How to Calculate Lean Body Mass

If you have a body fat reading, calculating lean body mass is a single multiplication and needs no formula at all. Your body is fat plus everything else, so the everything-else — the lean part — is simply the fraction of your weight that is not fat. Take someone who weighs 80 kg with 20% body fat: the lean fraction is 100% − 20% = 80%, so lean body mass is 80 × 0.80 = 64 kg, and their fat mass is the remaining 16 kg. That is the whole method, and because it starts from a real measurement of your body rather than a population average, it is the most accurate route there is. You can get the body fat figure it needs from calipers, a body-composition scale, a DEXA scan, or the circumference-based Body Fat % Calculator.

Most people do not have a body fat reading to hand, and this is where the formulas come in. Each takes your height, weight and sex and returns an estimate of lean mass. The Boer (1984) formula is a weighted sum of your weight and height with a sex constant; the James (1976) formula uses a weight-over-height-squared term that reads lower for heavier builds; and the Hume (1966) formula came out of total-body-water research. You run the same one number — your height and weight — through all three and get three slightly different answers. The tool above does this for you and shows each result alongside the average, so you can see the spread rather than trusting a single equation.

The three formulas rarely land on the same number, and that is expected: each was fitted to a different group of people decades apart, so each carries the quirks of its own study population. For a typical adult they usually sit within a few kilograms of one another, and taking the average is a simple way to hedge between them rather than betting on one being right. None of them can see your actual muscle or fat, which is why a measured body fat percentage always wins when you have it. The practical takeaway is to let the tool do the arithmetic: enter your height, weight and sex into the Lean Body Mass Calculator for all three estimates at once, add a body fat percentage if you have it for the exact figure, and then feed the result into the Katch-McArdle Calculator to turn your lean mass into a metabolism estimate.