FFMI Calculator

Fat-free mass index (FFMI) describes how much lean mass you carry relative to your height, in the same way that BMI relates total weight to height. Enter your weight, height and body fat percentage, and the calculator gives your fat-free mass, your FFMI and the height-normalized FFMI used in research and by many strength athletes. Because it removes fat from the calculation, FFMI separates a muscular person from someone of the same weight who carries more fat, which BMI cannot do.

fat-free mass (kg) = weight × (1 − body fat ÷ 100)

FFMI = fat-free mass ÷ height (m)²

normalized FFMI = FFMI + 6.1 × (1.8 − height in meters)

The formulas come from a 1995 study by Kouri, Pope, Katz and Oliva. The normalization adjusts values to a reference height of 1.8 meters, because taller people tend to have slightly lower FFMI for the same build.

80 kg, 180 cm, 15% body fat. Fat-free mass is 68 kg. FFMI is 68 ÷ 1.8² = 21.0, and since the height is exactly 1.8 m, the normalized value is the same.

70 kg, 170 cm, 20% body fat. Fat-free mass is 56 kg and FFMI is 19.4. The height adjustment adds 6.1 × 0.1, giving a normalized FFMI of 20.0.

Kouri and colleagues measured 157 male athletes, including users and non-users of anabolic steroids. The non-users had normalized FFMI values up to about 25, while many users exceeded that figure, so 25 is often quoted as an approximate upper limit for drug-free men. It is a guide from one study of men, not a hard rule: a small number of people exceed it naturally, measurement error can push a result over, and the study did not set comparable figures for women, who typically have lower values.

1. Weigh yourself at a consistent time, such as in the morning before eating.
2. Enter your height without shoes, in centimeters or inches.
3. Enter a body fat percentage from the most reliable method you have. If you do not have one, the body fat calculator estimates it from tape measurements.
4. Read the normalized FFMI, and recalculate every few months to track change rather than focusing on a single reading.

FFMI is most useful for tracking your own progress over time. If your weight rises while FFMI also rises, you are adding lean mass; if FFMI stays flat while weight rises, most of the gain is fat. Clinicians also use fat-free mass index to identify low muscle mass in conditions such as chronic illness, where very low values can signal malnutrition. For overall weight status, see the BMI calculator, and for lean mass estimates without a body fat measurement, the lean body mass calculator uses height-and-weight formulas.