The formula
lean mass = weight × (1 − body fat ÷ 100)
Where it comes from
Lean body mass is everything that is not fat: muscle, bone, organs, connective tissue and the water in all of them. It is the figure that matters when losing weight, because a diet that costs lean mass lowers your metabolic rate and makes the weight easier to regain.
How to work it out by hand
- Estimate body fat percentage from BMI, age and sex (Deurenberg)
- Multiply your weight by that percentage for fat mass
- Subtract it from your weight for lean mass
- Divide lean mass by height squared for the lean mass index
What is worth knowing
The lean mass index is the more useful figure of the two, because it does for lean tissue what BMI does for total weight: it makes people of different heights comparable. Typical values run about 16 to 20 for men and 14 to 17 for women, and trained lifters go higher. The practical point is what protects it during a deficit: enough protein, around 1.6 to 2.2 g per kilo, and resistance training. Without both, roughly a quarter of the weight lost on a diet is lean tissue.