FFMI Calculator (Fat-Free Mass Index)
Work out your Fat-Free Mass Index from weight, height and body-fat percentage, with the height-normalized FFMI, fat and lean mass, a rating for men or women, the lean mass you would need for a goal FFMI and how sensitive the result is to your body-fat estimate.
How it works
Fat mass = weight × body fat% ÷ 100 · Fat-free mass (FFM) = weight − fat mass · FFMI = FFM (kg) ÷ height (m)² · Normalised FFMI = FFMI + 6.1 × (1.8 − height in m) · Fat mass index = fat mass ÷ height² · FFM for a goal = goal FFMI × height² · Weight at the goal = goal FFM ÷ (1 − goal body fat%)
FFMI (Fat-Free Mass Index) takes the fat out of your weight and asks how much lean mass you carry for your height. It first finds fat-free mass, which is everything that is not fat (muscle, bone, organs and water), then divides it by height squared, the same way BMI divides total weight. The normalised value adds 6.1 × (1.8 − height) so that people of different heights can be compared on a 1.8 m scale; it adds a little for shorter people and takes a little off for taller ones. The idea comes from a 1995 study of drug-free and steroid-using male athletes, in which the drug-free group topped out at a normalised FFMI of about 25. Choose male or female to get the matching reference bands, enter weight in kg, lb or stone and height in cm, inches or metres. Besides FFMI the calculator shows fat mass, the fat mass index, BMI, a table of the lean mass each band means at your height and a table of how your FFMI changes if your body-fat reading is a few points off. Under More options you can set a goal FFMI to see how much lean mass, and what body weight, that goal means for you.
Worked example
An 80 kg man who is 178 cm tall with 15% body fat carries 80 × 0.15 = 12 kg of fat and 68 kg of fat-free mass. Height squared is 1.78² = 3.1684 m², so his FFMI is 68 ÷ 3.1684 ≈ 21.5, which is above average for men. Normalised, it is 21.5 + 6.1 × (1.8 − 1.78) ≈ 21.6. To reach a normalised FFMI of 25 he would need about (25 − 0.12) × 3.1684 ≈ 78.8 kg of fat-free mass, roughly 10.8 kg more lean mass. If his body-fat reading were really 19%, his FFMI would be 20.5 instead.
Sources: Kouri EM, Pope HG et al. (1995), Fat-free mass index in users and nonusers of anabolic-androgenic steroids, Clin J Sport Med 5(4) · Kyle UG, Schutz Y et al. (2003), Body composition interpretation: contributions of the fat-free mass index and the body fat mass index, Nutrition 19
Frequently asked questions
What FFMI counts as muscular?
For men, roughly 20–22 is above average, 22–23 excellent and 23–26 superior. In a large Swiss reference study, adults with a normal BMI had an FFMI of about 16.7–19.8 (men) and 14.6–16.8 (women). For women the bands are about 3–4 points lower than for men: 15–17 is average and 18–21 is very muscular. These are rough guides, not medical categories.
Is an FFMI above 25 proof of steroid use?
No. In the 1995 study behind the index, drug-free male athletes topped out near a normalised FFMI of 25, so values above it are uncommon without drugs. But a few people exceed it naturally, and an underestimated body-fat reading can push FFMI up by a point or more.
Should I look at FFMI or normalised FFMI?
Use normalised FFMI when you compare people of different heights, or yourself with the 25 benchmark, because it adjusts everyone to a 1.8 m frame. For tracking your own progress over time either works, since your height does not change.
Why use FFMI instead of BMI?
BMI counts all weight the same, so a lean, muscular person can read as overweight. FFMI ignores fat and measures lean mass for height, and the fat mass index shown next to it tells you how much fat you carry for your height. Together they describe body composition much better than BMI.
How accurate does my body-fat number need to be?
FFMI is only as good as the body-fat estimate. A reading that is 4 points too low raises FFMI by about 1. A DEXA scan is the most reliable; calipers and tape methods are fine for tracking trends if you measure the same way each time.
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Educational estimate, not medical advice. Consult a qualified professional for health decisions.