Lean Body Mass (James Formula) Calculator

How it is calculated

The James equation uses the ratio of weight to height squared in a quadratic term. For men: LBM = 1.1 × W − 128 × (W / H)². For women: LBM = 1.07 × W − 148 × (W / H)². W is weight in kilograms and H is height in centimetres. Fat mass is W − LBM and body fat is that figure divided by W, times 100. Any sex value other than female uses the male line. The tool gives no result for empty, zero or negative weight or height, nor when the lean mass would be negative or above total weight. Because the weight ratio is squared, lean mass can fall as weight rises at very high weights, so treat the output as a rough estimate.

Common mistakes

• Entering pounds or inches. The fields are kilograms and centimetres, and 165 cm typed as 65 (inches) makes the weight-to-height ratio huge and the tool returns nothing.
• Expecting more weight to always mean more lean mass. With the same 170 cm height, the engine gives 65.71 kg lean mass at 100 kg but only 42.84 kg at 200 kg, because the squared term grows faster than the weight term.
• Forgetting to pick the sex. Anything other than female uses the male coefficients, which give a higher lean mass.
• Treating body fat percent as a measured value. It is simply weight minus the estimate, so it ignores muscle mass, bone density and water.
• Entering height in metres (1.8). The result is blank, since the ratio becomes absurd.