Your lean body mass (LBM), also called fat-free mass, is everything in your body that is not fat — muscle, bone, organs and water. This calculator estimates it from your height, weight and sex using the Boer formula, a widely cited equation used in sports nutrition and clinical dosing. It is for adults tracking body composition, setting protein goals or estimating metabolic rate.
How it works
The tool converts your input to metric (kilograms and centimetres) if needed, then applies the Boer formula:
- Men: LBM = 0.407 × weight(kg) + 0.267 × height(cm) − 19.2
- Women: LBM = 0.252 × weight(kg) + 0.473 × height(cm) − 48.3
It then derives two more figures: fat mass = total weight − LBM, and body-fat % = fat mass ÷ total weight × 100. Imperial inputs are converted first (1 lb = 0.453592 kg, height in inches × 2.54 = cm) so the formula always runs on metric values.
Worked example
A 178 cm, 75 kg man:
LBM = 0.407 × 75 + 0.267 × 178 − 19.2 = 30.53 + 47.53 − 19.2 = 58.9 kg
| Result | Value |
|---|---|
| Lean body mass | 58.9 kg |
| Fat mass | 16.1 kg |
| Body-fat (approx) | 21.5% |
A 165 cm, 60 kg woman:
LBM = 0.252 × 60 + 0.473 × 165 − 48.3 = 15.12 + 78.05 − 48.3 = 44.9 kg
| Result | Value |
|---|---|
| Lean body mass | 44.9 kg |
| Fat mass | 15.1 kg |
| Body-fat (approx) | 25.2% |
Why lean mass matters more than total weight
Body weight alone conflates muscle with fat. Two people at the same weight can have very different body compositions — one with more muscle, one with more fat — and treating them identically leads to poor decisions. Lean body mass is a better anchor because:
- Protein targets: many nutrition protocols recommend 1.6–2.2 g of protein per kg of lean mass per day for muscle growth or retention, rather than per kg of total weight. Using LBM avoids over-prescribing protein to people carrying more fat.
- Resting metabolic rate (RMR): muscle is metabolically active tissue. Equations like Katch-McArdle estimate RMR directly from LBM and are generally more accurate for people at the lower or higher ends of body-fat percentage.
- Clinical dosing: some medications, particularly anaesthetics and certain antibiotics, are dosed to lean body mass to avoid overdosing in patients with high body fat.
Limitations
The Boer formula is a population estimate validated primarily in adults of typical body composition. It can underestimate lean mass in very muscular individuals and overestimate it in people with very low muscle mass (such as some elderly adults). For precise measurements, methods such as DEXA scanning, hydrostatic weighing or calibrated bioimpedance remain the gold standard.
The Boer formula runs entirely in your browser — none of your inputs are uploaded or stored.