Lean Body Mass Calculator

Lean body mass (LBM) is everything in your body that is not stored fat — muscle, bone, organs, connective tissue, and water. It is the counterpart to fat mass: total weight is LBM plus fat mass. Most of the metabolically active tissue in your body is lean mass, which is why it is closely tied to your resting energy expenditure.

From your weight and body-fat percentage: LBM = Weight × (1 − Body Fat% / 100), and Fat Mass = Weight × (Body Fat% / 100). A 180 lb person at 20% body fat has 36 lb of fat and 144 lb of lean mass. The accuracy of the LBM figure depends entirely on the body-fat percentage you feed in — estimate that with the Body Fat Estimator.

Because the scale cannot tell fat from muscle. During a fat-loss phase the goal is to lose fat while keeping lean mass — tracking LBM tells you whether you are doing that or losing muscle too. It also reveals recomposition, where weight barely moves but fat falls and muscle rises. Weight alone would call that “no progress”; LBM shows the real change.

There are no universal thresholds for LBM — a healthy value varies widely by sex, age, height, and activity. A tall, athletic person has far more lean mass than a shorter, sedentary one, and neither is “right” in isolation. LBM is most useful as a personal baseline you track over time; the direction it moves matters more than the absolute figure.

Two levers do most of the work: resistance training, which signals the body to build and retain muscle, and adequate protein, the building blocks. This matters most during a calorie deficit, when the body sheds muscle unless training and protein say to keep it. Crash diets without resistance work lose a large share of weight as lean mass — a slower, protein-forward, strength-based approach protects it.