Lean Body Mass Calculator (Boer, James, Hume)

Understanding the lean body mass calculator — concept and principle

Lean body mass (LBM) is everything in your body except stored fat: muscle, bone, organs, skin and body water. It is used to dose some drugs, to set protein targets and to see whether weight change comes from fat or lean tissue. This calculator estimates LBM from weight, height and sex with three classic equations: Boer, James and Hume.

The three formulas put lean mass between 54.45 and 58.99 kg of the 75 kg body.

How it is calculated

W = weight in kg, H = height in cm.

Boer (1984): men 0.407W + 0.267H − 19.2; women 0.252W + 0.473H − 48.3
James (1976): men 1.1W − 128 × (W ÷ H)²; women 1.07W − 148 × (W ÷ H)²
Hume (1966): men 0.32810W + 0.33929H − 29.5336; women 0.29569W + 0.41813H − 43.2933

Results are in kilograms. Subtracting LBM from weight gives an estimate of fat mass.

The principle

Each equation was fitted to people whose lean mass was measured by a reference method, such as body water dilution or body density. Taller people have longer bones and more muscle, so height adds lean mass. Extra weight is partly lean and partly fat, so each additional kilogram adds less than one kilogram of LBM. The James formula uses a squared term in W ÷ H, which bends the curve: in very heavy people it eventually predicts that LBM falls as weight rises, which is impossible. For men this turn happens at a BMI of about 43 and for women at about 36, so James should not be used in severe obesity.

Worked example

Inputs: male, weight 75 kg, height 175 cm.
Boer: 0.407 × 75 + 0.267 × 175 − 19.2 = 30.53 + 46.73 − 19.2 = 58.05 kg.
James: (75 ÷ 175)² = 0.1837; 128 × 0.1837 = 23.51; 1.1 × 75 − 23.51 = 58.99 kg.
Hume: 0.32810 × 75 + 0.33929 × 175 − 29.5336 = 24.61 + 59.38 − 29.53 = 54.45 kg.

Each point is the calculator re-run with only “Height (cm)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Height (cm)	Lean body mass (Boer)
43.8	23.01
87.5	34.69
131	46.37
175	58.05
219	69.73
262	81.41
350	104.77

How to read the result

The three answers span 54.5 to 59 kg, which implies body fat of roughly 21% to 27%. That spread is the honest uncertainty of a formula that knows nothing about your training. Boer is often preferred in pharmacology for adults of normal weight. If you know your body fat percentage from another method, weight × (1 − fat %) gives a more personal figure.

Common mistakes

Entering height in metres or inches; these equations expect centimetres.
Using James in people with obesity, where it underestimates LBM.
Treating LBM as muscle mass. Muscle is only part of it; bone, organs and water make up the rest.

Keep in mind: These are population estimates, not a diagnosis. They are less accurate for athletes, older adults, children and people with obesity or fluid retention. Drug doses must be set by a clinician, not by this calculator.

Frequently asked questions

What is the difference between lean body mass and fat-free mass?

Fat-free mass excludes all fat; lean body mass is often defined to include a small amount of essential fat.

Which formula should I use?

For a normal-weight adult, Boer is a reasonable default. Compare all three and treat the range as your estimate.

Can I increase my lean body mass?

Yes, mainly through resistance training with enough protein and energy.

Sources: Boer P, Estimated lean body mass as an index for normalization of body fluid volumes in humans, American Journal of Physiology (1984) · James WPT, Research on Obesity, HMSO, London (1976) · Hume R, Prediction of lean body mass from height and weight, Journal of Clinical Pathology (1966)

Checked 8 Nov 2026. Published formulas cited on the page. For information only — not medical advice.

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