Lean body mass is a genuinely useful number for setting protein targets and for a more accurate BMR estimate — but most people don't have a DEXA scan handy to measure it directly.
Boer formula calculator, estimating without a body fat test
The Boer formula estimates lean mass from just height, weight, and sex, using population-average body composition data. For an 80 kg, 180 cm man, it estimates 61.4 kg of lean mass — leaving an estimated 18.6 kg of fat mass.
Fat free mass calculator, why it's more useful than total weight
Total body weight includes fat, and fat tissue has very different metabolic and nutritional needs than muscle, bone, and organs. Protein recommendations are often expressed per kilogram of lean mass rather than total weight for exactly this reason.
Katch mcardle lean mass, the connection to BMR accuracy
If you know your actual lean mass (from a DEXA scan or a direct body fat percentage), the Katch-McArdle BMR formula uses it directly and tends to be more accurate than formulas based on total weight alone — especially for people with muscle mass well above or below average.
