Lean Body Mass (James Formula) Calculator

About this calculator

Lean body mass is total body weight minus fat mass — everything that isn't fat, including muscle, bone, organs, and water. The James formula is one of several published equations that estimate it from easily measured inputs: weight, height, and sex, using separate coefficients for men and women rather than requiring a body-fat measurement device.

Because lean mass drives resting metabolic rate more than total weight does, it's used by clinicians, dietitians, and sports scientists to set protein intake targets, dose certain medications by lean mass rather than total body weight, and track whether a training or diet program is preserving muscle or just shedding weight. This calculator takes your weight, height, and sex and returns your estimated lean body mass using the James equation.