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Lean Body Mass Calculator

Fat-free mass by three formulas, or from a body fat figure.

Health Calculators Free, no sign-up Runs in your browser

Optional. If you know it, this is the most reliable route.

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Lean body mass
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Fat mass
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Lean as a share of weight
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James formula
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Hume formula
MethodLean massFat mass
Lean body mass includes muscle, bone, organs and water, not muscle alone. It is the figure some drug doses and the Katch-McArdle metabolic equation are based on.

About the Lean Body Mass Calculator

Lean Body Mass Calculator estimates everything in your body that is not fat: muscle, bone, organs, blood and water. Three published formulas are shown -- Boer, James and Hume -- because estimating lean mass from height and weight alone is inherently imprecise, and the spread between them is the honest measure of that.

If you know your body fat percentage, that route is better than any of the formulas and the calculator uses it. Lean mass is simply weight times one minus the fat fraction, and a measured body fat figure, even from a tape method, carries more information about you than a formula fitted to a population.

Lean body mass matters for two practical reasons. Some drug doses are calculated from it rather than total weight, because fat tissue does not take up most drugs the same way. And the Katch-McArdle metabolic equation uses it instead of sex and height, which makes it the most accurate basal rate estimate for people at either extreme of body composition.

This is general information and not medical advice. The formula cannot know your medical history, your medication, your body composition or anything else specific to you. Speak to a doctor or another qualified professional about any question concerning your health, and never delay seeking advice because of a figure on this page.

Your measurements are processed inside your own browser. Height, weight, age and anything else you enter never leave your device, nothing is logged, and nothing survives closing the tab.

How to use the Lean Body Mass Calculator

  1. Choose sexThe fields below change to match what you pick, so only the figures that matter to your case are asked for.
  2. Enter weight (kg), height (cm) and body fat (%)Decimals are fine, and thousands separators are accepted and ignored.
  3. Read the lean body mass, fat mass and lean as a share of weightThe figures update as you type. The Calculate button is there if you would rather press something, but nothing waits for it.
  4. Copy or check the resultCopy the summary with the copy button, or open the table to check it row by row.

Features

  • Shows lean body mass, fat mass, lean as a share of weight, james formula and hume formula together, so you are not running it twice
  • A results table you can read across
  • Updates as you type, with no button to press and no page reload
  • Copy the result in one click
  • Free, no sign-up, and it runs entirely in your browser

Who uses this tool

The lean body mass calculator is used by writers, developers, students, marketers and anyone else who needs the job done once without installing software. Common cases include:

  • Getting lean mass for a Katch-McArdle BMR estimate
  • Tracking muscle retention during weight loss
  • Converting a body fat percentage into kilograms
  • Comparing the published lean mass formulas

Frequently asked questions

Is lean body mass the same as muscle mass?

No. It includes bone, organs, blood and water as well as muscle. Skeletal muscle is typically about half of it.

Which formula should I use?

Boer is the usual default and is used here. If you know your body fat percentage, that route is more reliable than any of them.

Why do the formulas disagree?

Because they were fitted to different populations decades apart. Height and weight simply do not determine body composition, so any formula based on them is an approximation.

If the lean body mass calculator is not quite what you need, these other free tools solve closely related problems, and there are 19 more in health calculators.

Body Fat Calculator

Body fat percentage by the US Navy tape method.

Health Calculators Use tool

BMR Calculator

Basal metabolic rate by three published equations.

Health Calculators Use tool

BMI Calculator

Work out BMI from height and weight, in metric or imperial units.

Health Calculators Use tool

Checked and rebuilt on 2026-10-14. The method this calculator uses is described in the section above, and corrections go through the contact page.