Baseball Glossary › Isolated Power
Isolated Power (ISO)
Isolated power (ISO) measures a hitter's raw power by counting only extra bases. The quickest formula is slugging percentage minus batting average. A hitter who singles a lot but rarely hits for extra bases has a low ISO, while a home run hitter has a high one, even if his batting average is modest.
Formula
Worked example (illustrative numbers): in 500 at-bats a hitter has 130 hits, including 25 doubles, 2 triples and 20 home runs.
Extra bases = 25 + (2 x 2) + (3 x 20) = 89
ISO = 89 / 500 = .178
Check: AVG = 130 / 500 = .260; total bases = 83 singles + 50 + 6 + 80 = 219, SLG = .438; .438 - .260 = .178
Athletics example: Mark McGwire
Mark McGwire, who broke in with Oakland, hit 252 doubles, 6 triples and 583 home runs in 6,187 career at-bats, with a .263 average and a .588 slugging percentage. See the full career line on the Mark McGwire player page.
ISO = (252 + 2 x 6 + 3 x 583) / 6,187 = .325
Shortcut: .588 - .263 = .325
The two methods agree once rounding is taken into account. Career MLB totals from our player database (MLB Stats API).
What to Know
ISO vs SLG: slugging percentage still counts every single as one base, so a high-average singles hitter can post a respectable SLG. ISO removes the singles, leaving only the extra bases from doubles, triples and home runs. Subtract batting average from SLG and you have it.
FanGraphs publishes a rule-of-thumb scale, noting that league averages shift from year to year: about .140 is average, .170 above average, .200 great and .250 excellent, while .100 is poor and .080 awful.
ISO pairs naturally with OPS. Two hitters with the same OPS can get there in different ways, and ISO shows how much of it comes from power.
Frequently Asked Questions
What is ISO in baseball?
ISO stands for isolated power. It measures a hitter's raw power by counting only the extra bases from doubles, triples and home runs per at-bat. The shortcut is slugging percentage minus batting average.
How do you calculate isolated power?
Subtract batting average from slugging percentage. Or add doubles, twice the triples and three times the home runs, then divide by at-bats. Both methods give the same answer apart from rounding.
What is a good ISO in baseball?
FanGraphs' rule of thumb puts about .140 at average, .170 above average, .200 great and .250 excellent. League-wide power varies by season, so compare a hitter with his league in the same year.
What is the difference between ISO and slugging percentage?
Slugging percentage counts singles as one base each. ISO removes singles entirely, so it only reflects extra-base power. A singles hitter can have a decent slugging percentage but a low ISO.
Can isolated power be higher than batting average?
Yes. A power hitter with a modest average can have an ISO close to or above his batting average, because ISO is driven by home runs and doubles rather than by how often he gets a hit.
Does ISO include walks?
No. Like slugging percentage, isolated power is based only on hits and at-bats. Walks and hit-by-pitches are not at-bats and add no bases to the calculation.
Related Terms
Sources
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