What is lean body mass?
Lean body mass (LBM) — also called fat-free mass — is your total body weight minus your body fat. It includes muscle, bone, organs, skin, blood and water. Muscle is the part you can change most, and it matters: more lean mass means a higher resting metabolism, better blood sugar control, stronger bones and better health as you age. For most men LBM is around 60–90% of body weight; for women about 55–80%.
This free lean body mass calculator estimates your LBM with three published formulas, or calculates it directly if you know your body fat percentage (try our body fat calculator).
How the lean body mass calculator works
| Formula | Men | Women |
|---|---|---|
| Boer (1984) | 0.407 × kg + 0.267 × cm − 19.2 | 0.252 × kg + 0.473 × cm − 48.3 |
| James (1976) | 1.1 × kg − 128 × (kg ÷ cm)² | 1.07 × kg − 148 × (kg ÷ cm)² |
| Hume (1966) | 0.32810 × kg + 0.33929 × cm − 29.5336 | 0.29569 × kg + 0.41813 × cm − 43.2933 |
| From body fat % | LBM = weight × (1 − body fat % ÷ 100) | |
Example: a man 180 cm and 80 kg has a Boer LBM of 0.407 × 80 + 0.267 × 180 − 19.2 ≈ 61.4 kg (77% of his weight), implying about 23% body fat.
Lean body mass ranges: low, normal and high
| LBM as % of body weight | Women | Men | What it suggests |
|---|---|---|---|
| Low | Under 68% | Under 75% | Higher body fat (over ~32% women / 25% men) — health risk rises |
| Normal / healthy | 68 – 79% | 75 – 86% | Healthy body composition |
| High (athletic) | 80 – 86% | 87 – 94% | Lean and muscular |
| Very high | Over 86% | Over 94% | Very low body fat — may affect hormones and health |
Why lean body mass matters
- Metabolism: lean tissue burns more energy at rest — the Katch-McArdle BMR formula is based on it.
- Protein targets: setting protein by lean mass avoids overestimating needs at higher body fat.
- Healthy weight loss: the goal is to lose fat, not lean mass — tracking LBM shows whether your diet is protecting muscle.
- Healthy ageing: keeping muscle prevents frailty, falls and bone loss.
- Medicine dosing: doctors use LBM to dose some medicines and anaesthetics.
Health warnings
- Formulas give population averages and can be several kilograms off for very muscular, very lean or very heavy people.
- The James formula becomes inaccurate at high BMI (it can even fall as weight rises).
- Not suitable for children, pregnancy, or people with fluid retention (heart, kidney or liver disease).
- Rapid weight loss diets without protein and strength training cause loss of lean mass.
How to build and keep lean mass
Do resistance training 2–3 times a week, eat enough protein spread through the day (the protein calculator helps), avoid crash diets, sleep 7–9 hours and get enough vitamin D. Read our guide to protein and sports nutrition.
Frequently asked questions
What is a good lean body mass percentage?
For women, around 68–79% of body weight is lean mass in a healthy range, and for men about 75–86%. Athletes are often higher.
Which lean body mass formula is most accurate?
The Boer formula is generally the most accurate of the three for most adults and is widely used in medicine. If you know your body fat percentage accurately, the direct method is better.
Is lean body mass the same as muscle mass?
No. Lean body mass includes muscle plus bones, organs, skin, blood and water. Muscle is roughly half of lean mass in most adults.
How can I increase my lean body mass?
Strength training, eating 1.6–2.2 g of protein per kg of lean mass when building muscle, a small calorie surplus and good sleep all help increase lean mass.
Can I lose fat without losing lean mass?
Mostly, yes — by losing weight slowly, eating plenty of protein and continuing strength training during your diet.
Sources
- Boer P. Estimated lean body mass as an index for normalization of body fluid volumes in humans. Am J Physiol 1984;247:F632–F636.
- James WPT. Research on obesity. London: HMSO, 1976.
- Hume R. Prediction of lean body mass from height and weight. J Clin Pathol 1966;19:389–391.