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Fat Mass and Lean Mass Calculator

Break your weight into fat mass and lean mass instantly.

How it is calculated

The split is one multiplication and one subtraction: fat mass = W × BF / 100 and lean mass = W − fat mass. W is total body weight in kilograms and BF is body fat as a percentage, so 22 means 22%, not 0.22. Fat mass is also shown in pounds, using 1 kg = 2.20462 lb. Lean mass is shown in kilograms only. The tool shows no result for a weight outside 2 to 650 kg or for a body fat below 0% or at 100% and above. A body fat of exactly 0 is accepted and gives all-lean mass. The accuracy of the split is only as good as the body fat figure you bring from a scan or tape formula.

Worked example

Someone weighs 75 kg and a body composition scan says 22% body fat. Fat mass is 75 × 22 / 100 = 16.50 kg. Lean mass is 75 − 16.50 = 58.50 kg. In pounds, the fat mass is 16.50 × 2.20462 = 36.38 lb. If a later scan shows the same 75 kg but 20% fat, fat mass drops to 15 kg, meaning 1.5 kg of fat was replaced by lean tissue.

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Common mistakes

  • Entering body fat as a fraction (0.22). The field is a percentage, so 0.22 is read as 0.22% and gives only 0.17 kg of fat for a 75 kg person, with no warning.
  • Mixing sources. A scan, a skinfold test and a tape formula can disagree by several points, so do not switch method between readings and compare the fat mass.
  • Entering pounds in the weight field. It is kilograms, so 165 lb typed as 165 gives a result about 2.2 times too high.
  • Reading lean mass as muscle only. It also includes bone, organs and body water, so it can change with hydration alone.
  • Typing 100 for body fat. A 100% share would mean no lean tissue at all, so the tool shows no result for it.

About this calculator

Total body weight is made up of two very different components: fat mass, the weight contributed by body fat, and lean mass (also called fat-free mass), which covers everything else — muscle, bone, organs, and water. Once you know your body-fat percentage, splitting the two is straightforward: fat mass = total weight × body-fat percentage, and lean mass = total weight − fat mass.

This breakdown matters more than total weight alone for tracking real physical progress, since two people at the same weight can have very different body compositions.

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