CALCULATOR
Lean Body Mass Calculator
Lean mass and fat mass from your body fat percentage, or from the Boer formula if you do not know it.
If you know your body fat percentage the result is far more accurate than any formula estimate.
Why lean body mass matters more than weight
Lean body mass is everything that is not fat: muscle, bone, organs, connective tissue and body water. It is the metabolically active part of you, which is why two people at identical bodyweight can have noticeably different calorie needs.
If you know your body fat percentage, lean mass is simple arithmetic and the result is reliable. If you do not, this calculator falls back on the Boer formula, which estimates from height and weight alone and is correspondingly less accurate for people at either extreme of muscularity.
Lean mass is also the input for the Katch-McArdle equation, the BMR formula that outperforms the others for lean and trained people precisely because it works from lean tissue rather than total weight.
How lean body mass is calculated
Fat mass = weight - LBM
Example: 80 kg at 20% body fat
LBM = 80 x 0.80 = 64 kg
Women: LBM = 0.252W + 0.473H - 48.3
If you have a body fat measurement, the first method is simple subtraction and as accurate as that measurement. The Boer formula estimates from height and weight alone, which works reasonably for average builds and poorly at the extremes: it will underestimate a heavily muscled lifter and overestimate someone carrying high body fat.
What lean body mass includes
| Component | Approximate share of LBM |
|---|---|
| Skeletal muscle | 40-50% |
| Body water | Large share, overlaps other tissues |
| Bone | 12-15% |
| Organs | 10-12% |
| Connective tissue and skin | Remainder |
This matters for interpretation: lean body mass is not the same as muscle mass. Gaining two kilograms of lean mass does not mean gaining two kilograms of muscle, because water shifts are part of the figure. That is also why hydration status moves bioimpedance readings so much.
Why lean mass drives your calorie needs
Lean tissue is metabolically active; stored fat is comparatively inert. Two people at 80 kg, one at 12 percent body fat and one at 30 percent, have meaningfully different resting energy requirements because their lean mass differs by around 14 kg. This is precisely the gap the Katch-McArdle equation closes, and why it outperforms weight-based formulas for trained people.
Same weight, different composition
| Profile | Weight | Body fat | Lean mass | Katch-McArdle BMR |
|---|---|---|---|---|
| Trained | 80 kg | 12% | 70.4 kg | 1,890 kcal |
| Average | 80 kg | 22% | 62.4 kg | 1,718 kcal |
| Higher body fat | 80 kg | 32% | 54.4 kg | 1,545 kcal |
Protecting lean mass while dieting
- Keep protein at 1.6 to 2.2 g per kilogram of bodyweight. This is the single most effective dietary lever.
- Lift at least twice a week. Training is what signals that the tissue is worth keeping.
- Avoid very aggressive deficits. Past roughly 1 percent of bodyweight per week, a larger share of loss comes from lean tissue.
- Sleep adequately. Short sleep measurably increases the proportion of weight lost as muscle rather than fat.
- Track lean mass rather than scale weight during a recomposition. The scale can hold steady while composition improves substantially.
Frequently asked questions
What is a good lean body mass?
Can you gain lean mass while losing fat?
Does lean body mass include water?
Should protein be based on lean mass or total weight?
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General information, not medical advice. Consult a qualified professional before changing your diet, especially with a medical condition.