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Fat-Free Mass Index (FFMI) Calculator

Enter your height, weight and body-fat percentage to calculate your FFMI, height-normalised FFMI, estimated fat-free mass and estimated fat mass.

Your details

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Not sure of your body-fat percentage? Use the body fat percentage calculator first. FFMI and the estimated mass values depend on the percentage entered.
Height-normalised FFMI
21.6 kg/m²
FFMI
21.5 kg/m²
Estimated fat-free mass
68.0 kg
Estimated fat mass
12.0 kg

Your height-normalised FFMI is 21.6 kg/m². It is calculated from the body-fat percentage entered and adjusted to a reference height of 1.80 m; it is not a direct measurement of muscle mass or an overall health assessment.

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.

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.

FFMI = Estimated fat-free mass [kg] ÷ Height [m]²
  • Estimated fat-free mass = Weight [kg] × (1 − Body-fat percentage ÷ 100)
  • Estimated fat mass = Weight [kg] − Estimated fat-free mass
  • Height-normalised FFMI = FFMI + 6.3 × (1.80 − Height in metres)

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

What is FFMI?

Fat-Free Mass Index (FFMI) is a formula-based estimate: your estimated fat-free mass in kilograms divided by your height in metres, squared. This calculator works it out from the height, weight and body-fat percentage you enter, and also shows the height-normalised version, your estimated fat-free mass and your estimated fat mass.

How is FFMI calculated?

Height is converted to metres and weight to kilograms. Estimated fat-free mass is weight × (1 − body-fat percentage ÷ 100). FFMI is that fat-free mass divided by height in metres squared. Height-normalised FFMI adds 6.3 × (1.80 − height in metres) to adjust the figure to a 1.80 m reference height.

What does height-normalisation do?

The height-normalisation step adjusts FFMI to what it would be at a reference height of 1.80 m, so figures for people of different heights can be read on the same scale. It is a mathematical adjustment, not a direct measure of muscle mass or a personal comparison standard.

Why does the body-fat percentage I enter matter so much?

FFMI, estimated fat-free mass and estimated fat mass are all derived from the body-fat percentage you enter. Even a small error in that figure moves every one of these results, so FFMI is only as reliable as that input and should be read as an estimate rather than a measured body-composition result.

Is FFMI a measure of muscle mass or health?

No. FFMI is calculated from height, weight and an entered body-fat percentage. It does not directly measure skeletal muscle, body fat, bone, water or organ mass, and it is not a measure of overall health, athletic ability or nutritional status. It should not be used to diagnose a condition or set a personalised weight, training or nutrition target.

Where do I get my body-fat percentage?

If you don't already know it, use the body fat percentage calculator first, then enter that figure here. FFMI and the estimated mass values depend entirely on the percentage entered.