What is lean body mass?

Lean body mass (LBM) is your total body weight minus fat mass. It includes muscle, bone, organs, water, and connective tissue — everything except stored body fat.

Which formula should I trust most?

All three formulas (Boer, James, Hume) are validated adult estimates. The average provides a reasonable middle ground. For precision, use DEXA or hydrostatic weighing.

Why do results differ between formulas?

Each formula was derived from different study populations and uses different regression coefficients. Variation of 1-3 kg between formulas is normal.

How is this different from body fat percentage calculators?

Body fat calculators typically require circumference measurements (Navy method). LBM calculators estimate fat-free mass from height and weight only — simpler inputs but less precise.

Can agents run this deterministically?

Yes. Supply sex, weight_kg, and height_cm as JSON to receive lbm_kg, body_fat_pct, and per-formula breakdowns deterministically.

Is this tool free and private to use?

Yes. AI Fit Hub tools are free, no-signup browser tools. Inputs stay in your browser unless you choose to share a URL.

Methodology: Lean Body Mass Calculator

Scope

Estimates lean body mass from height and weight using three adult-validated formulas (Boer, James, Hume).

Formula

Boer (M) = 0.407*W + 0.267*H - 19.2. Boer (F) = 0.252*W + 0.473*H - 48.3. James and Hume use different polynomial forms.

Coefficients

Parameter	Value	Note
Boer W coef (M)	`0.407`
Boer H coef (M)	`0.267`
Boer intercept (M)	`-19.2`

Data sources

Boer P. Estimated lean body mass as an index for normalization of body fluid volumes in humans. Am J Physiol. 1984;247(4):F632-F636. — PMID 6496691.
Hume R. Prediction of lean body mass from height and weight. J Clin Pathol. 1966;19(4):389-391. — PMID 5929341.
James WPT. Research on obesity: a report of the DHSS/MRC group. London: Her Majesty's Stationery Office, 1976. — UK government report; no stable peer-reviewed URL exists, but the James LBM formulas are routinely cited from this HMSO publication.

Assumptions

Height and weight measured, not estimated.

Approximation range

Three-formula spread is typically within about 5 kg for a given person; display the range, not a single formula's output.

Limitations

Formulas were fit on general-population data; heavily muscled athletes are systematically under-predicted.
Do not use LBM formulas to validate a body-fat measurement — the two would be circular.

Reproducibility

Male, 85 kg, 180 cm. Boer: 0.407*85 + 0.267*180 - 19.2 = 63.6 kg.

Change log

2026-04-24: methodology page first published.

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Waist-to-Hip Ratio Calculator — Calculate waist-to-hip ratio and assess body composition using WHO guidelines.

Worked example

Computed by the same engine bundle served at /engines/lean-body-mass-calculator.js. Re-runnable: the values below are the literal output of compute(engineInput).

Input

tool: lean_body_mass_calculator
sex: male
weight_kg: 80
height_cm: 178

Output

formulas: [{"name":"Boer","lbmKg":60.885999999999996,"bodyFatPct":23.892500000000005},{"name":"James","lbmKg":62.144678702184066,"bodyFatPct":22.319151622269917},{"name":"Hume","lbmKg":57.10802,"bodyFatPct":28.614974999999998}]
averageLbmKg: 60.046233
averageBodyFatPct: 24.942209
weightKg: 80

FAQ

What is lean body mass?: Lean body mass (LBM) is your total body weight minus fat mass. It includes muscle, bone, organs, water, and connective tissue — everything except stored body fat.
Which formula should I trust most?: All three formulas (Boer, James, Hume) are validated adult estimates. The average provides a reasonable middle ground. For precision, use DEXA or hydrostatic weighing.
Why do results differ between formulas?: Each formula was derived from different study populations and uses different regression coefficients. Variation of 1-3 kg between formulas is normal.
How is this different from body fat percentage calculators?: Body fat calculators typically require circumference measurements (Navy method). LBM calculators estimate fat-free mass from height and weight only — simpler inputs but less precise.
Can agents run this deterministically?: Yes. Supply sex, weight_kg, and height_cm as JSON to receive lbm_kg, body_fat_pct, and per-formula breakdowns deterministically.
Is this tool free and private to use?: Yes. AI Fit Hub tools are free, no-signup browser tools. Inputs stay in your browser unless you choose to share a URL.