Quick answer: Lean body mass (LBM) is everything you weigh that is not fat: muscle, bone, organs, and water. For a man, LBM = 0.407 x weight(kg) + 0.267 x height(cm) - 19.2; for a woman, LBM = 0.252 x weight(kg) + 0.473 x height(cm) - 48.3 (the Boer formula).
Last reviewed: October 2026
Key facts
- LBM = total weight minus fat mass; it drives most of your resting calorie burn.
- The widget uses the Boer formula, the most validated for adults (1984).
- From LBM you also get fat mass = weight - LBM and an estimated body fat %.
- Formulas estimate; a DEXA scan is the direct measurement.
A lean body mass calculator estimates your LBM from height, weight, and gender using the Boer formula. Enter your measurements in metric or imperial units, and it returns your LBM, your fat mass, and an estimated body fat percentage. It also derives fat-free mass index and shows how your LBM compares to population averages by age and sex.
This lean body mass calculator starts from one fact: LBM is the metabolically active part of your weight: fat tissue burns very few calories at rest, while muscle, organs, and the rest of your lean mass drive your basal metabolic rate. That is why protein targets are often set per kilogram of lean mass (around 1.6-2.2 g/kg for active people) rather than per kilogram of total weight. This free lean body mass calculator gives you an instant answer with no signup.
How to use the lean body mass calculator
- Choose your unit system: Metric (kg/cm) or Imperial (lb/in).
- Enter your weight and height.
- Select your gender (the Boer coefficients differ for men and women).
- Click Calculate to see your LBM, fat mass, and estimated body fat %.
Lean body mass formula (Boer)
Women: LBM = 0.252 x W + 0.473 x H - 48.3
W is weight in kilograms and H is height in centimeters. The Boer equation (1984) was developed in clinical pharmacology to normalize drug dosing by body size, and it remains the most validated LBM formula for healthy adults. Fat mass then follows as weight minus LBM, and body fat % as fat mass divided by weight times 100.
Lean body mass calculator worked examples
Man: 80 kg at 180 cm
LBM = 0.407 x 80 + 0.267 x 180 - 19.2 = 32.56 + 48.06 - 19.2 = 61.4 kg (rounded). Fat mass = 80 - 61.4 = 18.6 kg, so estimated body fat = 23.2%.
Woman: 60 kg at 165 cm
LBM = 0.252 x 60 + 0.473 x 165 - 48.3 = 15.12 + 78.05 - 48.3 = 44.9 kg (rounded). Fat mass = 60 - 44.9 = 15.1 kg, so estimated body fat = 25.2%.
LBM formula comparison
| Formula | Men | Women |
|---|---|---|
| Boer (1984) | 0.407 x W + 0.267 x H - 19.2 | 0.252 x W + 0.473 x H - 48.3 |
| James (1976) | 1.1 x W - 128 x (W/H)2 | 1.07 x W - 148 x (W/H)2 |
| Hume (1966) | 0.3281 x W + 0.33929 x H - 29.53 | 0.29569 x W + 0.41813 x H - 43.29 |
Frequently asked questions
What is lean body mass?
Lean body mass is your total weight minus your fat mass. It includes skeletal muscle, bone, organs, ligaments, and the water stored in them. For an 80 kg man with 23% body fat, LBM is about 61.4 kg.
What is the difference between lean body mass and fat-free mass?
They are close but not identical. Lean body mass includes a small amount of essential fat inside organs, while fat-free mass excludes all fat. The gap is about 2-3% in men and 5-12% in women, so the two terms are often used interchangeably.
Which LBM formula is best?
The Boer formula is the most widely used and validated for healthy adults. James and Hume are older alternatives that usually agree with Boer within 1-2 kg for normal body sizes. For true precision, a DEXA scan measures lean mass directly.
Why does gender change the result?
Men and women distribute muscle and fat differently at the same height and weight, so the Boer coefficients are sex-specific. The female equation weights height more and subtracts a larger constant, reflecting naturally higher body fat at the same weight.
How do I use my LBM number?
Set protein targets from LBM (about 1.6-2.2 g per kg of lean mass for active people), track whether scale changes come from muscle gain or fat loss, and calibrate calorie needs, since lean tissue drives your metabolism.
How accurate is the Boer estimate?
Within a few kilograms for most healthy adults. It breaks down at extreme body sizes and cannot distinguish muscle from bone or water. Treat it as a solid estimate, and use a DEXA scan when clinical precision matters.
How do I calculate LBM if I already know my body fat percentage?
Use LBM = weight x (1 - body fat % / 100), which is more direct than a formula estimate. For example, a person weighing 90 kg with 20% body fat has LBM = 90 x (1 - 0.20) = 90 x 0.80 = 72.0 kg, leaving 18.0 kg of fat mass.
What does a typical LBM result look like for a woman?
Take a woman weighing 65 kg at 165 cm. The Boer formula gives LBM = 0.252 x 65 + 0.473 x 165 - 48.3 = 16.38 + 78.05 - 48.3 = 46.1 kg. Her fat mass is 65 - 46.1 = 18.9 kg, or about 29.1% body fat, which sits in the typical range for adult women.
Sources
- pmc.ncbi.nlm.nih.gov: Peer-reviewed study comparing Boer and James lean body weight formulas for clinical dosing
Key takeaways
- Bookmark this lean body mass calculator for quick recalculations whenever your inputs change.
- LBM = total weight minus fat mass; it drives your resting calorie burn.
- Boer: men 0.407 x W + 0.267 x H - 19.2; women 0.252 x W + 0.473 x H - 48.3 (kg, cm).
- Fat mass = weight - LBM; body fat % = fat / weight x 100.
- Boer is an estimate; DEXA measures lean mass directly.
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