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Lean body mass calculator — Boer, James, Hume.
Your lean body mass (LBM) is everything you carry that isn't fat — muscle, bone, organs, and water. This calculator estimates it with the Boer formula by default, shows the James and Hume equations for comparison, and can use your body-fat % directly. It reports fat mass too, in metric or imperial, live, no signup.
All four estimates.
The three named formulas plus the direct body-fat method. They routinely disagree by 2–5 kg — the spread is your uncertainty.
| Method | Lean body mass | Notes |
|---|---|---|
| Boer (1984) | — | The common default; reliable across typical adults |
| James (1976) | — | Adds a weight-to-height correction term |
| Hume (1966) | — | Its own regression from a separate cohort |
| From body fat | — | Most accurate — needs your body-fat % |
The formula estimates only know height, weight, and sex — they can't see how muscular you actually are, so a heavily-trained lifter and a sedentary person of the same size get the same number. If you have a real body-fat reading, the direct method is the one to trust.
The lean-mass formulas, explained.
The real coefficients, male form shown. Weight in kg, height in cm.
LBM = 0.407 × kg + 0.267 × cm − 19.2 Female: 0.252 × kg + 0.473 × cm − 48.3. Boer, 1984 — the most widely used general estimate.
LBM = 1.1 × kg − 128 × (kg ÷ cm)² Female: 1.07 × kg − 148 × (kg ÷ cm)². James, 1976 — includes a squared correction term.
LBM = 0.3281 × kg + 0.33929 × cm − 29.5336 Female: 0.29569 × kg + 0.41813 × cm − 43.2933. Hume, 1966 — a separate regression.
Worked example (Boer, male): 178 cm, 80 kg → 0.407 × 80 + 0.267 × 178 − 19.2 = 32.56 + 47.53 − 19.2 = 60.9 kg of lean mass, leaving about 19 kg of fat. Add a 15% body-fat reading and the direct method gives 68 kg — the gap shows why a real body-fat number beats a population formula. Sources: Boer 1984; James 1976; Hume 1966.
Related body calculators.
Get a real body-fat reading, turn lean mass into FFMI, then track it in the browser.
Body-fat calculator
The US Navy method — the most direct way to get lean mass.
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Open toolNutrition tracker
Log food against your numbers, right in the browser.
Open toolLean body mass questions.
What is lean body mass?
Lean body mass (LBM), also called fat-free mass, is everything in your body that is not fat — muscle, bone, organs, connective tissue, and water. It typically makes up 60 to 90 percent of total bodyweight, higher in leaner and more muscular people. LBM is the tissue that burns most of your resting calories, so it is the number that really matters for metabolism and performance.
Which lean body mass formula is most accurate?
No single formula wins for everyone. The Boer formula is the most widely used default and performs well across typical adults. James and Hume were derived from different cohorts and can disagree by 2 to 5 kg at the same body size. If you know your body-fat percentage, the direct method — weight × (1 − body fat ÷ 100) — is usually the most accurate, because it uses your actual composition rather than a population average.
How do I calculate lean body mass from body fat?
Multiply your bodyweight by one minus your body-fat percentage divided by 100. For an 80 kg person at 15 percent body fat, LBM = 80 × (1 − 0.15) = 68 kg, leaving 12 kg of fat mass. This is the most direct method and the one to prefer when you have a reliable body-fat reading from the US Navy method, calipers, or a DEXA scan.
Why do the three formulas give different numbers?
Boer, James, and Hume were each built from a different study population using different reference techniques, so their coefficients differ. Boer uses simple weight and height terms; James adds a squared weight-to-height correction; Hume uses its own regression. Seeing all three side by side gives you a realistic range rather than a false-precision single figure — treat the spread as your estimate's uncertainty.
How is lean body mass different from FFMI?
Lean body mass is an absolute weight in kilograms. FFMI takes that lean mass and divides it by your height squared, turning it into a muscularity index you can compare across body sizes — much like BMI does for total weight. LBM tells you how much fat-free tissue you carry; FFMI tells you whether that is a lot for your height.
Is this lean body mass calculator free?
Yes. It runs in your browser, updates as you type, and needs no signup. Save your result to Nishaana to track lean mass and fat mass over time alongside your training.
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Free in your browser — Nishaana keeps your lean mass, fat mass, and weight trend together and charts them as you cut or build, no app store needed.
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