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FFMI Calculator

Find your fat-free mass index.

How it is calculated

The fat-free mass index compares your lean mass with your height. First the lean mass is found: FFM = weight × (1 − bodyfat ÷ 100), with weight in kg and body fat in percent. Then FFMI = FFM ÷ h² + 6.1 × (1.8 − h), with height h in metres (the form takes centimetres and divides by 100). The second term is the height-normalised (adjusted) version: it adds a small bonus for people shorter than 1.8 m and subtracts for taller ones. The result is rounded to one decimal. The tool does not classify the value; body-fat input is the weak point, since estimates from scales or calipers carry errors.

Worked example

Weight 80 kg, height 180 cm, body fat 15%. Lean mass: 80 × (1 − 0.15) = 68.0 kg. Height in metres: 1.80, so h² = 3.24. Raw index: 68 ÷ 3.24 = 20.99. The adjustment 6.1 × (1.8 − 1.8) is 0, because this person is exactly 1.8 m tall. The FFMI is 21.0, with 68.0 kg of fat-free mass.

Try this example →

Common mistakes

  • Entering height in metres or feet. The field expects centimetres; typing 1.8 would give an absurdly large index.
  • Entering body fat as a fraction (0.15) instead of a percent (15).
  • Using BMI-style total weight as lean mass. Only the fat-free part counts, so the body-fat figure matters a lot.
  • Trusting a rough body-fat estimate. A 3-point error in body fat shifts the FFMI by several tenths.

About this calculator

This calculator finds your Fat-Free Mass Index (FFMI) — your fat-free (lean) mass in kilograms divided by height in meters squared, the same idea as BMI but stripped of body fat. It usually also reports a height-normalized FFMI, which adjusts the score to what it would be at 1.8m, so people of different heights can be compared fairly.

Bodybuilders, strength athletes and coaches use FFMI to track how much muscle someone is actually carrying rather than just total weight, and natural lifters use it as a rough benchmark since a normalized FFMI above roughly 25 is unusual without performance-enhancing drugs. It needs your weight, height and body fat percentage to work.

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