Lean Body Mass Calculator
Estimate your lean body mass, everything you weigh that isn't fat, from your height and weight with three published formulas side by side. Know your body fat %? Add it for a fourth estimate. The math runs in your browser: nothing you type is sent anywhere.
Your estimated lean body mass
Fill in the form and press Calculate to see your lean body mass by each formula and the body fat each one implies.
- Boer formula
- James formula
- Hume formula
- From your body fat %
These are estimates, not medical or dietary advice. How we estimate, with sources
What lean body mass includes
Lean body mass is your weight minus your body fat: muscle, bone, organs, skin, blood and the water they hold. It isn't the same as muscle, though muscle is the part training changes most. If you know your body fat percentage, the math is simple: weight × (1 − body fat). The body fat calculator estimates that number with a tape measure.
Three formulas, three answers
The Boer, James and Hume formulas each predict lean body mass from height and weight, with constants fitted on different groups of people. For the same man, 180 cm (5 ft 11 in) and 80 kg (176 lb), they give:
Lean body mass for a man, 180 cm and 80 kg, by formula
| Formula | Lean body mass | Body fat |
|---|---|---|
| Boer (1984) | 61.4 kg | 23% |
| James (1976) | 62.7 kg | 22% |
| Hume (1966) | 57.8 kg | 28% |
None of them can see how much of your weight is muscle. If you lift and carry more muscle than average for your size, your real lean mass may be above all three. A body fat estimate, from a tape measure or a scan, brings the number closer to you.
Lean body mass vs muscle mass
Muscle is only part of lean body mass; bone, organs and water make up the rest. That's why an estimate of lean mass can move with water and food from one day to the next, while muscle changes slowly. Compare measurements taken the same way, at the same time of day, over weeks, and treat any single reading as a rough guide.
Why lean body mass matters when you diet
When you lose weight, the goal is to lose fat and keep lean mass. Re-check your body fat every few weeks during a cut: if your weight falls and your lean mass holds, the weight you lose is mostly fat. The protein calculator works from body weight, like the meta-analysis on our sources page, and the BMR calculator's Katch-McArdle estimate uses lean mass directly.
Track what you keep
One estimate is a snapshot. The BodyMacros app keeps your weight trend and your lifts in one place and re-estimates your calorie and macro targets each week, so you can see whether you're losing fat and keeping your strength.
FAQ
Which lean body mass formula is most accurate?
None is accurate for everyone. Boer, James and Hume were fitted on different groups, so they disagree by a few kilos for the same person, and none can tell muscle from fat by height and weight alone. If you know your body fat percentage, weight × (1 − body fat) usually reflects you better. Then follow the trend.
Is lean body mass the same as muscle mass?
No. Lean body mass is everything except fat: muscle, bone, organs, skin and water. Muscle is the largest part you can change with training, but it's only part of the total. That's why a lean mass number is always higher than muscle mass, and why water and food can move an estimate of it from day to day.
How do I calculate lean body mass from body fat percentage?
Multiply your weight by one minus your body fat as a decimal. At 80 kg and 20% body fat, that's 80 × 0.8 = 64 kg of lean mass. Enter a body fat % above to see this estimate next to the formulas, or use the body fat calculator first if you don't know yours yet.
Why does the calculator ask for sex?
Each formula has separate constants for men and women, because at the same height and weight women carry more body fat on average. The calculator uses the constants for the sex you choose. If neither fits you well, a body fat estimate from a tape measure or a scan gives a lean mass number that doesn't rely on those averages.
Track this in the app
Log your food and your lifts in one place. Your estimates adjust each week to how you eat and train.
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