Lean Body Mass Calculator — Free

The most direct way to find lean body mass is from a body composition measurement: once you know what fraction of your weight is fat, the rest is lean. That is why the measured method — weight × (1 − body fat% ÷ 100) — is the gold standard this calculator uses whenever you enter a body fat percentage. A DEXA scan, hydrostatic weighing or the US Navy circumference method all give you that percentage.

When no body fat figure is available, regression equations estimate LBM from the variables that correlate with it — height, weight and sex. The Boer (1984) formula was derived to normalise body-fluid volumes and is the most widely cited. The James (1976) formula uses a weight²/height² term, which pulls the estimate down at higher body-mass indexes (shorter or heavier builds). The Hume (1966) formula was built from total-body-water measurements and is still used in pharmacology for drug dosing. They were validated on different populations, so they rarely agree exactly — typically within a few kilograms — which is why showing all three plus their average is more honest than presenting one number as fact.

Lean body mass and fat mass are two halves of the same whole: lean mass % = 100 − body fat %. If you weigh 80 kg at 20% body fat, you carry 16 kg of fat and 64 kg of lean mass. This is why scale weight alone is misleading — two people at the identical weight can have very different amounts of muscle and fat, and look completely different.

You will also see lean body mass called fat-free mass (FFM), and for everyday use the two are interchangeable — so this tool doubles as a fat-free mass calculator. The only technical difference is that lean body mass counts the small amount of essential fat inside the bone marrow, nervous system and organs (about 2–3% of body weight), whereas fat-free mass excludes every gram of fat. The number you get above is your weight minus your fat mass either way.

For a precise lean-mass figure, measure your body fat first with the Body Fat % Calculator and enter the result above. Tracking BMI alongside it adds context — the BMI Calculator screens weight-for-height, but unlike lean body mass it can't tell muscle from fat, which is its main blind spot for athletes.

Lean mass is metabolically active, so it largely sets how many calories you burn at rest. That is exactly why the most accurate metabolism formula — Katch-McArdle — works from lean body mass rather than total weight:

1. Estimate your true metabolism. Feed your lean mass into the Katch-McArdle Calculator for a BMR and TDEE that reflect your body composition, not just your weight.
2. Set a deficit that protects muscle. Use the Calorie Deficit Calculator for a moderate target, then keep protein high and lift to hold onto lean mass while you lose fat.

The goal of a smart diet is fat loss, not weight loss: keeping lean body mass steady while fat mass drops is what produces a leaner, stronger, higher-metabolism result — and stops the scale from telling you a half-truth.