Lean Body Mass Calculator
This lean body mass calculator finds your fat-free mass and fat mass. Enter your body-fat percentage for an exact figure, or estimate your lean body mass from height and weight with the Boer formula.
What lean body mass is
Lean body mass (LBM), also called fat-free mass, is everything in your body that isn't fat — muscle, bone, organs, connective tissue, and water. It's the mass you generally want to preserve (or grow) while losing fat.
Two ways to find it
Exact (preferred): if you know your body-fat percentage, lean mass is simply weight minus fat:
LBM = weight × (1 − body-fat % ÷ 100)
Fat mass = weight − LBM
Estimate (Boer, 1984): Men: LBM = 0.407·kg + 0.267·cm − 19.2
Women: LBM = 0.252·kg + 0.473·cm − 48.3
Worked example
Someone weighing 80 kg at 20% body fat: LBM = 80 × (1 − 0.20) = 64 kg, with 16 kg of fat mass. Without a body-fat reading, the Boer estimate for an 80 kg / 180 cm man is about 61.4 kg.
Why lean body mass matters
Lean mass largely sets your resting metabolism and your daily protein needs, and it's the tissue you're trying to keep during a cut or build during a recomp. It also feeds the Katch–McArdle equation in a BMR calculator.
Frequently asked questions
What is lean body mass?
- Your total weight minus fat mass — muscle, bone, organs, and water.
How do I calculate lean body mass?
- Multiply your weight by (1 − body-fat %/100); the remainder is fat mass. No body-fat reading? Estimate it from height and weight with the Boer formula.
Lean body mass vs fat-free mass — are they the same?
- They're used interchangeably here; strictly, fat-free mass excludes the small amount of essential fat inside cells.
Why does lean body mass matter?
- It drives your resting metabolism and protein needs and is the mass you want to keep while losing fat.
What's a good lean body mass?
- There's no single target — track whether it holds steady or rises while body-fat % falls.
Disclaimer. This calculator provides estimates for general information and is not a clinical body-composition measurement or medical advice.