Lean Body Mass Calculator
Lean body mass is everything except stored fat mass (bones, muscle, organs, water, etc.—including essential fat in organs). This tool applies several published estimating equations side by side. Use metric inputs internally; weights shown in your chosen unit.
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| Formula | Lean body mass | Est. body fat % |
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Adult equations: Boer (1984), James (1976), Hume (1966). Child equation: Peters et al. (2011). Body fat % ≈ (weight − LBM) / weight. LBM and fat-free mass are related but not identical.
What lean body mass includes
Lean body mass (LBM) is your total body weight minus stored (non-essential) fat mass — it includes muscle, bone, organs, water, and the small amount of essential fat found in organs and cell membranes. Each formula here (Boer, James, Hume) uses a different regression derived from body-composition studies, which is why comparing all three side by side gives a useful sense of the range rather than relying on a single number.
Frequently asked questions
Is lean body mass the same as "fat-free mass"? They're closely related but not identical — LBM technically includes a small amount of essential fat, while fat-free mass excludes all fat. In practice, the two terms are often used interchangeably and the difference is small.
Why do the formulas disagree with each other? Each was derived from a different research sample using different measurement methods (e.g., some based on hydrostatic weighing, others on different body composition references), so a few kilograms of disagreement between formulas is normal and expected.
Why is there a separate formula for children? Children's body composition and growth patterns differ substantially from adults, so the Peters et al. equation was specifically derived from pediatric data rather than adapting an adult formula.