Slugging percentage (SLG) is total bases divided by at bats — a single counts once, a double twice, a triple three times, a home run four — per the MLB glossary definition. Despite the name, it isn't a percentage at all: it's average bases per at bat, with a theoretical maximum of 4.000.
SLG = (1B + 2×2B + 3×3B + 4×HR) ÷ AB
A worked example
A hitter's season line: 165 hits — 35 doubles, 3 triples, 27 home runs — over 550 at bats.
Stat sheets list hits, not singles, so the first step is always the subtraction: singles = 165 − 35 − 3 − 27 = 100. Then:
Total bases = 100 + (2 × 35) + (3 × 3) + (4 × 27)
= 100 + 70 + 9 + 108 = 287
SLG = 287 ÷ 550 ≈ .522
Against the .400–.420 league-average range of recent MLB seasons, .522 is legitimate power: .450+ is solidly above average, .500+ marks a real power threat, and .550+ over a full season is elite. The Slugging Percentage Calculator runs this arithmetic — including a total-bases readout — from the four hit types and at bats.
What the denominator leaves out
SLG divides by at bats, not plate appearances. Walks, hit-by-pitches, sacrifice flies, and sacrifice bunts simply don't exist in this stat — a hitter who walks 100 times has the same SLG denominator treatment as one who never walks. That's a feature, not an oversight: SLG is trying to isolate what happens when the bat decides the outcome. The reaching-base skill lives in on-base percentage, and the two stats are designed to be complementary halves — which is exactly why they're summed into on-base plus slugging (OPS).
Why "percentage" is a misnomer
A percentage can't exceed 100; SLG tops out at 4.000, the mark of a hitter who homers in every at bat. Real single-game lines break the percentage illusion constantly — go 2-for-4 with two home runs and you slugged 2.000 for the day. The name stuck from an era when baseball called every rate stat a percentage (fielding percentage has the same problem in reverse — it genuinely is one).
The .522 in the example above reads: this hitter averaged a bit more than half a base per at bat.
SLG's blind spot: it can't tell power from volume
Two hitters can share a .450 SLG while being completely different players — one stacking 180 singles-heavy hits, the other hitting 40 home runs with a low batting average. SLG rewards both paths equally because singles still count.
Isolated power (ISO) strips the ambiguity: ISO = SLG − AVG, which zeroes out every single and counts only extra bases. The .522 slugger above hits .300 (165 ÷ 550), so his ISO is .222 — comfortably power-hitter territory (league-average ISO typically runs in the .140–.160 range). When the question is specifically "how much power," ISO answers it; when the question is "how much damage per at bat," SLG does. And when the question is the hitter's whole offensive profile, the OBP/SLG split — explored in the OPS lineup-construction guide — matters more than either number alone.