BMI divides weight by height squared and treats every kilogram alike, so it cannot tell a muscular athlete from a heavy person with little muscle. The fat-free mass index fixes part of this problem. It removes the fat from the weight first, then divides the remaining lean mass by height squared. The result describes how much muscle and other fat-free tissue you carry for your height, and it is widely used in sports science and by lifters to track muscular development.
This calculator gives both the raw index and the height-normalized version, so you can compare yourself with published reference points.
How to use the FFMI calculator
- Enter your body weight in kilograms or pounds.
- Enter your height in metres, centimetres, inches or feet.
- Enter your body fat percentage from a caliper test, a scan or an estimate such as the Skinfold Body Fat Calculator.
- Read your lean mass, fat mass, FFMI and normalized FFMI.
FFMI formula
FFMI = lean mass (kg) ÷ height (m)²
Normalized FFMI = FFMI + 6.1 × (1.8 − height (m))
The index and its height adjustment were described by Kouri EM, Pope HG, Katz DL and Oliva P in Clinical Journal of Sport Medicine 1995;5:223–228, a study of male athletes that compared users and non-users of anabolic steroids. The adjustment accounts for lean mass not rising exactly with the square of height; the choice of 1.80 m is a reference height.
Worked example
A man weighing 80 kg, 1.80 m tall, with 12% body fat.
- Lean mass = 80 × (1 − 0.12) = 70.4 kg; fat mass = 9.6 kg
- FFMI = 70.4 ÷ 1.80² = 21.7
- Normalized FFMI = 21.7 + 6.1 × (1.8 − 1.8) = 21.7 (same, since he is 1.80 m)
A man at 1.88 m, 85 kg and 10% fat has an FFMI of 21.6 and a normalized value of 21.2.
Lean mass at common FFMI values
| Height | Lean mass at FFMI 18 | FFMI 20 | FFMI 22 | FFMI 25 |
|---|---|---|---|---|
| 165 cm | 49.0 kg | 54.4 kg | 59.9 kg | 68.1 kg |
| 170 cm | 52.0 kg | 57.8 kg | 63.6 kg | 72.2 kg |
| 175 cm | 55.1 kg | 61.2 kg | 67.4 kg | 76.6 kg |
| 180 cm | 58.3 kg | 64.8 kg | 71.3 kg | 81.0 kg |
| 185 cm | 61.6 kg | 68.5 kg | 75.3 kg | 85.6 kg |
| 190 cm | 65.0 kg | 72.2 kg | 79.4 kg | 90.2 kg |
How to read your FFMI
- Higher means more lean tissue for your height. The values are not a score of health or fitness.
- The 25 benchmark. Kouri et al. found that non-steroid-using athletes in their sample rarely exceeded a normalized FFMI of about 25. It is a rough observation from a small group, not a law.
- Everyday ranges are far lower. Most untrained adults have FFMI values in the high teens to low twenties for men and mid-teens to high teens for women.
- Use the trend. If your FFMI rises while body fat holds steady, you are building lean tissue.
Limits
FFMI is only as accurate as the body fat figure behind it. Fat-free mass also includes bone, organs and water, so a high value can reflect hydration or a large frame as well as muscle. It was developed in men and is less well studied in women, children and older adults. To estimate lean mass from height and weight alone use the Lean Body Mass Calculator; for an overall view see the Body Fat Calculator and BMI Calculator. Protein intake advice for building muscle is in the Protein Intake Calculator, and strength estimates are in the One Rep Max Calculator.
This page is for information only and is not medical or dietary advice. Speak to a doctor or registered dietitian about your own situation.
Sources
- Kouri EM, Pope HG, Katz DL, Oliva P. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clin J Sport Med 1995;5:223–228.
- VanItallie TB, Yang MU, Heymsfield SB, Funk RC, Boileau RA. Height-normalized indices of the body’s fat-free mass and fat mass: potentially useful indicators of nutritional status. Am J Clin Nutr 1990;52:953–959.
Frequently asked questions
What is FFMI?
Fat-free mass index is lean mass in kilograms divided by height in metres squared. It is the body-composition equivalent of BMI but excludes fat, so it is a better yardstick for how much muscle someone carries relative to their size.
What is normalized FFMI?
Taller people have a slightly higher raw FFMI at the same level of development, so Kouri and colleagues (1995) proposed adjusting the value to a height of 1.80 m: normalized FFMI = FFMI + 6.1 × (1.8 − height in metres). It allows fairer comparison between people of different heights.
Is there a natural limit to FFMI?
In the 1995 study of 157 male athletes, the non-steroid-using athletes had normalized FFMI values that did not exceed about 25, while some steroid users went higher. This has become a popular rule of thumb, but it comes from a small sample of men and should not be treated as a hard limit or used to judge individuals.
How accurate is the result?
It depends entirely on the body fat figure you enter. Estimates from skinfolds or tape formulas can be off by several percentage points, and that error flows straight into the lean mass. DEXA scans are more reliable.
Does FFMI apply to women?
The formula works for anyone, but the 25 rule of thumb came from men. Women typically have lower FFMI values at the same fitness level because of lower average lean mass, and there is no equivalent well-established limit.