Lean body mass (LBM, or fat-free mass) is the weight of everything in your body that isn't fat — muscle, bone, organs and water. It's a more useful training metric than scale weight because it separates muscle from fat. This calculator estimates it with the Boer formula from sex, weight and height, then derives fat mass (weight − LBM) and body-fat percentage. Because two people at the same weight can carry very different amounts of muscle, LBM is widely used in fitness, nutrition and clinical dosing.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: the Boer lean-body-mass formula. Not medical advice.
The lean body mass equations
The Boer equation weights your mass and height with sex-specific coefficients to predict the fat-free portion of your body. Subtracting that from total weight gives the fat mass, and dividing fat mass by total weight gives the body-fat percentage. Because the formula only knows weight, height and sex, it assumes an average build — so it predicts well for typical bodies but under- or over-estimates for very muscular or very lean individuals.
Worked example — an 80 kg man
Scenario: male, 80 kg, 180 cm.
His lean body mass is about 61.4 kg — roughly 77% of his weight — leaving about 18.6 kg of fat and a body-fat estimate near 23%. Build and sex shift the result: a 70 kg, 175 cm man comes out near 56.0 kg LBM, while a 60 kg, 165 cm woman is about 44.9 kg and a 70 kg, 170 cm woman about 49.8 kg. Tracking LBM over a training block shows whether the scale's movement is muscle or fat — something total weight alone can't reveal.
Frequently Asked Questions
Weight minus fat — muscle, bone, organs, water. Weight = LBM + fat mass.
Boer: ♂ 0.407·kg + 0.267·cm − 19.2; ♀ 0.252·kg + 0.473·cm − 48.3. 80/180 ♂ → 61.4 kg.
(weight − LBM) ÷ weight × 100. 80 kg, LBM 61.4 → ≈ 23%.
Sets protein needs, doses some drugs, tracks muscle vs fat change during training.
A formula estimate — off for very muscular/lean people. DEXA or calipers measure directly.