Lean Body Mass Calculator

Lean body mass is everything in your body that isn't fat: muscle, bone, organs and water. Choose male or female, then enter height in centimetres and weight in kilograms. The tool applies a formula it labels Boer, with separate versions for men and women, and shows three lines:

• Lean body mass in kilograms.
• Body fat mass, which is simply your weight minus the lean mass.
• Lean mass as a percentage of your weight.

The defaults, a man at 175 cm and 70 kg, give 56.0 kg lean, 14.0 kg fat mass and 80.0%. These are estimates from two measurements, not a body scan and not a diagnosis.

A woman who is 165 cm tall and weighs 62 kg gets 45.4 kg of lean mass, 16.6 kg of fat mass and a lean share of 73.2%. The same height and weight entered as male gives a noticeably higher lean figure, because the two versions of the formula weight height and body weight differently.

Height counts for a lot in this formula. Two people of the same weight but different heights get different lean estimates, with the taller one assigned more lean mass.

The estimate moves in a straight line with your weight. Take the default man and raise his weight from 70 kg to 90 kg at the same height: lean mass goes from 56.0 to 64.2 kg, fat mass from 14.0 to 25.8 kg, and the lean share drops to 71.3%. Of the 20 kg gained, the formula counts 8.2 kg as lean.

That split is built into the formula, not measured. It will give the same answer whether the extra weight is muscle or fat, so it can't tell you which one you gained or lost.

The Body Fat Percentage Calculator on this site works differently, from neck and waist measurements. For the default man with a 38 cm neck and 85 cm waist it estimates 16.9% fat, which works out at about 58.1 kg of lean mass, against 56.0 kg here. Neither number is the true value. The tape method at least responds to a change in your waist, so it can register a change in shape that this formula can't.