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Lean Body Mass Calculator
Enter your weight and body fat percentage to get lean mass, fat mass and two formula comparisons.
Calculate lean body mass
Lean body mass: — · Fat mass: —
Lean percentage: — · Fat percentage: —
Boer formula: — · James formula: —
—
How to use the Lean Body Mass Calculator
- Enter your body weight, then choose kilograms or pounds.
- Add your height in centimetres and your body fat percentage.
- Read your lean and fat mass alongside the Boer and James estimates.
Features
- Lean mass and fat mass from body fat percentage
- Boer and James formula comparisons
- Metric or imperial units
- Copyable summary of every figure
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Frequently Asked Questions
Common questions about the free online lean body mass calculator.
Lean body mass is everything in your body that is not fat, including muscle, bone, organs and
water. It is often called fat free mass.
The calculator multiplies your weight by your body fat percentage to get fat mass, then
subtracts that fat mass from your total weight.
The Boer formula estimates lean mass from weight and height, using different coefficients for
men and women. It is widely used in clinical work.
The James formula is another height and weight estimate of lean mass. Comparing it with Boer
shows how much the two approaches differ.
The formulas predict lean mass from height and weight alone, while your result uses a
measured body fat percentage, so small gaps are normal.
Use a figure from a reliable measurement such as skinfold calipers, a smart scale trend or a
DEXA scan rather than a rough guess.
Yes. Choose kilograms or pounds and the calculator converts the results so you can read them
in the units you prefer.
No. All of the calculations run locally in your browser, so your weight, height and body fat
figures never leave your device.
Yes. The calculator is fully responsive and works in any modern browser on desktop, tablet
and phone.
Last updated: October 10, 2026