• Expected statistics: versions of batting average, slugging and weighted on-base average that estimate what a batted ball would typically produce given its exit velocity and angle, and in some versions the batter's speed.

These measurements matter because they separate what the hitter did from what the fielders and parks did with it. A hitter who barrels many balls but has a low home run total may have been unlucky, or may be playing in a park that suppresses fly balls. An expected slugging number higher than the actual figure is a candidate to improve; a much lower one is a candidate to decline. This does not make the prediction certain.

The historical record has none of this. It has no Statcast measurements, no exit velocities and no launch angles, which is why this article's historical comparisons use ISO and rate stats and its modern discussion is descriptive. When you read a barrel rate, remember that it exists only since 2015 and that the leagues' own definitions can be revised.

Park, ball and context

All of these measures are sensitive to environment. Altitude and air density affect how far a ball travels; fences vary in distance and height; and the manufacturing of the baseball has changed over time, as the debate about the live-ball era shows. A hitter in an extreme hitter's park will post a higher ISO than the same player in a pitcher's park, so comparisons across clubs call for park factors. Adjusted numbers solve the problem by comparing a hitter with his league and park. Unadjusted ISO is a description, not a verdict.

Using power numbers in fantasy and scouting

In fantasy baseball, ISO and barrel rate are useful as warning lights. A hitter with a good batting average and a very low ISO may be a bad bet for home runs. A hitter with a high ISO and a high strikeout rate will hurt you in average. The combination to look for is power with plate discipline, which you can check against the pair of numbers in strikeout and walk rates. Injuries also matter: a hitter returning from a wrist or hand injury is often reported to show less power for a time, which is one reason to read managing injuries in fantasy baseball alongside the numbers.

Scouts use the same ideas in other words. Raw power is how far a hitter can drive the ball in batting practice; game power is how much of that shows up in games. ISO is a way to see the game power in the record.

Common questions

Is ISO the same as slugging percentage? No. Slugging counts singles; ISO removes them. Slugging is a measure of total production on the bases, and ISO is a measure of extra-base production.

What is a good ISO? Compare with the league. The league figure in the combined American and National Leagues was .159 in 2025, so a hitter well above that is a power hitter, and one near .200 is excellent.

Does ISO include walks? No. It divides by at-bats, so walks do not appear in either part.

Can a speedy hitter have a high ISO? Yes, because doubles and triples count. A fast hitter in a big ballpark can pile up extra-base hits without hitting many home runs.

How are barrels different from home runs? A barrel describes the quality of contact, whether or not the ball clears the fence. A home run is a result. Barrels help predict results, but they are not the same thing.

Where to go next

You can calculate ISO yourself with the site's stat calculator, and see how the league's best hitters rank on the leaders pages. For more on how statistics describe players, the stats and analytics topic page collects further reading. Totals markets in betting depend partly on run environments like these; the article on moneyline, run line and totals explains the vocabulary, and anyone considering a wager should know that gambling carries risk, is for adults of legal age where it is legal, and has a US help line at 1-800-GAMBLER.