Find your lean body mass and fat mass.
Body fat percentage is subtracted from the whole, and the remainder is applied to your total weight.
Lean mass: Weight × (1 − body fat%)
Lean body mass is everything in your body that is not fat: muscle, bone, organs, skin, blood and water. This calculator splits your total weight into lean mass and fat mass using your body fat percentage. If you weigh 70 kg with 20% body fat, 14 kg is fat and 56 kg is lean.
Knowing your lean mass helps you see what weight change is made of. Two people at the same weight can have very different amounts of muscle, and a person who loses 3 kg may have lost mainly fat or a worrying amount of muscle. Coaches use lean mass to set protein targets, some calorie formulas use it to estimate metabolism, and people doing strength training use it to check that they are gaining muscle rather than just weight.
1. Weigh yourself in kilograms.
2. Find your body fat percentage from a scan, calipers, a smart scale or an estimate such as the Deurenberg formula.
3. Fat mass = Weight × Body fat % ÷ 100.
4. Lean body mass = Weight − Fat mass, or Weight × (1 − Body fat % ÷ 100).
5. Repeat under the same conditions every few weeks to track changes in each part.
The calculation treats the body as two parts: fat and everything else. Body fat percentage tells you what share of the total is fat, so the rest must be lean. The arithmetic is exact once the percentage is known. The weakness is upstream: every error in the body fat figure carries straight into lean mass. A 3 percentage point error in body fat at 76 kg shifts lean mass by more than 2 kg.
Strictly, fat-free mass excludes all fat, while lean body mass includes a small amount of essential fat found in cell membranes, nerves and bone marrow. In everyday use and in most calculators the two terms are treated as the same, and the difference of a few percent of body weight does not matter for tracking. Lean mass is not the same as muscle: organs, bone and body water are a large part of it.
Muscle and organs use far more energy at rest than fat, so lean mass predicts resting metabolism well. The Katch-McArdle equation estimates BMR as 370 + 21.6 × lean body mass in kg, which can suit athletes better than weight-based formulas. Protein targets are sometimes set per kg of lean mass for people with high body fat, to avoid overestimating needs. In both cases the answer is only as good as the body fat estimate behind it.
Prakash weighs 76 kg and his gym's body composition scale reads 24% body fat, and he wants to know how much of him is lean tissue before starting a strength programme.
Answer: Lean body mass 57.8 kg; Fat mass 18.2 kg
Treating lean body mass as pure muscle, when it includes bone, organs and water.
Comparing readings from different devices or at different times of day, which vary with hydration.
Entering body fat as a decimal such as 0.24 instead of 24.
Assuming a smart scale's body fat reading is precise enough to track small weekly changes.
Checking whether weight loss is coming mainly from fat.
Tracking muscle gain during strength training.
Setting protein targets based on lean mass.
Estimating BMR with lean-mass formulas such as Katch-McArdle.
Why does lean body mass matter?
It is often used to work out calorie needs and protein targets more precisely than just your total weight, since muscle burns more calories than fat does.