Your lean body mass is your total body weight minus body fat. It’s very hard to calculate this number perfectly without advanced testing, but you can try. Figure out your body fat percentage from measuring using a handheld device, a digital scale that tracks body fat, or get a local trainer at your gym to measure your body fat. If none of these are options, you can search online for body fat measurement pictures. Your total body weight times your body fat percentage will tell you how many pounds of fat you carry, and your total weight minus fat calculation gives you your lean body mass weight.