Lean body mass:
how to work out what your weight is made of

What is lean body mass?

Lean body mass is your total body weight minus the weight of body fat. It includes muscle, bones, organs, skin, connective tissue and the water held in them. It is not the same as muscle mass, which is only one part of it.

Why does it matter? Two people can weigh the same and have very different lean mass. And one person can lose weight on the scale while losing fat, lean mass or a mixture of both. Splitting weight into fat and lean parts shows which one is moving.

You will need an estimate of body fat to calculate it, or you can use a formula that works from height and weight alone. Both routes are covered below, with the limits of each.

A simple way to picture it: a person weighing 80 kg with 25% body fat has 20 kg of fat and 60 kg of everything else. That 60 kg is the lean body mass. Water is a large part of it, which is why hydration affects some readings.

Lean body mass formula from body fat

1. 1Take your body weight in kilograms.
2. 2Convert the body fat percentage to a decimal by dividing by 100.
3. 3Subtract that decimal from 1 to get the lean share.
4. 4Multiply your weight by the lean share.

The result is only as good as the body fat figure behind it. If a scale overstates body fat by 3 points, it understates lean mass by about 2 kg at this weight. That is why the same device and conditions matter when you track changes.

Estimating lean body mass without body fat: the Boer formula

When you do not have a body fat reading, the Boer equation estimates lean mass from weight, height and sex. It is one of several published equations of this type and is widely used in clinical and research settings.

For a woman of 162 cm and 58 kg, the women's formula gives 0.252 × 58 + 0.473 × 162 − 48.3 = 42.9 kg. In both cases the figure is an estimate for a typical body of that size, not a measurement of you.

Both versions are linear in weight and height. Each extra kilogram of weight adds 0.407 kg of lean mass for men and 0.252 kg for women, and each extra centimetre of height adds 0.267 kg and 0.473 kg respectively. The formulas assume that most of any weight difference between people of the same height is lean tissue.

Because the formula never sees how much fat you carry, two people of the same height and weight get the same answer. Use it when nothing better is available, and prefer a measured body fat percentage when you have one.

Reading weight change: lean versus fat

Weight alone cannot say what changed. The table compares three situations for a person who starts at 80 kg with 25% body fat, which is 60 kg lean and 20 kg fat. The numbers are illustrative arithmetic, not typical results.

Both later rows show 6 kg lost. In the first, about 5.2 kg came from fat and 0.8 kg from lean mass. In the second, about 3.0 kg came from fat and 3.0 kg from lean mass. The scale reads the same in both cases, but the bodies are not the same.

This is why people who track composition look at weight, body fat percentage and lean mass together. A fall in lean mass during weight loss is worth discussing with a doctor or dietitian, especially for older adults or anyone with a medical condition.

Lean body mass and resting energy use

Lean tissue uses more energy at rest than fat does, so some equations estimate basal metabolic rate (BMR) from lean mass. The Katch–McArdle equation is one of them: BMR = 370 + 21.6 × lean body mass (kg).

Each extra kilogram of lean mass adds about 21.6 kcal a day in this model. The BMR calculator on this page uses a different equation, Mifflin–St Jeor, which starts from weight, height, age and sex, so its figure will not match exactly. All of these are estimates and are not a prescription for how much to eat.

The practical point is the direction, not the decimals. If lean mass falls during a period of weight loss, resting energy use tends to fall with it. Keeping lean mass while losing fat is a common aim, but how to do it depends on the person and is best discussed with a professional.

Mistakes people make with lean mass

• Calling lean body mass "muscle mass". Lean mass also includes bone, organs and water.
• Using a body fat reading from one device and a lean mass reading from another.
• Treating a formula estimate, such as Boer, as if it were a scan.
• Reading a one-day change as a gain or loss of muscle when it is mostly water.
• Setting a target that assumes lean mass never changes, when training and food intake both affect it.
• Ignoring age and illness. Lean mass can fall with age or during illness, and standard formulas may not apply well to older adults or people with medical conditions.

For the fat side of the same arithmetic, see the guide to body fat percentage. The BMI guide covers the height-and-weight measure that many lean mass formulas build on.