# Lean body weight or mass,  a waste of arithmetic

Sometimes, people ask me to explain what is "lean body weight".  It's not what you think.  It has nothing to do with your ideal weight, or what your body should be like if you were "lean".

Instead,  it refers to the sum of the weight of your bones, muscles and organs... basically the sum of everything other than fat in your body.

But who would like to have a body with ZERO fat?  Nobody.  In fact, you'd probably die if you had no fat.

So why is it included along with BMI on the body surface area calculator page?  Because it's kind of fun to think about,  thats all.   Also, some medical formulas use lean body mass for various calculations of organ function and medication doses.

Maybe you are wondering why your lean body mass goes up,  as your weight goes up,  even though your height is unchanged.  The reason is that the skeletal muscle mass tends to increase as body fat increases.  It takes extra muscle to carry extra fat around, right?

Hope this helps.

The formula for lean body mass using the method of James1,2 is:

Lean Body Weight (men) = (1.10 x Weight(kg)) - 128 x ( Weight2/(100 x Height(m))2)
Lean Body Weight (women) = (1.07 x Weight(kg)) - 148 x ( Weight2/(100 x Height(m))2)

An alternate formula for lean body mass using the method of Hume3 is

For men over the age of 16: lean body mass in kilograms = (0.32810 * (body weight in kilograms)) + (0.33929 * (height in centimeters)) - 29.5336

For women over the age of 30: lean body mass in kilograms = (0.29569 * (body weight in kilograms)) + (0.41813 * (height in centimeters)) - 43.2933

These formulas are trusted and highly scientific, based on various types of measurements of human body composition, including dual energy X-ray absorptiometry (DEXA). However, remember that they are based on "averages". They predict the lean body weight "average" of a group of people with similar height and weight.

But you, as an individual, might have more muscle than others in your group, or bigger internal organs, or denser bones, or any number of factors that make you a little different than others. So even the lean body weight formulas are estimates.

References:

1. Hallynck TH Soep HH et al. Should clearance be normalised to body surface or to lean body mass? Br J Clin Pharmacol. 1981; 11: 523-526.
2. James WPT. Research on obesity. London. Her Majesty's Stationery Office.
3. Hume R. Prediction of lean body mass from height and weight. J Clin Path. 1966; 19