Lean Body Mass Calculator
This calculator is for informational purposes only and is not a substitute for professional medical advice. Consult a healthcare provider for personal health decisions.
LBM = weight × (1 − body fat%) or clinical formulas
How Lean Body Mass Is Calculated
Lean body mass (LBM) is total body weight minus fat mass — including muscle, bone, organs, and water. When body fat percentage is known: LBM = weight × (1 − body fat % ÷ 100). A 80 kg person at 15% body fat has LBM = 80 × 0.85 = 68 kg and fat mass = 12 kg.
Without body fat measurement, clinical formulas estimate LBM from weight, height, and sex. Boer formula: male LBM = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2; female LBM = 0.252 × weight(kg) + 0.473 × height(cm) − 48.3. James formula: male LBM = 1.1 × weight − 128 × (weight/height)²; female LBM = 1.07 × weight − 148 × (weight/height)². Hume formula: male LBM = 0.3281 × weight + 0.33929 × height − 29.5336; female LBM = 0.29569 × weight + 0.41813 × height − 43.2933.
These formulas were derived from different populations and methods — results typically vary 2–5 kg between formulas for the same person. Direct body fat measurement (DEXA, hydrostatic weighing, BIA) with the body fat formula usually provides the best individual estimate when available.
Implied body fat % = (fat mass ÷ weight) × 100 when using formula estimates. LBM is useful for setting protein targets (often expressed per kg LBM in research), calculating creatinine clearance, and tracking muscle changes during training or aging. Fat-free mass and lean body mass are related but not identical — LBM includes essential fat in organs.
Use comparisons to understand estimate range rather than treating any single formula as ground truth. Body composition shifts with training, diet, hydration, and age — remeasure periodically for meaningful trends.
Examples
| Example | Result |
|---|---|
| 80 kg, 15% body fat | LBM 68 kg, fat mass 12 kg |
| Male 80 kg, 180 cm (Boer) | LBM ~66 kg |
| Female 65 kg, 165 cm (James) | LBM ~47 kg |
| Compare all 3 formulas | Side-by-side comparison table |
Frequently asked questions
Lean body mass is everything in your body that isn't fat — muscle, bone, organs, and fluids. LBM = total weight minus fat mass.
If you know body fat %, the direct formula is most accurate for you. Among estimation formulas, Boer is commonly used clinically; comparing all three shows typical range.
LBM includes muscle plus bone, organs, and water. Muscle mass is a subset of LBM. Consumer scales estimate muscle but with variable accuracy.