To calculate your lean body mass, multiply your body weight by your body fat percentage, and subtract the result from your body weight. The answer is your lean body mass. A lean body mass calculator is available on BodyBuilding.com, as of July 2015.
To use the body mass calculator at Bodybuilding.com, visit the site's home page, and place the cursor over Articles & Videos. On the drop-down menu that appears, click on Workouts. On the side menu, click on Tracking under the Workouts heading. Click on Measuring Body Fat under the Tracking heading. Select Calculate Your Lean Body Mass! Free Calculator.
Enter your body weight, body fat percentage and select either pounds or kilograms. Click on Calculate for your lean body mass.
Lean body mass refers to any body weight that isn't fat. This doesn't mean lean body mass is completely free of fat. Bone marrow, internal organs and the central nervous system are all considered lean body mass but contain small amounts of fat.
The only accurate way to determine lean body mass is by using body fat percentage. The most accurate methods of determining body fat percentage are DEXA scans and hydrostatic weighing. Skin fold calipers are a cheaper, more convenient method of measuring body fat that tend to be accurate when used correctly. Other methods of measuring body fat include bioelectric impedance analysis and body circumference measurements, but these methods are less accurate.