What lean body mass is
Lean body mass (LBM) is total weight minus fat mass: muscle, bone, organs, skin, blood and water. It is the metabolically active part of the body, which is why the most accurate BMR formula (Katch-McArdle) uses it, why protein recommendations for athletes are sometimes given per kilogram of lean mass, and why some medications are dosed on it. For a healthy adult it is typically 70–90% of body weight; the rest is essential and storage fat.
Without a body-composition measurement, LBM is estimated from height, weight and sex. The three formulas below give the example man's lean mass (5′10″, 170 lb).
| Formula | Lean body mass | Implied body fat |
|---|---|---|
| Boer (1984) | 131.5 lb | 23% |
| James (1976) | 133.9 lb | 21% |
| Hume (1966) | 123.7 lb | 27% |
From a body fat percentage
If you have a body fat measurement — from a DEXA scan, hydrostatic weighing, calipers or a bioimpedance scale — lean mass is simply weight × (1 − body fat). This is far more accurate than any formula, because the formulas assume an average body composition for your height and weight and cannot tell a muscular person from an overweight one. Enter the percentage in the optional field and the calculator uses it.
| Body fat | Lean mass | Fat mass |
|---|---|---|
| 8% | 156 lb | 14 lb |
| 12% | 150 lb | 20 lb |
| 15% | 144 lb | 26 lb |
| 20% | 136 lb | 34 lb |
| 25% | 128 lb | 42 lb |
| 30% | 119 lb | 51 lb |
| 35% | 110 lb | 60 lb |
Lean mass by weight
The formulas attribute most extra weight to fat: in Boer's equation each extra kilogram of body weight adds only 0.41 kg (men) or 0.25 kg (women) of lean mass. That is a reasonable population average — heavier people do carry more lean tissue, but proportionally less — and it is exactly why the formulas misjudge athletes.
| Weight | Men: LBM | Body fat | Women: LBM | Body fat |
|---|---|---|---|---|
| 130 lb | 115 lb | 11% | 96 lb | 26% |
| 150 lb | 123 lb | 18% | 101 lb | 33% |
| 170 lb | 132 lb | 23% | 106 lb | 38% |
| 190 lb | 140 lb | 26% | 111 lb | 42% |
| 220 lb | 152 lb | 31% | 118 lb | 46% |
What to do with the number
Lean mass is the input to several other calculations, and the figure to track when the goal is body recomposition rather than weight loss alone.
| Use | Rule | Example |
|---|---|---|
| Katch-McArdle BMR | 370 + 21.6 × LBM (kg) | 1,659 kcal for the example |
| Protein target | 1.8–2.2 g per kg of LBM when dieting | 107–131 g |
| Drug dosing | Some anaesthetics and chemotherapy dose on LBM | Clinical use |
| Progress tracking | LBM rising while fat mass falls = recomposition | Needs a body fat measurement |
How the formulas differ
Boer (1984) was derived for drug-dosing in adults and is the most widely used clinically. James (1976) was built into many early clinical calculators but behaves poorly at very high weights, where its quadratic term can even make LBM fall as weight rises. Hume (1966) is close to Boer for average builds. The calculator reports all three and uses Boer as the headline when no body fat is entered.
- Boer (men): 0.407 W + 0.267 H − 19.2; (women): 0.252 W + 0.473 H − 48.3 (kg, cm).
- James (men): 1.1 W − 128 (W/H)²; (women): 1.07 W − 148 (W/H)².
- Hume (men): 0.3281 W + 0.33929 H − 29.5336; (women): 0.29569 W + 0.41813 H − 43.2933.
- From body fat: LBM = W × (1 − BF%).
Limits
Formula estimates are population averages accurate to perhaps ±5 kg for individuals; they are least reliable for very muscular, very lean or very heavy people and for older adults, who lose muscle relative to weight. Lean mass also includes water, which shifts by a kilogram or two with hydration, glycogen and sodium — so day-to-day changes in a scale's lean-mass readout are mostly water, not muscle.