Lean Body Mass Calculator

Lean body mass, or fat-free mass, is everything in your body that is not fat: muscle, bone, organs, skin and water. It is a more meaningful number than total weight for anyone who lifts, diets or simply wants to age well, because muscle and bone are what keep you strong and metabolically healthy, while fat mass is the part that rises and falls with calorie balance.

The most reliable way to get it is to know your body-fat percentage. Lean body mass is then simply your weight multiplied by one minus the fat fraction. At 80 kg and 20% body fat, lean mass is 80 × 0.8 = 64 kg and fat mass is 16 kg. Both numbers matter, and the calculator shows both so you can tell whether a drop on the scale came from fat or from muscle.

When you do not know your body fat, the Boer formula estimates lean mass directly from height and weight. It treats the body as a predictable mix of tissue and assumes that taller and heavier people carry proportionally more lean mass, with separate coefficients for men and women. The estimate is reasonable for a general population but, like every equation of its kind, it can be off by several kilograms for athletes, people with obesity, or anyone at the extremes of height.

The practical use is tracking. During a fat-loss phase you want lean mass to stay flat while fat mass falls. If lean mass is dropping quickly, the usual culprits are too steep a calorie deficit, too little protein, or no resistance training. Keeping protein high and lifting weights while you diet is the most reliable way to protect lean mass. Re-measure every four to six weeks rather than daily; body water swings are large enough to hide a month of real progress.

This calculator returns estimates for general information only. It is not medical advice, diagnosis or treatment, and it cannot replace an assessment by a qualified health professional. Individual results vary, and figures such as calorie needs, body-fat percentage and heart-rate zones are approximations based on population averages rather than measurements of you.