Lean Body Mass Calculator

Lean body mass (LBM) is everything in your body that is not fat: muscle, bone, organs, skin and water. This calculator estimates it from your sex, height and weight with three published equations, Boer, James and Hume, and shows the matching fat mass. If you know your body fat percentage, it also calculates lean mass directly, which is usually more accurate.

Lean mass is used to set protein targets, to estimate energy needs with the Katch-McArdle equation and, in medicine, to dose certain drugs such as anesthetics that do not distribute into fat.

Boer, men: 0.407 × W + 0.267 × H − 19.2; women: 0.252 × W + 0.473 × H − 48.3

James, men: 1.1 × W − 128 × (W ÷ H)²; women: 1.07 × W − 148 × (W ÷ H)²

From body fat: LBM = W × (1 − body fat % ÷ 100)

W is weight in kilograms and H is height in centimeters. The Hume formula has the same linear form as Boer with different coefficients. All three were fitted to measurements in adults of typical build.

A man, 80 kg and 180 cm. Boer gives 61.4 kg, James 62.7 kg and Hume 57.8 kg, so about 77% of his weight is lean by the Boer estimate.

A woman, 140 lb and 5 ft 5 in. Boer gives about 101 lb of lean mass and 39 lb of fat mass.

A man, 185 lb with 15% body fat. Lean mass is 185 × 0.85 = 157 lb, compared with 138 lb from the Boer formula.

Boer is the most widely used in clinical practice and gives sensible values across a broad range of body sizes. James is often used in anesthesia, but it has a known flaw: at high body mass index the squared term grows so fast that the estimate starts to fall as weight rises, and it can give implausible results in severe obesity. For that reason, many anesthesia references now prefer the Janmahasatian formula for such patients. Hume tends to give the lowest values of the three. When you have a body fat measurement, the direct method beats all of them.

Dual-energy X-ray absorptiometry (DEXA) is the most common reference method outside research labs and separates fat, lean tissue and bone. Bioelectrical impedance scales are convenient but are sensitive to hydration and can be off by several percentage points. Skinfold calipers in trained hands and the Navy tape method give reasonable estimates for tracking change over time. Whatever method you use, measure under the same conditions each time and watch the trend over several weeks rather than any single reading, since day-to-day swings in water can mask real change.

Sports nutrition guidelines often express protein needs per kilogram of body weight, about 1.4 to 2.0 g/kg for active people, but some coaches use lean mass instead, which avoids overestimating needs for people with more body fat. Lean mass is also the input to the Katch-McArdle BMR equation. Tracking lean mass during weight loss shows whether you are losing fat or muscle; strength training and enough protein help preserve it. Lean mass also declines with age, by a few percent per decade after about 30 in inactive adults, which lowers resting energy use and strength. Regular resistance exercise slows that loss at any age, and older adults gain meaningful strength even when they start late.