Lean body mass is everything you are made of except fat, and the awkward truth about calculating it is that the standard formulas cannot see fat at all. Boer, James and Hume each take exactly three inputs: weight, height and sex. That means an elite athlete and a sedentary person of identical build...
These are estimates from height and weight, not measurements of your body. All three formulas see only those two numbers and your sex, which means an athlete and a sedentary person of identical build receive identical answers — the equations cannot see the very thing they are estimating. If a figure matters, a DEXA scan measures it. And although these equations are used clinically for drug dosing, the version here is a population estimate rather than a dosing calculation, which is a decision for whoever is prescribing.
WHICH FORMULA
LEAN BODY MASS, BOER 1984
59.8 kg
76.7% of your body weight, leaving 18.2kg the formula attributes to fat. The three formulas differ by 4.8kg on these same inputs.
LEAN
59.8kg
FAT
18.2kg
LEAN SHARE
76.7%
| Formula | Lean | Fat | Lean % |
|---|---|---|---|
| Boer 1984 | 59.8 kg | 18.2 kg | 76.7% |
| James 1976 | 60.9 kg | 17.1 kg | 78.1% |
| Hume 1966 | 56.1 kg | 21.9 kg | 71.9% |
Why they disagree, and what to do about it. Each was fitted to a different cohort using a different reference method — skinfolds, total body water, electrolyte dilution — so a few kilograms of spread is the normal state rather than a sign that one is broken. Boer is the usual clinical default. What matters more than the choice is consistency: switching formula midway through tracking manufactures a change that did not happen to you.
Lean body mass and fat-free mass are not quite the same thing. Fat-free mass excludes all fat. Lean body mass conventionally includes the essential fat in nerve tissue, bone marrow and organs — a few percent in men and rather more in women. The two terms are used interchangeably almost everywhere, including by calculators, and the difference is small enough not to matter for most purposes and real enough to be worth knowing when a source seems to disagree with another by a couple of kilograms.
THE FORMULAS
Two of these are straight lines and one is an arch. Boer and Hume rise steadily with weight. James is a downward parabola, so it climbs, flattens and then falls — at your height it turns over around 135kg. Past that point it reports less lean mass for more body weight, which is a property of the equation rather than of anybody.
Boer 1984 · James 1976 · Hume 1966 · live calculation
Enter weight, height and sex. That is genuinely all these formulas take, which is both why they are convenient and why they cannot distinguish two bodies of the same size and different composition.
Pick a formula, or leave it on Boer. The choice matters less than consistency — switching formula partway through tracking manufactures a change that did not happen to you.
If you have a measured body fat percentage from a DEXA scan, put it in. The page will then stop estimating and simply compute weight times one minus the fat fraction, which is better than anything the equations can offer.
Look at the spread between the three before treating any single figure as precise. Three to five kilograms of disagreement on the same body is the normal state.
Last updated: August 1, 2026 · Boer 1984, James 1976 and Hume 1966, each reproducing its published worked example · An estimate from height and weight, not a measurement of your body.