Isolated Power (ISO) and Extra-Base Rate Calculator
Isolated Power (ISO) and Extra-Base Rate Calculator
Isolated power is slugging percentage minus batting average, and that is exactly extra bases per at-bat. This page takes the four kinds of hit separately rather than a total-bases figure, so it can also give the extra-base hit share — which, as the slugging page says in its own worked example, no total-bases total can recover.
ISO, SLG, AVG and the extra-base share
Extra bases ÷ at-bats112 singles, 30 doubles, 3 triples and 25 home runs in 550 at-bats — the same 170 hits and 281 total bases as the slugging page’s own default line
One figure, three published forms, and the one thing slugging cannot tell you
ISO = (2B + 2×3B + 3×HR) ÷ AB
ISO = (total bases − hits) ÷ AB
- all three are one expression
- FanGraphs’ library prints all three — “ISO = SLG – AVG”, “ISO = ((2B) + (2*3B) + (3*HR)) / AB” and “ISO = Extra Bases / At-Bats”. Slugging percentage is total bases over at-bats and a batting average is hits over at-bats, so subtracting them leaves total bases minus hits over at-bats, which is the extra bases. The card computes the third form and prints the first beside it as a check
- the subtraction is legitimate and the addition is not
- both fractions are over the SAME denominator, the at-bats, so their difference is a rate in the same units as either. OPS adds slugging to on-base percentage, whose denominator is at-bats plus walks plus hit-by-pitches plus sacrifice flies, and that sum is not a rate at all. One of the two operations on these statistics is arithmetic and the other is not
- a double is one extra base, not two
- slugging percentage prices a double at two and a home run at four. Isolated power prices them at one and three, because the base every hit is worth has been removed. So a home run is three times a double here and twice a double there
- league average, the absent band, and the absent glossary
- FanGraphs puts league-average ISO at “around .140” — one publisher’s figure for an unstated set of seasons rather than a governing body’s threshold, so this card prints no verdict and no band. MLB’s own glossary entry for isolated power could not be read at all, because the fetcher refused it, so no governing-body wording for ISO is quoted anywhere here
- what ISO does not add, in its publisher’s words
- FanGraphs is candid that isolated power “doesn’t tell you anything you can’t learn from AVG and SLG together, but it saves you a step”. True of the headline, and not true of the extra-base share beneath it — which is why this page is about the decomposition rather than the single number
Worked example
112 singles, 30 doubles, 3 triples and 25 home runs in 550 at-bats — the same 170 hits and 281 total bases as the slugging page's own default line
Hits = 112 + 30 + 3 + 25 = 170, and total bases = 112 + 60 + 9 + 100 = 281
ISO = (281 − 170) ÷ 550 = 111 ÷ 550 = .202
The identity: slugging = 281 ÷ 550 = .511, batting average = 170 ÷ 550 = .309, and .511 − .309 = .202 exactly. Both fractions are over the same 550, which is what makes the subtraction legitimate and is what cannot be said of adding them
Extra-base hits = 30 + 3 + 25 = 58, which is 34.1% of the 170 hits. This is the figure total bases alone cannot give: 281 total bases from 170 hits is also what 133 singles and 37 home runs would produce, and that line has 37 extra-base hits for a share of 21.8% — with an identical .309 average, an identical .511 slugging and an identical .202 isolated power
Double every count, to 224 singles, 60 doubles, 6 triples, 50 home runs and 1,100 at-bats, and nothing moves: 222 ÷ 1,100 is .202 again. It is a rate
Move all 30 doubles into home runs and ISO RISES to 171 ÷ 550 = .311 while the batting average does not move at all — still 170 hits in 550 at-bats, still .309. That is the argument for the statistic in one line
Set every hit to 0 and ISO is .000 rather than nothing — a hitter with no hits took no extra bases. Set the at-bats to 0 and the page returns nothing instead
Enter 600 singles in 550 at-bats and the page refuses rather than printing a figure, because every hit is also an at-bat
Two hitters, identical on four statistics, different on the fifth
| Hitter A, the default line | Hitter B | Agree? | |
|---|---|---|---|
| Singles | 112 | 133 | no |
| Doubles | 30 | 0 | no |
| Triples | 3 | 0 | no |
| Home runs | 25 | 37 | no |
| Hits | 170 | 170 | yes |
| Total bases | 281 | 281 | yes |
| Batting average | .309 | .309 | yes |
| Slugging percentage | .511 | .511 | yes |
| Isolated power | .202 | .202 | yes |
| Extra-base hits as a share of hits | 34.1% | 21.8% | NO — and only the four hit counts can tell |
What each statistic prices each kind of hit at
| Single | Double | Triple | Home run | |
|---|---|---|---|---|
| Batting average | 1 | 1 | 1 | 1 |
| Slugging percentage | 1 | 2 | 3 | 4 |
| Isolated power | nothing | 1 | 2 | 3 |
| Extra-base hit share | nothing | 1 | 1 | 1 |
A subtraction that is allowed, and a share that needs four fields
Isolated power is the part of slugging percentage that a batting average cannot already see. Every hit is worth at least one base, so subtracting the batting average from the slugging percentage leaves only the bases beyond that first one, divided by at-bats. FanGraphs’ library prints the figure three ways — “ISO = SLG – AVG”, “ISO = ((2B) + (2*3B) + (3*HR)) / AB” and “ISO = Extra Bases / At-Bats” — and all three are the same expression rearranged, which the card above demonstrates by computing one of them and printing another beside it.
Rule 9.21 of Major League Baseball’s Official Baseball Rules, 2024 Edition, is the one place these percentages are defined by the body that runs the competition rather than by statistical custom. It sets out winning percentage at 9.21(a), batting average at (b), slugging percentage at (c), fielding average at (d), earned-run average at (e) and on-base percentage at (f). Isolated power is not among them. It is slugging percentage at (c) minus batting average at (b), so both of its ingredients are defined by the body that runs the competition and the figure itself is not — Major League Baseball publishes it in its statistics glossary, and that glossary entry could not be read for this page because the fetcher refused it, so no governing-body wording for isolated power is quoted here at all. The definition above is FanGraphs’ library’s, and it is labelled as such.
The subtraction is legitimate, and the addition that produces OPS is not. Slugging percentage and batting average are both over at-bats — 550 of them on the line above — so their difference is a rate in the same units as either, and .511 minus .309 really is .202 extra bases per at-bat. On-base percentage is over at-bats plus walks plus hit-by-pitches plus sacrifice flies, so OPS adds two fractions that do not share a denominator and the sum is not a rate at all. That flaw is worked in full on the batting average, OBP and SLG calculator and is not repeated here.
Why this page exists beside the slugging page. That page takes total bases as a single field, and its own worked example says the counts of doubles, triples and home runs cannot be recovered from it. That is exactly right, and it is why this one takes four hit counts. The consequence is in the table above: 170 hits and 281 total bases describe both a hitter with 30 doubles, 3 triples and 25 home runs and one with 37 home runs and nothing else, and those two share a batting average, a slugging percentage and an isolated power. What separates them is the extra-base hit share — 34.1% against 21.8% — which no page working from a total-bases figure can produce. ISO is a statement about a total, not about a hitter’s shape.
What isolated power does not do, in its own publisher’s words. FanGraphs says it “doesn’t tell you anything you can’t learn from AVG and SLG together, but it saves you a step”, and puts league-average ISO at “around .140” without naming the seasons that holds over. Hence no band here: a threshold drawn from one publisher’s unstated sample would be a distribution presented as a standard. Compare within one competition and season, and remember that a ballpark matters more for extra-base hits than for singles — the park-adjusted versions of all this need annual league tables and per-park factors, which is the wall described on the wOBA calculator and, built higher, on the WAR explainer. For the version that prices each hit by a published run value rather than by its bases, see that wOBA page; and for what happened to the balls this hitter put into the field rather than over the fence, the BABIP calculator, which removes home runs from both halves of its fraction for the opposite reason this page keeps them.
Frequently asked questions
How do you calculate isolated power?
Slugging percentage minus batting average, which is total bases minus hits over at-bats. 112 singles, 30 doubles, 3 triples and 25 home runs in 550 at-bats gives 281 total bases and 170 hits, so ISO is 111 ÷ 550 = .202. FanGraphs’ library prints all three forms as one expression.
Why can you subtract batting average from slugging percentage but not add on-base percentage to it?
Because slugging and batting average share a denominator — at-bats — so their difference is a rate in the same units. On-base percentage is over at-bats plus walks plus hit-by-pitches plus sacrifice flies, so OPS adds two fractions that do not share one and the sum is not a rate.
How is this different from the slugging percentage calculator?
That page takes total bases as one field, and says itself that the counts of doubles, triples and home runs cannot be recovered from it. This page takes the four hit types separately, so it also gives the extra-base hit share: two hitters with 170 hits and 281 total bases share an average, a slugging and an ISO and can have shares of 34.1% and 21.8%.
What is a good ISO?
No governing body publishes a threshold and none is invented here, so there is no band. FanGraphs puts league average at “around .140” without naming the seasons — an anchor, not a standard. Compare within one competition and season, since extra-base hits are more park-sensitive than singles.
Does isolated power tell me what kind of hitter someone is?
Less than it is usually taken to. It is extra bases per at-bat and nothing else, so a hitter with 30 doubles and 25 home runs and one with 37 home runs and no doubles have exactly the same ISO off the same 170 hits. The extra-base share on this card separates them; ISO does not.
Why does a home run count as three in ISO and four in slugging?
Because isolated power has removed the one base every hit is worth. A home run is four bases of which three are extra; a double is two of which one is extra. So a home run is four times a double in slugging and three times a double here, and the two order a doubles hitter against a home-run hitter differently.
Related calculators
References
- FanGraphs Library. ISO — gives isolated power three ways, “ISO = SLG – AVG”, “ISO = ((2B) + (2*3B) + (3*HR)) / AB” and “ISO = Extra Bases / At-Bats”, says “ISO tells you the average number of extra bases a player gets per at bat”, puts league average at “Around .140”, and is candid that it “doesn’t tell you anything you can’t learn from AVG and SLG together, but it saves you a step”. library.fangraphs.com (read 15 Oct 2026). MLB’s own glossary entry for isolated power COULD NOT BE READ — the fetcher refused mlb.com/glossary/advanced-stats/isolated-power with a permission error — so no governing-body wording for ISO is quoted anywhere here.
- Official Baseball Rules, Rule 9.21, Determining Percentage Records — (a) winning percentage; (b) batting average, hits ÷ times at bat; (c) slugging percentage, total bases on safe hits ÷ at-bats; (d) fielding average, putouts plus assists ÷ total chances (putouts, assists, errors); (e) earned-run average, earned runs × 9 ÷ innings pitched, with a Comment working 3 earned runs in 9 1/3 innings as 2.89; (f) on-base percentage, hits plus walks plus hit-by-pitches ÷ at-bats plus walks plus hit-by-pitches plus sacrifice flies. Read on baseballrulesacademy.com, 12 Oct 2026: the official 2024 Edition PDF was fetched whole and its extracted text of 109,942 characters stops inside Rule 5.09, while its own contents page lists Rule 9.21 on page 141. Every formula taken from it is corroborated against MLB’s glossary.
- Major League Baseball. Glossary: Slugging Percentage (SLG) — “the total number of bases a player records per at-bat”, as “(1B + 2Bx2 + 3Bx3 + HRx4)/AB”; it “deals only with hits and does not include walks and hit-by-pitches in its equation”. mlb.com (read 12 Oct 2026).
- Major League Baseball. Glossary: Batting Average (AVG) — “batting average is determined by dividing a player’s hits by his total at-bats”, “for a number between zero (shown as .000) and one (1.000)”; it “doesn’t take into account the number of times a batter reaches base via walks or hit-by-pitches” or “hit type”. mlb.com (read 12 Oct 2026).
- Office of the Commissioner of Baseball. Official Baseball Rules, 2024 Edition (ISBN 978-1-63727-386-9), fetched as a PDF 12 Oct 2026. Its extracted text is 109,942 characters and stops inside Rule 5.09, while its own contents page lists Rule 9.16 on page 132 and Rule 9.21 on page 141 — so the scoring chapter is cited below through a reproduction and MLB’s own glossary, not from this document.