Lean Body Mass Calculator — Boer, James & Hume
Estimate your lean body mass — everything except fat — with three published formulas side by side, or directly from a known body fat percentage, plus your fat-free mass index (FFMI).
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Formula Comparison
Lean mass, fat mass and implied body fat from each method for your inputs.
| Method | Lean Mass | Fat Mass | Body Fat | FFMI |
|---|
How this calculator works
Lean body mass (LBM) includes muscle, bone, organs and water — everything that isn't fat tissue. Each formula estimates it from weight (W, kg) and height (H, 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
- From body fat %: LBM = W × (1 − body fat ÷ 100) — the most direct option if you have a measured body fat figure.
Fat-free mass index is FFMI = LBM ÷ height (m)². The height-normalized version adds 6.1 × (1.8 − height in m), as proposed by Kouri et al. (1995) to compare people of different heights.
By Suhaib Hassan · Reviewed 28 September 2026 against the original published formulas · How CalculatePilot verifies formulas →
Why lean body mass matters
Lean body mass is a useful figure for setting protein intake targets and for tracking body composition changes during training. Two people with the same total weight can have very different lean body mass, which is part of why weight alone is a limited measure of fitness or health. Clinically, lean mass is also used for dosing some medications.
A worked example
A man weighing 80 kg at 180 cm. Boer: 0.407 × 80 + 0.267 × 180 − 19.2 = 32.6 + 48.1 − 19.2 = 61.4 kg lean mass, leaving 18.6 kg fat mass (about 23% body fat). James gives 62.7 kg and Hume 57.8 kg for the same inputs.
His FFMI is 61.4 ÷ 1.8² = 19.0; at exactly 1.8 m, the height-normalized FFMI is the same.
Limitations
Formula estimates become less precise at the extremes — very high muscle mass or very high body fat. The James formula in particular starts to fall as weight rises at high BMI, so it is unreliable above a BMI of about 35. Kouri et al. reported that FFMI values above about 25 were rare among drug-free male athletes, but FFMI calculated from a formula-based LBM carries the formula's error. DEXA scans and hydrostatic weighing measure body composition directly and are more accurate for people outside typical ranges.
Sources
- Boer P. Estimated lean body mass as an index for normalization of body fluid volumes in humans. Am J Physiol 1984;247:F632–F636.
- James WPT. Research on obesity: a report of the DHSS/MRC group. HMSO, London, 1976.
- Hume R. Prediction of lean body mass from height and weight. J Clin Pathol 1966;19:389–391.
- Kouri EM, Pope HG, Katz DL, Oliva P. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clin J Sport Med 1995;5:223–228.
Frequently asked questions
How is lean body mass different from muscle mass?
Lean body mass includes muscle, but also bone, organs, and water — it's a broader measure than muscle mass alone, which is why LBM is typically higher than an estimate of muscle tissue by itself.
Why does the formula use height as well as weight?
Height helps account for overall body frame size, improving the accuracy of the estimate compared to using weight alone.
Which lean body mass formula is most accurate?
No single formula is best for everyone. Boer is widely used and generally reasonable for typical builds; if you have a measured body fat percentage (for example from DEXA), calculating lean mass directly from it is more accurate than any height-and-weight formula.
What is FFMI?
Fat-free mass index is lean mass divided by height in metres squared — like BMI, but counting only lean tissue. It helps compare muscularity between people of different heights.
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