The Lean Body Mass Calculator splits your body weight into lean body mass (LBM) — your muscle, bone, organs and water — and fat mass. LBM is sometimes called fat-free mass, and tracking it is useful when you want to build muscle or make sure you keep it while losing fat.

The formulas

This tool offers three well-known equations that estimate LBM from your height, weight and gender: Boer (1984), often the default choice; James (1976); and Hume (1966). They give slightly different numbers, so it helps to compare them. Once LBM is known, your fat mass is simply your weight minus your lean mass, and your body fat % follows from that.

Muscle vs fat

Two people of the same weight can have very different body compositions. Someone with more lean mass and less fat tends to look leaner, be stronger, and have a healthier metabolism. That is why body composition is often more meaningful than weight alone.

A guide, not medical advice

These are estimates based on population averages and work best for people of average build; they can be off for very muscular or very lean bodies. For a precise reading, a DEXA scan or a quality body-composition scale is better. Nothing you enter is uploaded — the calculation runs entirely in your browser.