What lean body mass tells you
Lean body mass (LBM) is the weight of everything except body fat: muscle, bone, organs, and the water they hold. It is a useful reference point because most fitness goals act on lean tissue and fat separately. Two people at the same body weight can carry very different amounts of muscle, and lean body mass is what separates them.
When you do not have a body fat measurement, LBM can be estimated from height, weight, and sex. Several researchers have published equations for this. This calculator runs the three that clinicians and dietitians reach for most.
The three formulas
Each formula was fitted to a different reference population, so they disagree by a small margin. The sources are Boer P (1984, American Journal of Physiology), Hume R (1966, Journal of Clinical Pathology), and James WPT (1976), the equation the Royal Marsden uses.
- Boer, men: LBM = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2; women: 0.252 × weight + 0.473 × height − 48.3
- Hume, men: LBM = 0.3281 × weight + 0.33929 × height − 29.5336; women: 0.29569 × weight + 0.41813 × height − 43.2933
- James, men: LBM = 1.1 × weight − 128 × (weight ÷ height)²; women: 1.07 × weight − 148 × (weight ÷ height)²
Worked example: an 80 kg man at 180 cm gets a Boer estimate of 0.407 × 80 + 0.267 × 180 − 19.2 = 61.4 kg, about 77 percent of his body weight. The Hume and James estimates for the same man land near 57.8 kg and 62.7 kg, so the three span roughly 58 to 63 kg. That gap is the honest margin of a height-and-weight estimate.
Reading your result
The headline number is the Boer estimate, with the Hume and James figures beside it and the share of your body weight that is lean. Watch the trend rather than a single reading. If you lose fat while keeping the same training and protein, total weight drops but lean mass should hold, so its percentage rises.
| Source | Year | Notes |
|---|---|---|
| Boer | 1984 | Common default, sits mid-range for most adults |
| Hume | 1966 | Often reads lowest of the three |
| James | 1976 | Used by the Royal Marsden for dosing |
Where lean mass fits your goals
Lean body mass gives you a base for two decisions. First, protein: many guidelines set a daily target per kilogram of lean mass, and the protein calculator turns that into grams. Second, a target weight: the ideal weight calculator gives a height-based range you can pair with your lean figure. If you also want a height-adjusted muscularity score, the FFMI calculator builds on the same idea.
