💪 Lean Body Mass Calculator
Enter your bodyweight, body fat percentage and height to calculate fat mass, lean body mass and your Fat-Free Mass Index (FFMI).
Four numbers from weight, body fat and height
Bodyweight is split into fat and everything else, then the remainder is divided by height squared. The last line is the Kouri normalisation, which rescales FFMI to a 1.8 m frame:
fat mass = weight × bodyfat% / 100
lean mass = weight - fat mass
FFMI = lean mass / height(m)²
normalised FFMI = FFMI + 6.1 × (1.8 - height(m))At 80 kg, 15% body fat and 180 cm: 12.0 kg fat, 68.0 kg lean, FFMI 68 / 1.8² = 21.0. At exactly 1.8 m the normalised value is 21.0 as well. A 170 cm lifter with the same lean mass scores 23.5 raw, 24.1 normalised.
What the inputs demand
- Metric only. Kilograms and centimetres; 176 pounds returns nonsense rather than an error.
- Body fat is your figure. Nothing here estimates it from measurements, so the result is only as good as the caliper, scale or scan reading you type.
- Lean mass is not muscle. It bundles bone, organs, blood and the water and glycogen that shift a kilo across a week.
Frequently asked questions
How do I calculate lean body mass?
Multiply bodyweight by your body fat percentage, then subtract that fat mass. At 80 kg and 15% it is 12 kg of fat and 68 kg of lean mass.
What is a good FFMI?
Around 19 is typical for untrained men and 22 to 23 reflects years of serious training. The drug-free ceiling near 25 that gets quoted came from a small 1995 study of male bodybuilders. Women's values run several points lower.
Why are there two FFMI numbers?
Raw FFMI favours short people, because lean mass does not scale with height squared alone. The normalised figure puts everyone on a 1.8 m reference.