Why lean body mass matters
Lean mass drives your metabolism and strength. Tracking it tells you whether weight changes are fat or muscle, and it's used to set protein targets and medication doses.
The formulas
- Boer (1984): generally the most accurate of the three for adults.
- James (1976): widely used, but unreliable at very high body weights.
- Hume (1966): an older formula that tends to agree closely with Boer.
If you know your body fat, lean mass = weight × (1 − body fat %), which beats any of the formulas.
Frequently asked questions
Is lean body mass the same as muscle mass?
No. Lean mass includes muscle plus bone, organs, skin and water. Skeletal muscle is typically around half of lean mass.
How can I increase lean body mass?
Progressive strength training, enough protein (about 1.6–2.2 g/kg) and adequate calories. Expect slow gains: a few pounds of muscle per year after the first year.
Which formula should I use?
Boer is the best default. Use your body fat percentage instead if you have a reasonable estimate.
These results are estimates for healthy adults and are not medical advice. If you are pregnant, breastfeeding, under 18, or managing a medical condition, talk to your doctor or a registered dietitian.