Lean Body Mass Calculator

Lean body mass is everything you carry that is not fat: muscle, bone, organs, water. Scale weight hides it. Two people at 80 kg can hold wildly different amounts of muscle, and the scale never tells you which. Enter your sex, weight and height above and the calculator returns your lean mass. Add a body fat % and it splits your weight into lean and fat for you.

How the calculator works

With no body fat figure, it uses the Boer formula. Boer estimates lean mass from sex, weight and height, and it lands close for most people in a normal body composition range. For a 180 cm man at 80 kg it returns about 61.4 kg of lean mass.

The moment you type a body fat %, the math changes. Lean mass becomes weight times one minus your body fat fraction, and the rest is reported as fat mass. That reading beats any formula, because it is tied to your real composition instead of a population average. If you do not have a body fat number yet, get one with the body fat calculator first.

Why the number matters

Lean mass is the part of your weight that burns calories at rest and moves the barbell. When you diet, the goal is to drop fat while holding lean mass. When you bulk, the goal is the reverse. Tracking lean mass instead of scale weight tells you which way your body is moving.

It also feeds other metrics. Your FFMI is lean mass scaled to your height, and it shows where you sit against the natural muscle ceiling.

Reading your result

Retest at the same body fat range every few weeks. Lean mass shifts slowly, so a single jump usually means a noisy body fat read, not new muscle overnight. Look at the trend across a month or two.

To turn this into a plan that adds muscle while keeping fat in check, read the body recomposition guide.