Lean Body Mass Calculator
How much of your weight is lean mass — muscle, bone and water — and how much is fat. Add your body fat % for the most accurate result.
Your results
Shop proteinWhat is lean body mass?
Lean body mass (LBM) is everything you weigh that isn't fat — muscle, bone, organs and water. This lean body mass calculator estimates yours two ways: precisely from your body fat percentage if you know it, or from the Boer formula if you don't. It's arguably a more useful number than body weight, because it's the part of you that lifts, burns calories and shows up in the mirror.
How it works
- With body fat %: LBM = weight × (1 − body fat / 100). Direct arithmetic — the most accurate option.
- Without: the Boer formula (men: 0.407×weight + 0.267×height − 19.2; women: 0.252×weight + 0.473×height − 48.3), a validated clinical prediction used in medicine when body fat isn't known.
Why lean mass matters
- Protein targets scale with it: research on dieting athletes recommends 2.3–3.1 g of protein per kg of lean mass — more precise than scaling with total weight, especially at higher body fats.
- It predicts your metabolism: the Katch–McArdle BMR formula runs entirely on lean mass.
- It's the true progress metric: weight going up means little; lean mass going up means the bulk is working.
Frequently asked questions
How much of my lean mass is actually muscle?
Roughly half. The rest is bone, organs and water. That's why you can't gain lean mass as fast as total weight — muscle is the slow-growing part.
Why did my lean mass jump in one week?
Water. Creatine, carbs and training status all shift intramuscular water by a kilogram or more. Judge lean mass changes over months, not days.
References
- Boer P. Estimated lean body mass as an index for normalization of body fluid volumes in humans. Am J Physiol. 1984;247(4):F632–F636.
- Helms ER, et al. Int J Sport Nutr Exerc Metab. 2014;24(2):127–138.
Related
These results are estimates for healthy adults and are not medical advice. Consult a health professional before making major changes to your diet or training.
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