Lean Body Mass Calculator
Estimate lean body mass and body fat percentage using the Boer formula.
Lean body mass
61.4 kg
Estimated with the Boer formula
Estimated body fat
23.2%
Of total body weight
Lean mass index
19
Lean mass per square metre
The Boer formula is an estimate based on weight, height and gender — it is not a measurement. For accuracy, consider body composition testing.
Lean Body Mass Calculator on True Calculator gives you an instant, accurate answer with no sign-up and no app install. Estimate lean body mass and body fat percentage using the Boer formula. Every result shows the formula and a worked example so you can verify the calculation yourself, and all values are computed in your own browser — your numbers never leave your device.
Popular uses: lean body mass calculator · lbm calculator · muscle mass calculator
How We Calculate
This calculator uses medically reviewed formulas verified by our team. All calculations are performed instantly in your browser using JavaScript — no data is sent to any server.
We use WHO-recommended formulas and medical standards for health calculations. Results are estimates, not medical advice — always consult a doctor for health decisions.
When to Use This Calculator
Use the lean body mass calculator to separate muscle from fat in your mind and in your tracking. Gym-goers use it to confirm training is adding lean mass, weight-losers use it to ensure they are losing fat rather than muscle, and anyone curious about body composition gets a fast estimate without equipment. Knowing your lean mass also improves nutrition decisions: protein needs, for example, are sometimes expressed per kilogram of lean mass rather than total weight. The lean mass index adds a height-adjusted view, useful for comparing builds across people. Because the formula only needs weight, height and gender, it works anywhere — between sets at the gym, at home, or before a diet-planning session — and it pairs naturally with the BMI and body fat calculators on this site.
How to Use This Calculator
- Step 1: Choose your gender.
- Step 2: Enter your weight in kilograms.
- Step 3: Enter your height in centimetres.
- Step 4: Read your estimated lean body mass, body fat percentage and lean mass index.
Worked Example
For an 80 kg man who is 180 cm tall, the Boer formula gives a lean body mass of 61.4 kg. Since his total weight is 80 kg, the difference implies about 18.6 kg of fat, or 23.2% body fat. A 60 kg woman at 165 cm comes out at 44.9 kg of lean mass — 74.8% of her weight — with a lean mass index of 16.5 kg per square metre. These numbers are direct outputs of the Boer equations, which use only weight, height and gender, so they are useful references rather than measurements.
Tips and Common Mistakes
- •Tip 1: Track lean mass monthly instead of chasing the scale — muscle gain with fat loss can keep weight flat while composition improves.
- •Tip 2: Use the same formula and the same time of day for every comparison so differences mean something.
- •Tip 3: For precise numbers, consider a DEXA or bio-impedance test occasionally to calibrate this estimate.
- ✗Mistake 1: Reading the body fat percentage as a true measurement — it is derived from the lean mass estimate, so the two share the same margin of error.
- ✗Mistake 2: Comparing your number with someone else's without noting gender, height and weight, which all change the result.
Frequently Asked Questions
What is lean body mass?
It is everything in your body except fat — muscles, bones, organs, skin and fluids. Knowing it helps separate 'how much you weigh' from 'how much of you is actually muscle'.
Which formula does this calculator use?
The Boer formula, which estimates lean mass from weight, height and gender. For a man, lean mass ≈ 0.407 × weight (kg) + 0.267 × height (cm) − 19.2; the female equation uses different constants.
How accurate is it compared to a body composition scan?
It is a well-validated estimate for average adults, but not a measurement. Athletes, older adults and people with unusual builds can see larger differences versus DEXA or BIA scans.
Why does body fat % appear in the results?
Once lean mass is estimated, the difference from total weight is assumed to be fat. So body fat % = (weight − lean mass) ÷ weight × 100. Both numbers come from the same estimate.
How can I use this for fitness goals?
Track lean mass over months rather than chasing the scale. If your weight is steady but lean mass is rising and fat is falling, your training and diet are working even when the scale does not move.
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