Fat-Free Mass Index (FFMI) Calculator

How to use this calculator

1. Enter your height, and pick its unit — centimetres, metres, feet, inches, or feet and inches.
2. Enter your weight, and pick its unit — kilograms, pounds, or stones and pounds.
3. Enter your body-fat percentage. If you don't know it, use the body fat percentage calculator first.
4. Read your height-normalised FFMI, FFMI, estimated fat-free mass and estimated fat mass.

Nothing is submitted anywhere. The calculation runs in the page as you type, and the link in your address bar carries your figures, so it can be shared or bookmarked.

What do I need to enter?

• Height — from 130 to 220 cm (or the equivalent in metres, feet, inches, or feet and inches). Used squared in the FFMI and to adjust it to 1.80 m.
• Weight — from 30 to 300 kg (or the equivalent in pounds, or stones and pounds). Your total body weight, fat included.
• Body fat percentage — from 2% to 70%. It sets how much of your weight counts as fat, so every result moves with it.

What do the results show?

• Height-normalised FFMI — the main figure in kg/m², adjusted to a reference height of 1.80 m.
• FFMI — fat-free mass divided by height squared, with no height adjustment.
• Estimated fat-free mass — your weight minus estimated fat, in the weight unit you chose.
• Estimated fat mass — your weight multiplied by the body-fat percentage, in the same unit.

How FFMI is calculated

The calculator converts height to metres and weight to kilograms. It then estimates fat-free mass from the body-fat percentage entered.

The height-normalisation formula adjusts FFMI to a reference height of 1.80 m. It is a mathematical adjustment, not a direct measure of muscle mass or a personal comparison standard.

FFMI depends on the body-fat percentage entered. It is therefore only as reliable as that input and should be interpreted as an estimate rather than a measured body-composition result.

Worked example. A 178 cm, 80 kg person at 15% body fat has an estimated fat-free mass of 80 × (1 − 0.15) = 68.0 kg and an estimated fat mass of 80.0 − 68.0 = 12.0 kg. FFMI is 68.0 ÷ 1.78² = 21.5 kg/m², and height-normalised FFMI is 21.5 + 6.3 × (1.80 − 1.78) =21.6 kg/m².

FFMI vs. lean body mass vs. BMI

The lean body mass calculator estimates the same fat-free weight this calculator starts from, but from height and weight alone rather than a body-fat percentage. The BMI calculator also divides by height squared, but uses total weight rather than fat-free mass — which is why a very muscular person can have a high BMI and an unremarkable FFMI at the same time.

Limitations of FFMI

• The result is only as accurate as the body-fat percentage entered — every FFMI and mass figure moves with it.
• It does not directly measure skeletal muscle, body fat, bone, water or organ mass, and can be affected by hydration, recent weight change, pregnancy and body composition unlike the population a body-fat estimate was developed on.
• It is not a diagnosis, a body scan, a fitness score or medical advice, and says nothing about overall health, athletic ability, nutritional status or muscular potential.

Frequently asked questions