Estimate your lean body mass and body fat from height, weight and sex, using several well-known formulas side by side.
Lean body mass is everything in your body except fat: bones, muscle, organs, blood, skin and water. It is your total weight minus your body fat weight. For most adults it falls between 60% and 90% of body weight, and men usually have a higher share than women. Doctors use lean body mass to set doses of some drugs, and people who train use it to follow muscle gain separately from fat loss.
There is no single agreed way to estimate lean body mass without a scan, so several formulas exist. The Boer and Hume formulas add up weight and height with different weights for men and women. The James formula starts from weight and takes off an amount that grows with the square of the ratio of weight to height. The calculator shows all three side by side so you can see the range. For children aged 14 or younger it adds the Peters formula, which was developed for them.
For each formula the calculator shows your lean body mass, what percentage of your weight it is, and the body fat percentage that is left over. The formulas are population averages built from height and weight alone, so they cannot tell a muscular person from a heavy one. Treat the result as an estimate, and expect it to drift for very small, very large or very muscular bodies, where a formula can give values that do not make sense.
Lean body mass includes a small amount of essential fat inside organs, while fat free mass excludes all fat. The difference is about 2% to 3% of body weight in men and 5% to 12% in women.
None is best for everyone. Boer is often used in medical settings, James and Hume are older formulas. Looking at all three together gives you a range instead of a single number.
Each one was fitted to a different group of people and uses height and weight in a different way, so the results differ by a few kilograms.
The formulas are straight-line or simple curve fits to typical adults. Outside the range they were built for, they can give a lean mass above your body weight or below zero.
It is a formula for children, which estimates lean body mass from an estimate of the volume of fluid outside the cells, based on weight and height. It is meant for children about 13 to 14 years old or younger.
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