Lean body mass calculator
Lean body mass is everything you are made of except fat: muscle, bone, organs, water. Enter a body fat percentage for a direct figure, or leave it blank and three published formulas estimate it from height and weight. All three are shown, because they disagree.
Lean mass is the basis for the Katch-McArdle energy equation and for protein targets set per kilogram of lean tissue rather than total bodyweight.
- Fat mass
- 20 kg
- Boer (1984)
- 60.9 kg
- James (1976)
- 62.1 kg
- Hume (1966)
- 57.1 kg
An estimate from a published formula, not a measurement. Use it as a consistent yardstick for tracking change.
Two ways to get the number
If you know your body fat percentage, lean mass is simple subtraction: total weight minus fat mass. That figure is only as good as the body fat estimate behind it, but it is the more direct route and this tool uses it whenever you supply one.
Without a body fat figure, the three formulas predict lean mass from height, weight and sex. They were fitted on different populations decades apart, which is why they can disagree by several kilograms for the same person.
What separates the three formulas
- Boer (1984) is the most widely used of the three today and behaves reasonably across a broad weight range.
- James (1976) includes a squared weight-to-height term, so it turns down at higher bodyweights and reads low for heavy, muscular people.
- Hume (1966) is the oldest and was fitted on a small sample. It is included because it is still widely quoted, not because it is the best.
When they cluster, the estimate is probably fine. When they spread widely, you are outside the population any of them were fitted on, and a body fat measurement is worth more than picking a favourite.
What lean mass is used for
Three practical things. It drives the Katch-McArdle resting energy equation, which is the better one for lean, muscular people. It gives protein targets a sensible basis when body fat is high. And it is the term in fat-free mass index, which is the muscle-aware replacement for BMI.
It is also the number that should be protected in a deficit. Weight loss that comes disproportionately out of lean mass is the failure mode every high-protein, keep-lifting recommendation exists to prevent.
Lean mass is not muscle mass
Skeletal muscle is roughly half of lean mass in a typical adult. The rest is bone, organs, connective tissue and body water. That last one moves quickly: creatine, carbohydrate intake and hydration can shift measured lean mass by a kilogram or more without a single muscle fibre changing.
Questions people ask
Which lean body mass formula is most accurate?
Boer is generally the best of the three for adults across a normal weight range. But any formula is a prediction from height and weight, and a body fat measurement you trust will beat all of them.
Why do the three results differ so much for me?
Because you are probably outside the population one of them was fitted on. James in particular reads low for heavy, muscular people because of its squared weight term. A wide spread is a signal to measure rather than estimate.
Can I gain lean mass while losing weight?
Yes, in the situations where recomposition works: new lifters, returning lifters, and people with higher body fat. It is slow, and it needs high protein and hard training to happen at all.
How fast can lean mass increase?
For a first-year lifter, perhaps a kilogram a month under good conditions. After a few years, a few hundred grams. Any tool that promises faster is describing weight gain, not lean tissue.
Related tools
The training is what changes it
Mekimeki tracks every set and every session so you can see whether the work behind your lean mass is actually progressing.
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