What Is Lean Body Mass?

Lean body mass is your total weight minus your fat: muscle, bone, organs, blood, and water. It is the part of you that drives your metabolism, and a more informative figure than total weight for several purposes.

The Boer equation

We estimate LBM using the Boer formula, published in the American Journal of Physiology in 1984:

Men:   LBM(kg) = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2
Women: LBM(kg) = 0.252 × weight(kg) + 0.473 × height(cm) − 48.3

For someone 170 cm tall weighing 68 kg:

Male:   0.407×68 + 0.267×170 − 19.2 = 27.68 + 45.39 − 19.2 = 53.9 kg
Female: 0.252×68 + 0.473×170 − 48.3 = 17.14 + 80.41 − 48.3 = 49.2 kg

That is about 79% and 72% of total body weight respectively — a difference reflecting typical body-composition differences rather than anything about those two individuals. Our Lean Body Mass Calculator handles both.

What it's useful for

• Understanding your metabolism. Lean tissue is metabolically active; fat much less so. LBM is the main driver of your BMR, which is why two people of the same weight can have genuinely different resting burns — see why BMR estimates vary.
• Setting protein targets. Protein needs track lean mass more closely than total weight, which matters if your body composition is unusual — see body weight or lean mass.
• Interpreting weight change. Losing weight is not automatically good if a large share is lean tissue. Tracking LBM alongside total weight tells you more than the scale alone.
• Clinical dosing. The Boer equation was originally developed for normalising body-fluid volumes, and LBM estimates are used in some drug-dosing contexts.

An estimate, not a measurement

The equation uses height, weight, and sex — and nothing that actually observes your body composition. It predicts a population-typical lean mass for someone with your measurements.

That means it cannot distinguish a heavily muscled person from a less muscled person of identical height and weight, which is precisely the distinction you might be asking about. We label it moderate confidence for that reason.

A more direct route

If you want a better figure, measure rather than infer. The Body Fat Calculator estimates your body fat percentage from tape measurements, and lean mass follows directly:

LBM = weight × (1 − body fat% / 100)

That is still an estimate — the body-fat figure carries its own ±3–4% uncertainty — but it is based on measurements of your actual body rather than on population averages for your height and weight. When the two disagree, the circumference-based route is generally the more informative.

Other equations exist too: Hume (1966) and James are documented alternatives to Boer and give slightly different figures, much as ideal weight formulas do.

The bottom line

Lean body mass is everything that isn't fat, and the Boer equation estimates it from height, weight, and sex — roughly 54 kg for a 68 kg, 170 cm man. Use it for metabolic context and protein targets, and prefer a body-fat-derived figure when you have one. Start with the Lean Body Mass Calculator.