Lean Body Mass Calculator
Lean mass is everything that is not fat — muscle, bone, organs and water. Its practical use is as a denominator: protein targets and calorie floors both work better against it than against total weight.
Lean mass
60.6kg
- Boer formula
- 61.4 kg
- James formula
- 62.7 kg
- Hume formula
- 57.8 kg
- Add a body fat percentage if you have one — measured is always better than estimated, because these formulas know only your height and weight.
- Lean mass is everything that is not fat: muscle, bone, organs and water. It is not a muscle measurement, and a large part of it is not trainable.
- Its practical use is as a denominator. Protein targets and calorie floors both work better against lean mass than against total weight.
About lean body mass
The best way to get this number is not a formula. If you know your body fat percentage from any source, lean mass is simply what is left — 80kg at 18% fat is 65.6kg of lean tissue, and no estimate is needed. Enter a percentage above and that is the figure this leads with.
Without one, three published formulas estimate it from height and weight alone. Boer, James and Hume give 61.4, 62.7 and 57.8 kilograms for an 80kg man of 180cm — a spread of nearly five kilos, which is the usual honest caveat about formulas that cannot see body composition.
The James formula in particular breaks down at higher body weights: it has a squared term that eventually causes the estimate to fall as weight rises, which is obviously wrong. It is included because it is widely cited, not because it should be trusted at the extremes.
Lean mass is not a muscle measurement. Bone, organs, blood and water are all in it, and most of that is not trainable. What responds to training is a fraction of the total, which is why lean mass moves slowly even when training is going well.
What it works out
- Lean mass from a measured body fat percentage
- Three published estimates from height and weight
- Fat mass alongside it
The formula
From body fat: LBM = Weight × (1 − Body fat %) Boer: LBM = 0.407w + 0.267h − 19.2
The first line is not really a formula — it is a definition. Lean mass is what is not fat, so 80kg at 18% fat is 80 × 0.82, which is 65.6kg. Anything else on this page is an attempt to guess that number without knowing the percentage.
Boer takes weight and height with fixed coefficients: 0.407 × 80 plus 0.267 × 180 less 19.2, which is 61.4kg. Hume uses different constants and gives 57.8. James includes a squared weight-over-height term and gives 62.7.
The spread between them is nearly five kilograms on the same person. That is not a flaw in any one formula so much as a statement about what height and weight alone can tell you — which is why a measured percentage, however roughly measured, beats all three.
James is worth a specific warning. Its squared term means that past a certain weight the estimate starts falling as weight rises, which cannot be right. It is here because it is widely quoted.
- w
- Weight in kilograms.
- h
- Height in centimetres.
- Body fat %
- From a tape measurement, calipers or a scan. Any of them beats a formula.
A worked example
A man of 80 kilograms and 180 centimetres, with body fat unknown.
That works out to 60.6 kg.
- Boer formula
- 61.4 kg
- James formula
- 62.7 kg
- Hume formula
- 57.8 kg
Questions
What is lean body mass?
Everything in your body that is not fat: muscle, bone, organs, blood and water. On an 80kg man at 18% body fat it is 65.6 kilograms.
Is lean body mass the same as muscle mass?
No, and the difference is large. Bone, organs and body water are all included, and skeletal muscle is typically around half of lean mass. Most of the rest cannot be trained.
Why does it matter?
It is a better denominator than total weight. Protein needs scale with the tissue that uses it, and calorie floors work better against lean mass — which matters most for anyone carrying a lot of fat, where total weight badly overstates both.
Which formula is most accurate?
Boer is generally regarded as the best of the three for most adults. But all three are estimates from height and weight, and any measured body fat percentage will beat them.
How fast can I gain lean mass?
Slowly. A well-trained beginner might add a kilogram or two of muscle a month in the first year, and that rate falls sharply afterwards. Anything faster is mostly water, glycogen and food weight.
Does lean mass fall when I lose weight?
Some, almost always. Adequate protein and resistance training minimise it, and an aggressive deficit maximises it — which is the main argument against very fast weight loss.