Lean body mass · first principles
Lean Body Mass Calculator
Lean body mass is every gram of you that isn't fat: muscle, bone, organs, skin, water. Knowing it lets you set smarter nutrition and training targets.
What is lean body mass?
Lean body mass (LBM) is your weight minus fat mass. It's the engine that drives your metabolism, strength and function.
If you know your body fat %, the simplest formula is weight × (1 − bf/100). When you don't, formulas like Boer and Hume estimate LBM from height and weight alone.
Where lean mass sits on the muscle spectrum
Adjusted FFMI normalizes lean mass for height. Numbers are lower bounds for each band.
Bands are research-informed orientation, not normative targets. Genetic ceiling sits 1 to 2 points below the elite-natural threshold for most lifters.
Four ways to estimate lean mass
Direct uses your body fat. Boer, Hume and James estimate lean mass from height and weight when you don't have a body-fat reading.
The example numbers come from a 1.80 m, 80 kg male at 18% body fat. The 4 kg spread across formulas is the cost of not measuring body fat.
Why scale weight hides the lean-mass signal
During recomposition, fat goes down and lean mass goes up at roughly the same rate. The scale stays flat. Lean mass is the receipt that proves the trade actually happened.
Bodyweight is the sum of two stocks moving in opposite directions. Cutting half a kilo of fat while building half a kilo of muscle reads as zero on the scale, even though body composition has shifted measurably.
Lean body mass separates the two stocks. Multiplying weight by (1 − body-fat percentage) gives a kilogram-precise read on muscle, bone, organs and water. Track that number monthly and the recomposition is no longer invisible.
Lean mass also drives the calorie equation. Katch-McArdle BMR uses lean mass directly, sidestepping the age and gender approximations of Mifflin-St Jeor. The leaner you are, the more accurate Katch becomes relative to Mifflin.
Lean mass and recomposition, explained
Questions people search when the scale stays flat but the mirror keeps moving.
Recompose with Recomp AI
Lean mass is the receipt of recomposition. We track it daily.
Nutrition from first principles, guided by the best AI nutrition agent. Recomp AI ties lean mass to body fat, macros and weight trend in one daily chat, so the slope is always right there.