Lean Body Mass (LBM) Calculator
Estimate lean body mass and fat mass from a known body-fat percentage, or from weight and height using the Boer, James, or Hume regression equations — with a labeled composition breakdown and step-by-step working.
Background
Lean body mass (LBM) is everything in the body that isn't fat — muscle, bone, organs, water. The most direct route is LBM = Weight × (1 − BF%/100) when body-fat percentage is already known. When it isn't, decades-old regression equations (Boer, James, Hume) estimate LBM from just weight and height, using sex-specific coefficients fitted to population data.
How to use this calculator
- Body-Fat % is the most direct route — if you know your body-fat percentage, this needs no height and no formula.
- Boer, James, and Hume each estimate LBM from weight, height, and sex when body-fat % isn't known — they were fit to different population samples and can disagree by a few kilograms.
- Click Calculate to see the composition breakdown, LBM%, and the full working for whichever mode you picked.
How this calculator works
Body-Fat % mode: LBM = Weight × (1 − BF%/100) — a direct calculation that doesn't depend on sex or height.
Boer (1984): Male LBM = 0.407W + 0.267H − 19.2; Female LBM = 0.252W + 0.473H − 48.3 (W in kg, H in cm).
James (1976): Male LBM = 1.1W − 128(W/H)²; Female LBM = 1.07W − 148(W/H)².
Hume (1966): Male LBM = 0.32810W + 0.33929H − 29.5336; Female LBM = 0.29569W + 0.41813H − 43.2933.
Fat mass is Weight − LBM, and LBM% is LBM ÷ Weight. At extreme height-to-weight ratios, James and Hume can occasionally return an LBM above total body weight — a known limitation of those regression equations, flagged with a note rather than hidden.
Formula & Equations Used
From body-fat %: LBM = W × (1 − BF%/100)
Boer (1984): Male 0.407W + 0.267H − 19.2; Female 0.252W + 0.473H − 48.3
James (1976): Male 1.1W − 128(W/H)²; Female 1.07W − 148(W/H)²
Hume (1966): Male 0.32810W + 0.33929H − 29.5336; Female 0.29569W + 0.41813H − 43.2933
All formulas: Fat mass = W − LBM; LBM% = (LBM ÷ W) × 100
Example Problems & Step-by-Step Solutions
These cover cases the Quick Examples chips above don't already demonstrate.
Example 1 — Body-fat % (male)
75 kg, 18% body fat.
Step: LBM = 75 × (1 − 0.18) = 61.5 kg. Fat mass = 13.5 kg. LBM% = 82%.
Example 2 — Boer, imperial units (male)
190 lb, 70 in.
Step: Convert: 86.18 kg, 177.80 cm. LBM = 0.407(86.18) + 0.267(177.80) − 19.2 ≈ 63.35 kg.
Example 3 — Hume, imperial units (female)
140 lb, 64 in.
Step: Convert: 63.50 kg, 162.56 cm. LBM = 0.29569(63.50) + 0.41813(162.56) − 43.2933 ≈ 43.46 kg.
Example 4 — When a formula hits its limit
Male, 40 kg, 259 cm, James formula.
Step: LBM ≈ 40.95 kg — above the 40 kg body weight entered. The calculator flags this rather than showing a silently impossible number; Boer or a known body-fat % would be more reliable here.
Frequently Asked Questions
Which method should I choose?
If you have a reliable body-fat %, use it — it's the most direct route. Otherwise Boer is the most widely used clinically; James and Hume are older but still common.
Why did switching modes change my result without me touching my weight?
Each mode is a fully separate calculation method (a different formula, or the direct body-fat route), so the mode you have selected determines exactly which equation runs — nothing overrides silently.
What units do I enter?
Weight in kg or lb, height in cm or in. Everything converts internally to kg/cm for the formulas, and results are shown back in the weight unit you chose.
Is LBM the same as fat-free mass?
They're often used interchangeably; small definitional differences exist in research contexts.
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