About the Lean Body Mass Calculator
Lean body mass is everything in your body that is not fat: muscle, bone, organs, skin and water. This calculator estimates it from your sex, height and weight using three well-known formulas, Boer, James and Hume, and averages them. It then shows the fat mass and body fat percentage that each result implies. You can enter metric or imperial units. The numbers are estimates from equations, not a scan, and they are most useful for tracking your own changes over time.
Updated October 2026
How to use the Lean Body Mass Calculator
- 1Choose your sex and your preferred units.
- 2Enter your weight and height.
- 3Read the average lean body mass in the highlighted box.
- 4Compare the three formulas and the body fat each one implies.
- 5Press Copy result to save the figures.
Why lean mass is worth knowing
Scale weight alone cannot tell you whether you are losing fat or muscle. Lean body mass can. When you diet, you want weight to fall while lean mass stays the same, and when you train for strength you want lean mass to rise. Metabolism also depends on lean tissue, which is why the Katch–McArdle BMR formula uses it.
Reading the results
- Lean body mass: weight of everything except fat
- Fat mass: your total weight minus lean mass
- Body fat percentage: fat mass as a share of total weight
- Average: a balanced middle of the three formulas
Using it to track progress
Because these formulas rely only on height and weight, any change in the result simply reflects a change in your weight. That makes them poor at showing muscle gain on their own. For progress tracking, combine them with waist measurements, photos, strength numbers and, if you can, a repeatable body-composition test taken at the same time of day.
Related tools
The Body Fat Calculator estimates body fat from measurements, the BMR Calculator can use lean mass through the Katch–McArdle formula, and the Protein Calculator helps set intake. The BMI Calculator and Ideal Weight Calculator compare weight with height, and the TDEE Calculator estimates daily calories.
Features
- Boer, James and Hume formulas, plus their average
- Lean mass, fat mass and body fat percentage
- Metric and imperial units
- Checks for impossible heights and weights
- No sign-up and nothing stored
- Runs in your browser
Frequently asked questions
What is lean body mass?
Lean body mass is your total weight minus the weight of your body fat. It includes muscle, bones, organs, blood and water. It is used in medicine for drug dosing and in fitness to track muscle gain and fat loss.
Which formula is the best?
No single formula is best for everyone. Boer and James are widely used in clinical settings, and Hume is common in research. Their answers usually differ by a few kilograms, which is why this page shows all three and an average.
How accurate is the body fat percentage shown?
It is a rough estimate. The formulas use only height, weight and sex, so they cannot tell a muscular person from someone with the same measurements and more body fat. For a better number use skinfold calipers, a bioimpedance scale or a DEXA scan.
How do I find my lean body mass by hand?
If you know your body fat percentage, multiply your weight by that percentage to get fat mass, then subtract it from your weight. A person of 80 kg with 20 percent body fat has 16 kg of fat and 64 kg of lean mass. This calculator works the other way round, from height and weight.
Why is my result very different from my scale?
Home scales estimate body composition from a small electric current, and their readings change with hydration. Formula results stay the same for the same numbers. Differences of several kilograms between methods are common, so pick one and use it consistently.
Can I use it for children or athletes?
These formulas were developed for adults of typical build, so results for children, bodybuilders and very tall or short people are less reliable. The tool asks for adult-range heights and weights and warns about impossible numbers.