The measure of lean body weight vs. fat. Lean body weight is the measure of non-fat tissue in the body such as bones, organs, and muscle. The most accurate method for determining body composition involves calculating the overall density of the body (weight / volume), and then applying a formula to get the overall fat percentage. Other, simpler methods also exist which can be applied by an individual. Generally, I believe (and many other experts agree) that body composition is a more important measure of overall fitness than weight. Read more.