Lean Body Mass Estimate Calculator View Calculator Get an estimate of your body's lean mass by entering your height and weight. This calculator uses the James formula which applies to adults and children of all ages. Your lean body mass is the sum of all your nonfat parts including muscle, tissue, organs, blood, and water. Don't confuse lean body mass with ideal weight - essential fat is 10% for women and 2% for men, and acceptable fat is up to 31% for women and 25% for men. Learn more about putting this calculator or other calculators on your website.