Updated September 2026 Everything that is not fat — muscle, bone, organs and water — by three published formulas
What lean body mass is, and why it matters
Lean body mass is everything in your body that is not fat: skeletal muscle, bone, organs, connective tissue and the water they contain. It is the tissue that does the metabolic work, which makes it the number behind several other calculations.
- It drives your resting metabolism. Equations like Katch-McArdle and Cunningham predict BMR from lean mass alone, and for lean or muscular people they outperform height-and-weight equations.
- It is the better basis for protein. Fat tissue has almost no protein requirement, so at higher body fat, setting protein per kilogram of lean mass beats setting it per kilogram of bodyweight.
- It is what you are trying to keep while dieting. A 10 kg weight loss of which 2 kg is lean mass is a considerably worse outcome than one of which 0.5 kg is, at the same scale reading.
The three formulas
All three predict lean mass from height, weight and sex. W is weight in kilograms, H is height in centimetres.
Men: LBM = (0.407 × W) + (0.267 × H) − 19.2
Women: LBM = (0.252 × W) + (0.473 × H) − 48.3
Men: LBM = (1.1 × W) − 128 × (W ÷ H)²
Women: LBM = (1.07 × W) − 148 × (W ÷ H)²
Men: LBM = (0.32810 × W) + (0.33929 × H) − 29.5336
Women: LBM = (0.29569 × W) + (0.41813 × H) − 43.2933
Boer is the most widely used in clinical settings and is a reasonable default. James behaves poorly at higher body weights — its squared term causes the estimate to flatten and eventually fall as weight rises, which is a known limitation rather than a bug in our implementation. Hume sits between the two.
The better method, if you have a body fat percentage
Every formula above is inferring composition from height and weight, which is exactly the information that cannot distinguish muscle from fat. If you have a body fat estimate, use it directly:
A 90 kg person at 20 percent body fat has 72 kg of lean mass. No regression required. Estimate your body fat with our body fat calculator, and read how the measurement methods compare for how much to trust the input.
Fat-free mass index (FFMI)
FFMI is lean mass divided by height squared — BMI for the tissue that matters. It answers a question raw lean mass cannot: is this a lot of muscle for someone of your height?
Normalised FFMI = FFMI + 6.1 × (1.8 − height in m)
| Normalised FFMI | Men | Women |
|---|---|---|
| Below average | Under 18 | Under 14 |
| Average | 18–20 | 14–16 |
| Above average | 20–22 | 16–18 |
| Well trained | 22–24 | 18–20 |
| Exceptional | 24 and above | 20 and above |
The normalisation term adjusts for the fact that taller people have a mathematical advantage in any index divided by height squared. A frequently cited figure puts the natural ceiling for men at around 25, based on analysis of pre-steroid-era bodybuilders — treat that as a rough guide rather than a hard limit, since it rests on a small and unusual sample.
A worked example
A man of 180 cm and 85 kg at 15 percent body fat.
Boer: (0.407 × 85) + (0.267 × 180) − 19.2 = 63.6 kg
FFMI = 72.3 ÷ 1.8² = 22.3 — well trained
Note the 8.7 kg gap between the measured-composition figure and the Boer estimate. That gap is the muscle this man carries above what height and weight alone would predict — and it is the clearest illustration of why a body fat input beats a regression whenever you have one.
What to do with the number
Set protein from it — 2.0 to 2.5 g per kilogram of lean mass is a well-supported range, and our protein target table covers the alternatives. Use it in a lean-mass BMR equation if you are unusually lean or muscular; the BMR formula comparison shows how much difference that makes. And track it across a diet: lean mass holding steady while weight falls is the definition of a cut going well.
Lean body mass calculator — FAQ
If you have a body fat percentage, none of them — use weight multiplied by (1 minus body fat) instead, which measures rather than infers. Among the regressions, Boer is the most widely used clinically and a reasonable default; James is known to behave poorly at higher body weights.
For men, roughly 18 to 20 is average, 20 to 22 above average, and 22 to 24 well trained. For women the corresponding bands are about 14 to 16, 16 to 18 and 18 to 20. Use the normalised figure, which adjusts for the advantage taller people get from any height-squared index.
No. Lean body mass includes bone, organs, connective tissue and water as well as skeletal muscle — muscle is roughly half of it. Muscle mass specifically requires imaging such as DEXA or MRI to measure.
Each was fitted against a different reference population using different methods and decades apart. They are all inferring composition from height and weight, which is precisely the information that cannot see body composition — so a several-kilogram spread is expected.
Bodyweight works fine for most people and is what the studies used. Switch to lean mass if you carry a lot of body fat, since fat tissue has little protein requirement — 2.0 to 2.5 g per kilogram of lean mass is the usual range.