Lean Body Mass Calculator: Boer, James and Hume

How to use this lean body mass calculator

Choose Metric (kilograms and centimetres) or Imperial (pounds, feet and inches).
Pick man or woman, and enter your weight and height.
Read the large figure: the Boer estimate of your lean body mass, with the James and Hume estimates alongside.
Use the spread between the three as your honest range — a DEXA scan is the way to measure it directly.

What lean body mass actually is

Lean body mass is your total weight minus your fat mass. What remains is the working machinery: skeletal muscle, bones, organs, blood, skin and body water. It is the part of you that burns energy at rest, carries you around and recovers from training — which is why two people at the same weight can look and perform very differently.

This page estimates it from height and weight alone, using the three formulas hospitals and researchers have used for decades: Boer (1984), James (1976) and Hume (1966). Boer is the headline because it tracks measured values best in modern validation studies; the other two are shown so you can see the honest spread instead of a false single truth.

The formulas

Boer (1984)

Men: 0.407 × kg + 0.267 × cm − 19.2 · Women: 0.252 × kg + 0.473 × cm − 48.3

The headline estimate, and the best-validated of the three against measured lean mass.

James (1976)

Men: 1.1 × kg − 128 × (kg ÷ cm)² · Women: 1.07 × kg − 148 × (kg ÷ cm)²

Built from body-density studies. It tends to read high in women and low at very high weights.

Hume (1966)

Men: 0.328 × kg + 0.339 × cm − 29.53 · Women: 0.296 × kg + 0.418 × cm − 43.29

The oldest of the three, derived from total body water measurements.

Examples

Person	Weight	Height	Lean mass (Boer)	Lean share
Man, 80 kg, 178 cm	80 kg	178 cm	60.9 kg	76%
Woman, 65 kg, 165 cm	65 kg	165 cm	46.1 kg	71%
Man, 95 kg, 185 cm	95 kg	185 cm	68.9 kg	72%

Three mistakes people make with lean body mass

Treating an estimate as a measurement

Height-and-weight formulas can miss your real lean mass by several kilos, especially if you are very muscular or carry weight unusually. Use them for trends and comparisons, not verdicts.

Confusing lean mass with muscle mass

Muscle is only part of lean mass. Bones, organs and water together weigh far more than most people expect, so a lean mass of 60 kg does not mean 60 kg of muscle.

Chasing the scale instead of the share

Losing 5 kg of fat while keeping lean mass raises your lean share and changes how you look far more than losing 5 kg of mixed tissue. The percentage matters as much as the kilos.

Frequently asked questions

What counts as lean body mass?

Everything that is not fat: skeletal muscle, bones, organs, blood, skin and body water. If you know your weight and your body fat percentage, lean mass is simply the weight that remains.

Which of the three formulas is most accurate?

Boer (1984) matches measured lean mass best in modern validation work, which is why it is the headline here. James and Hume are shown alongside so you can see the realistic spread — a few kilos either way is normal for formula estimates.

Is lean body mass the same as muscle mass?

No. Muscle is the largest single part of lean mass, but bones, organs and water are in there too. A lean mass of 60 kg might contain roughly 25 to 30 kg of skeletal muscle.

Why do the formulas disagree with each other?

Each was fitted to a different group of people measured a different way — body water for Hume, body density for James, a large mixed dataset for Boer. Different reference groups produce slightly different equations for the same body.

How can I increase my lean body mass?

Progressive resistance training plus enough protein and sleep is the reliable route; this site's protein calculator gives the daily target most guidelines use. Gains are slow — a few kilos of lean mass in a year is a strong result for a beginner.

Is lean body mass the same as fat-free mass?

Almost. Strictly, fat-free mass excludes even the essential fat your body must keep, while lean body mass includes it — so lean body mass reads slightly higher. In everyday use, including most calculators, the two terms are used interchangeably.

Privacy and important information

This calculator runs entirely in your browser: your numbers are never sent to a server, stored, or shared. Lean body mass from a formula is an estimate for tracking and comparison — if you need a clinical value, a DEXA scan or a clinician's assessment is the right tool.

Part of RahaKit's free health calculators.

Lean Body Mass Calculator