Estimate Fat-Free Mass Index from height, weight and body-fat percentage, with an optional height-normalized FFMI value.
FFMI is a body-composition index, not a diagnosis. Consumer body-fat measurements can have meaningful error, so small changes in the result may reflect measurement error rather than a real change in lean mass.
FFMI (Fat-Free Mass Index) answers a question BMI cannot: how much of your body is lean tissue, adjusted for your height? While BMI divides total weight by height squared, FFMI first strips out body fat, then divides only the fat-free mass by height squared. Two people can share a BMI of 28 yet have very different FFMIs — one muscular, one carrying more body fat — and this calculator tells them apart.
The calculator uses three steps:
Weight: 150 lb = 68.0 kg. Fat-free mass: 68.0 × (1 − 0.25) = 68.0 × 0.75 = 51.0 kg (112.5 lb). Height: 5 ft 4 in = 1.63 m; height squared = 2.64. FFMI: 51.0 ÷ 2.64 = 19.31. Height-normalized FFMI: 19.31 + 6.3 × (1.80 − 1.63) = 19.31 + 6.3 × 0.17 = 19.31 + 1.10 = 20.41. The normalization lifts the score by more than a full point here, because at 5 ft 4 in the raw FFMI understates lean mass relative to the 5 ft 11 in reference height.
The ranges below are the ones most commonly cited from the body-composition literature (Kouri et al., 1995). They describe trained men; women typically score lower at the same training level because essential body fat is higher (roughly 10–13% for women vs. 2–5% for men), so interpret any score in the context of sex, training history, and how body fat was measured.
| FFMI | Common interpretation |
|---|---|
| Below 18 | Below average — typical of untrained adults |
| 18–20 | Average — normal for active adults |
| 20–22 | Above average — regular strength training shows here |
| 22–23 | Excellent — well-developed musculature |
| 23–25 | Superior — years of dedicated lifting |
| Above 25 | Uncommon — often discussed alongside the debated “natural limit” idea |
One honest caveat about the “25 natural limit”: it comes from a small 1995 study and is debated, not settled science. FFMI alone cannot determine how anyone built their muscle, and it should never be used to accuse or diagnose. Treat scores above 25 as “rare and worth double-checking the body-fat input” rather than as proof of anything.
FFMI rises when fat-free mass rises relative to height, so the levers are the familiar ones: progressive strength training (gradually heavier weights over months and years), enough protein (sports-nutrition guidance commonly cites 1.6–2.2 g per kg of body weight per day for people building muscle), and patience — lean mass accrues slowly, on the order of a few pounds per year for experienced lifters. Cutting body fat also raises FFMI slightly by improving the denominator of the body-fat equation, but the durable way up is more muscle, not just less fat.
The weakest link in any FFMI number is the body-fat estimate you enter. Smart scales and handheld devices can be off by several percentage points; skinfold calipers depend on the tester; DEXA is the most consistent but costs more. Whatever method you use, re-measure with the same method, at the same time of day, every 8–12 weeks — the trend matters more than any single reading. If you want your own body-fat figure first, try our body fat calculator, compare against BMI, and plan protein with the protein calculator.
Further reading: the original FFMI study (Kouri et al., 1995) is on PubMed. This page is for general information only and is not medical advice.
Maintained by Live Fitness Check. Last updated: August 13, 2026. How we test our calculators .
FFMI relates estimated fat-free mass to height, providing a way to compare lean mass across people of different heights.
Body-fat percentage is used to estimate fat-free mass before dividing that mass by height squared.
It is a height-adjusted version of FFMI using the correction described in the Kouri study to normalize values toward a height of 1.80 meters.
