What lean body mass is
Lean body mass (LBM) is everything in your body except fat: muscle, bones, organs, skin, blood and water. It drives most of the calories you burn at rest, and doctors use it to dose some drugs and anesthetics. Without a body scan, it can be estimated from height and weight with formulas fitted on large groups of people.
The formulas
With weight in kilograms and height in centimeters:
| Formula | Men | Women |
|---|---|---|
| Boer (1984) | 0.407 W + 0.267 H − 19.2 | 0.252 W + 0.473 H − 48.3 |
| James (1976) | 1.1 W − 128 (W ÷ H)² | 1.07 W − 148 (W ÷ H)² |
| Hume (1966) | 0.3281 W + 0.33929 H − 29.5336 | 0.29569 W + 0.41813 H − 43.2933 |
The calculator also shows the implied body fat (1 − LBM ÷ weight) and the fat-free mass index, FFMI = LBM ÷ height in meters².
Worked example: a man, 180 lb and 5 ft 10 in
- Weight 81.6 kg, height 177.8 cm.
- Boer: 0.407 × 81.6 + 0.267 × 177.8 − 19.2 = 61.5 kg, or 135.6 lb.
- Implied body fat: 1 − 61.5 ÷ 81.6 = 24.7%.
- James: 138.5 lb. Hume: 126.9 lb.
- FFMI: 61.5 ÷ 1.778² = 19.5.
Lean mass by weight (men, 5 ft 10 in)
| Weight | Lean mass (Boer) | Implied body fat |
|---|---|---|
| 140 lb | 119.3 lb | 14.8% |
| 160 lb | 127.5 lb | 20.3% |
| 180 lb | 135.6 lb | 24.7% |
| 200 lb | 143.7 lb | 28.1% |
| 220 lb | 151.9 lb | 31.0% |
The formulas assume extra weight is mostly fat. That is often true, but not for people who train with weights.
Which formula?
Boer is the most widely used in medicine and gives sensible values across most body sizes. James was common in the past but gives unrealistic results at high body weight, because of the squared term. Hume tends to come out lower. Seeing all three gives you a range rather than false precision.
Better measurements
DEXA scans, bioelectrical impedance scales and skinfold calipers measure body composition more directly. For a quick estimate from a tape measure, the body fat calculator uses the US Navy method, which accounts for waist and neck size.
Using lean body mass
- Protein targets: some coaches base protein on lean mass, around 2 g per kg of LBM, which avoids overestimating needs for people with higher body fat.
- Tracking progress: if weight drops while lean mass holds steady, the loss is mostly fat.
- FFMI: helps compare muscularity between people of different heights.
These are population estimates and do not replace a clinical assessment.