Baseball ISO Calculator

ISO is power measured in extra bases per at bat

ISO (Isolated Power) is slugging percentage minus batting average. A hit already earns one base in batting average, so ISO counts only the bases beyond that: one for a double, two for a triple and three for a home run, divided by at bats. In other words, it shows how many extra bases a hitter collects per at bat, which separates raw power from the ability to put the ball in play.

Take the 2026 MLB ISO leader, Pete Crow-Armstrong (CHC): 626 AB, 175 H, 31 2B, 8 3B and 45 HR. His extra bases are 182 (31 + 2 × 8 + 3 × 45), and 182 ÷ 626 gives an ISO of .291. That is the highest among the 135 qualified MLB hitters and 1.86 times the league average of .156. His AVG was .280 and his SLG .570. Steven Kwan (CLE) hit almost the same .282 but had an ISO of only .067.

Why look at ISO on its own

Batting average and slugging percentage blend hitting for contact and hitting for power into one number. ISO pulls out the power half, which helps in situations like these.

• Splitting two hitters with the same average: In 2026 MLB, Crow-Armstrong (.280) and Kwan (.282) are almost level in AVG, yet their ISO is .291 and .067, which ranks 1st and 134th among 135 qualified hitters. The average hides that one of them supplies the extra bases and the other mostly singles.
• Same slugging, different hitters: Cam Smith (HOU) and Nick Gonzales (PIT) slug .390 and .388. Smith hit .210 with an ISO of .180, Gonzales hit .303 with an ISO of .085. In KBO, Kim Jae Hwan (SSG) and Kim Min Suk (Doosan) both slug .404, with ISO of .186 and .095. Slugging contains batting average, so a singles hitter can post a high SLG.
• Lineup roles: In amateur or fantasy leagues, a spot that needs on-base skill at the top and a spot that needs extra-base hits to drive in runners can be told apart by where a hitter sits on the AVG × ISO map.
• Tracking changes in power: Comparing a hitter's ISO from season to season shows a change in power apart from a change in average. FanGraphs suggests roughly 550 plate appearances before drawing conclusions from ISO.

How to use the calculator

1. Pick a league to compare with (MLB, KBO or NPB). The league average and the dots on the map (2026 regular season, qualified hitters) switch to that league.
2. Enter at bats, hits, doubles, triples and home runs. Hits means all hits, extra-base hits included. Walks, hit by pitches and sacrifices are not at bats, so leave them out. Blank fields count as 0. You can copy the AB, H, 2B, 3B and HR columns straight from MLB.com, Baseball-Reference or the KBO records page.
3. Read the ISO, the FanGraphs tier and the gap to the league average. On the ISO diamond, the thickness of each segment shows whether the extra bases came from doubles or from home runs.
4. On the AVG × ISO map, check the hitter type quadrant, the position among qualified hitters and the ISO of hitters with a similar average. Hover over a dot to see the player and the line.
5. The example buttons load the ISO leader, a contact hitter and a power hitter for each league. Copy result and Copy link let you reopen the same calculation later.

How ISO is calculated

1. Extra bases = 2B + 2 × 3B + 3 × HR. These are the bases left after removing the first base each hit already earns.
2. ISO = extra bases ÷ AB.
3. The same value comes from SLG − AVG, because SLG = total bases ÷ AB and total bases = hits + extra bases.
4. The result is rounded to three decimals, like baseball stats, and shown as .291.

The ISO diamond in the calculator draws this formula. All hits pass from home to first, doubles or better pass from first to second, triples or better from second to third, and only home runs from third to home. Dividing each count by at bats gives the table below. The last three segments add up to ISO, and all four add up to slugging. For Crow-Armstrong:

One rounding detail: subtracting the rounded figures gives .570 − .280 = .290, which is .001 off the .291 from the exact counts. When the two methods differ, the calculator marks the formula line with "≈".

League averages and qualified hitters (2026 regular season)

A league average is computed by adding up the team totals of the whole league. In 2026 the Central League of NPB has pitchers batting, so Central League pitchers' batting is removed from the NPB figures.

The KBO league ISO rose by .009, from .127 in 2025 to .136 in 2026, while MLB slipped from .159 to .156. In MLB and KBO the median of the qualified hitters sits above the league average, because players who start all season tend to have above-average power as well.

What counts as a good ISO

This is the rough scale FanGraphs gives, with the number of 2026 qualified hitters in each tier. The calculator assigns the highest tier whose value the ISO reaches. For example .199 is Above Average, .200 is Great, and anything under .100 counts as Awful.

FanGraphs notes that the scale is approximate and that league ISO changes from year to year (the article dates from 2010). The 2026 MLB average of .156 is indeed above the scale's "Average" of .140. Use the table as a rough ruler and look at the gap to the same season's league average as well. In KBO, 15 qualified hitters sit under .100, so many regulars there reach base with hits rather than extra bases.

For an extreme example, Barry Bonds (SF) in 2001 hit 73 home runs in 476 at bats for 411 total bases, an ISO of .536. That is about 1.8 times the 2026 MLB leader's .291.

Same average, same slugging, different ISO

These pairs come from the 2026 numbers: two hitters with nearly the same batting average, and two with nearly the same slugging percentage. On the map they are dots on the same vertical line (AVG) or the same diagonal (SLG) but far apart in height.

Kim Jae Hwan and Kim Min Suk have the same slugging, so SLG alone makes them look alike. But .310 of Kim Min Suk's .404 comes from hits themselves, while Kim Jae Hwan had fewer hits and still turned 34 of his 97 into extra-base hits. To compare power, use ISO; to compare the ability to get hits, look at batting average separately.

ISO and other hitting stats

Common mistakes

• Multiplying home runs by 4: Counting a home run as four bases, as in slugging, inflates ISO by home runs ÷ AB. For Crow-Armstrong that gives (182 + 45) ÷ 626 = .363, which is .072 above the correct .291.
• Dividing by plate appearances: Dividing by PA, which includes walks, hit by pitches and sacrifices, makes ISO look smaller. Using his 726 PA gives .251, not the .291 you get from at bats.
• Entering only singles as hits: ISO itself stays at extra bases ÷ AB whatever the hit total, but AVG and SLG come out wrong. Enter all hits, including doubles, triples and home runs.
• Subtracting rounded figures: Subtracting two values rounded to three decimals can be off by .001. The calculator shows "≈" on the formula line when the two methods differ.
• Judging a short stretch: FanGraphs points out that a .550 ISO after two weeks of a season is too early to expect to continue. Wait for something like 550 plate appearances before drawing a conclusion.
• Comparing ISO across leagues and seasons as is: Even in 2026 the league averages differ: MLB .156, KBO .136 and NPB .126. Compare with the gap to the league average or the multiple of it.

Limitations

• ISO is not adjusted for ballpark or league environment. The same ISO is worth more when it was earned in a pitcher-friendly park.
• It does not give doubles and home runs separate run values the way wOBA does, and it ignores situations such as runners, outs and score.
• Walks and hit by pitches, strikeouts and baserunning are not part of ISO. Look at OBP, wOBA and OPS+ together for a full picture of a hitter.
• The tiers are the FanGraphs rough scale based on MLB. For KBO and NPB, look first at the league average and the rank among qualified hitters.
• The "To the next tier" line is the minimum count assuming a home run in each of the next at bats. It is not a projection of future performance.
• The NPB 2026 league totals were rebuilt because the official site (npb.jp) could not be reached: team player lines from Sportsnavi plus players who changed teams in midseason. Compared with the pitcher totals, the rebuild has 31 fewer plate appearances and 6 fewer hits, with the same home runs. No NPB 2025 average is shown because the Central League pitcher batting total could not be found.

As of The formulas and the rough scale were checked against the FanGraphs Library and the official MLB Glossary on 2026-10-23. League totals and qualified hitters are calculated from the final 2026 regular season figures downloaded on 2026-10-22 from the KBO records page, the MLB Stats API and NPB records.

Sources

• 01FanGraphs Library — ISOISO = SLG − AVG = (2B + 2×3B + 3×HR) ÷ AB, rough scale (.250 Excellent to .080 Awful), 550 PA suggested, no park or league adjustment
• 02MLB Glossary — Isolated Power (ISO)Measures raw power by counting extra bases only, (1×2B + 2×3B + 3×HR) ÷ AB or SLG − AVG
• 03KBO official records (Korean) — team hitting2025 and 2026 regular season totals of AB, H, 2B, 3B and HR for the 10 teams
• 04KBO official records (Korean) — player hitting49 hitters at or above the 2026 qualifying 446 PA
• 05MLB Stats API — team hitting2025 and 2026 regular season totals for the 30 teams
• 06MLB Stats API — 2026 qualified hitters135 hitters at or above the qualifying 502 PA
• 07NPB — 2026 Central League qualified hitters (archived)45 qualified hitters together with the Pacific League table (bat_p), archived 2026-10-08
• 08Sportsnavi (Yahoo! JAPAN) — NPB team battingLeague totals rebuilt from the player lines of the 12 NPB teams in 2026
• 09MLB Stats API — Barry Bonds year by year2001: 476 AB, 156 H, 32 2B, 2 3B, 73 HR, 411 total bases

Frequently asked questions

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