Lean body mass calculator

Body weight is two very different tissues added together. Splitting them apart changes what the number means.

What lean body mass includes

Lean body mass is everything that is not fat: muscle, bone, organs, skin, connective tissue and the water in all of them. It is not the same as muscle mass — skeletal muscle is usually a little under half of LBM in an adult. Some texts distinguish LBM from fat-free mass, since LBM technically includes the small amount of essential fat inside organs and cell membranes, but in practice the two terms are used interchangeably.

The three formulas

Boer, James and Hume all estimate LBM from height, weight and sex alone. Boer (1984) is the most widely used and is the default here. James (1976) tends to read lower at higher body weights. Hume (1966) is the oldest and sits between them for most inputs.

All three share the same fundamental weakness: with only height and weight to work from, they cannot distinguish a muscular person from a heavier, less muscular one of the same dimensions. Two people at 180 cm and 80 kg get identical answers even if one is a rugby player and the other has never trained.

If you have a reliable body fat measurement — DEXA, a good calliper reading, or hydrostatic weighing — the fourth option uses it directly, and that will always beat an estimate from height and weight.

Why it is worth knowing

Protein targets scale with lean mass rather than total weight. Common guidance of 1.6-2.2 g per kg is more sensibly applied to LBM, especially at higher body fat, where scaling to total weight overshoots substantially.
Drug dosing for several anaesthetics and chemotherapy agents uses LBM, because those drugs distribute into lean tissue rather than fat.
Tracking a diet is far more informative in these terms. Losing 5 kg means one thing if lean mass held steady and something quite different if half of it was muscle.
Metabolic rate correlates better with lean mass than with body weight, which is why the Katch-McArdle BMR equation uses LBM directly.

Limits worth respecting

These equations were fitted on general adult populations and are unreliable at the extremes — very lean athletes, people with obesity, and anyone with significant oedema or muscle wasting. They are not validated for children or adolescents. Pregnancy makes them meaningless.

For tracking change over time, keep the same formula and the same scale, weigh at the same time of day, and look at the trend rather than any single reading. Day-to-day fluctuation in body water easily swamps a real week of change.

Common questions

Is lean body mass the same as muscle mass?

No. Lean body mass includes muscle, bone, organs, skin and body water. Skeletal muscle is typically a little under half of it, so a lean mass of 60 kg does not mean 60 kg of muscle.

Which formula is most accurate?

Boer is the most widely used and is a reasonable default. But all three work from height and weight alone, so none can tell a muscular person from a less muscular one of the same size. A real body fat measurement beats any of them.

Should I set protein intake from lean mass or total weight?

Lean mass is the better basis, particularly at higher body fat percentages. Scaling 2 g per kg to total body weight can overshoot considerably for someone carrying a lot of fat, since fat tissue has little protein requirement.

How do I measure body fat accurately?

DEXA is the practical gold standard for most people, followed by hydrostatic weighing and air displacement. Bioimpedance scales are heavily affected by hydration and are best used only for tracking trends on consistent conditions.