Lean Body Mass Calculator
Enter your sex, height and weight to estimate your lean body mass with the Boer, James and Hume formulas. If you know your body fat percentage, add it for a direct measurement-based figure.
Result
- Boer (most accurate)
- 61.4 kg · 23.2% body fat
- James
- 62.7 kg · 21.6% body fat
- Hume
- 57.8 kg · 27.8% body fat
- Fat mass (Boer)
- 18.6 kg
What is lean body mass?
Lean body mass (LBM) is your total body weight minus your body fat. It includes muscle, bone, organs, skin, connective tissue and the water they contain, which together make up roughly 70 to 90% of a healthy adult's weight. LBM matters because it is the metabolically active part of the body: it drives resting energy use, determines how much protein you need, and is the tissue that most drugs distribute into. Clinicians dose some anaesthetics and chemotherapy by lean mass, dietitians set protein targets with it, and lifters track it to check that weight gained is muscle rather than fat.
The formulas and a worked example
Without a body composition scan, LBM is estimated from sex, weight (W, kg) and height (H, cm) using regression equations fitted to measured populations:
| Formula | Men | Women |
|---|---|---|
| Boer (1984) | 0.407W + 0.267H − 19.2 | 0.252W + 0.473H − 48.3 |
| James (1976) | 1.1W − 128(W ÷ H)² | 1.07W − 148(W ÷ H)² |
| Hume (1966) | 0.3281W + 0.33929H − 29.5336 | 0.29569W + 0.41813H − 43.2933 |
Example: a man who weighs 80 kg and is 180 cm tall. Boer: 0.407 × 80 + 0.267 × 180 − 19.2 = 32.56 + 48.06 − 19.2 = 61.4 kg. James: 1.1 × 80 − 128 × (80 ÷ 180)² = 88 − 25.3 = 62.7 kg. Hume: 0.3281 × 80 + 0.33929 × 180 − 29.5336 = 26.25 + 61.07 − 29.53 = 57.8 kg. Dividing by body weight gives the body fat each formula implies: Boer suggests 1 − 61.4 ÷ 80 = 23.2%, James 21.6% and Hume 27.8%.
If you know your body fat percentage, the direct calculation is simpler and more reliable: LBM = weight × (1 − body fat ÷ 100). At 80 kg and 15% body fat that is 80 × 0.85 = 68 kg.
How to use this calculator
- Select your sex and the units you prefer.
- Enter your height and weight. Use a morning weight for consistency if you plan to track changes.
- Optionally enter a measured body fat percentage to add a measurement-based lean mass figure; the body fat calculator can estimate one from tape measurements.
- Read the Boer estimate as the headline figure and compare it with James and Hume; a large spread suggests the formulas fit your build poorly.
Typical values
| Group | Body fat | Lean mass as % of weight |
|---|---|---|
| Men, athletic | 6 – 13% | 87 – 94% |
| Men, average | 18 – 24% | 76 – 82% |
| Women, athletic | 14 – 20% | 80 – 86% |
| Women, average | 25 – 31% | 69 – 75% |
Women carry more essential fat than men, so for the same height and weight the female formulas return a lower lean mass. Lean mass peaks in the late twenties to thirties and declines by roughly 3 to 8% per decade after 40 unless resistance training and adequate protein slow the loss.
Limitations and common mistakes
- Height and weight cannot see composition. The formulas assume an average build; a muscular person is underestimated and a person with a high body fat percentage overestimated, especially by James.
- Extremes of size: the equations were fitted to adults of typical stature and become unreliable for children, very short or very tall people, and BMIs above about 40.
- Water shifts: lean mass includes water, so dehydration, a salty meal or a carbohydrate refeed changes it by a kilogram or more without any change in muscle.
- Mixing methods: if you track progress, use the same formula (or the same body fat method) each time rather than switching between them.
- Related measures: BMI and ideal weight look at total weight only; lean mass adds the composition dimension they lack.
These lean body mass figures are estimates from population equations and are not medical advice.
Frequently asked questions
What is the difference between lean body mass and muscle mass?
Lean body mass is everything that is not fat: muscle, bone, organs, skin and water. Skeletal muscle is only about half of it in a typical adult. So a gain in lean mass on the scale is not all muscle, and water shifts from salt, carbohydrate intake or a hard workout can move it by a kilogram from one day to the next.
Which formula should I trust?
Boer is usually the closest to DEXA scans across the general population, which is why it is the headline figure. James tends to overestimate lean mass in people with obesity, and Hume is the oldest and least validated. If the three disagree by more than a few kilograms, a measured body fat percentage will settle it.
Why is lean body mass used for drug dosing?
Many drugs, including some anaesthetics and chemotherapy agents, distribute mainly into lean tissue rather than fat. Dosing by total weight would overdose heavier people with a high body fat percentage, so clinicians dose per kilogram of lean mass instead, often using the Boer or James formula.
How much protein do I need per kilogram of lean mass?
A common target for people who train is 1.6 to 2.2 g of protein per kilogram of body weight, which works out around 2 to 2.8 g per kilogram of lean mass. Using lean mass avoids inflated targets for people carrying a lot of fat. The macro calculator can turn that into a daily plan.