FFMI Calculator

BMI’s biggest limitation, as our own BMI Calculator explains, is that it can’t tell fat from muscle. FFMI exists specifically to answer the question BMI can’t: how much of your body, relative to your height, is actually lean mass. Think of it as BMI’s counterpart for muscularity instead of overall size.

What This Calculator Measures

This estimates your fat-free mass relative to height, height-adjusted so taller and shorter people can be fairly compared. It says nothing about your body fat percentage directly, pair it with the Body Fat Calculator for that.

The Formula

Fat-Free Mass (kg) = Weight (kg) × (1 − Body Fat% ÷ 100)

FFMI = Fat-Free Mass (kg) ÷ Height (m)²

Normalized FFMI = FFMI + 6.3 × (1.8 − Height in m), a height adjustment introduced by the researchers who developed the metric so that people of different heights carrying proportionally similar amounts of muscle land on comparable numbers¹.

How This Calculator Computes Your Result

Enter your height, weight, and body fat percentage above. We calculate your fat-free mass, divide by height squared for raw FFMI, then apply the height normalization to give you a number comparable across different heights.

What Your Result Means

This range is drawn from two separate data sources. A large reference sample of apparently healthy adults found a median FFMI of about 18.9 for young men and 15.4 for young women², which anchors the average band. The upper end comes from the original research establishing FFMI as a screening measure, which examined 157 competitive athletes and found that drug-free competitors topped out around 25, while many steroid users exceeded it, some by a considerable margin¹. For women, typical values run several points lower across the same categories, reflecting average differences in muscle mass.

The labels above 22 are conventions used by FFMI calculators, not cut-offs from a study. The calculator labels results above 27 “Suspicion of steroid use”, but a number alone cannot show that. In the original study, 20 Mr. America winners from the pre-steroid era (1939 to 1959) had an estimated average FFMI of 25.4¹.

Worked Example

Take a man who is 5 ft 10 in (1.778 m), weighs 180 lb (81.6 kg) and has 15% body fat. His fat-free mass is 81.6 × 0.85 = 69.4 kg, and his raw FFMI is 69.4 ÷ 1.778² = 22.0. Because he is slightly shorter than 1.8 m, the normalization adds 6.3 × (1.8 − 1.778) = 0.1, for a normalized FFMI of 22.1, which the calculator labels Excellent.

Height matters more further from 1.8 m. At 15% body fat, a person who is 170 cm and 75 kg has a raw FFMI of 22.1 and a normalized FFMI of 22.7, while a person who is 190 cm and 95 kg has a raw FFMI of 22.4 and a normalized FFMI of 21.7. The shorter person moves up and the taller person moves down.

Limits of the Height Correction

The 6.3 × (1.8 − height) term is a slight, straight-line correction that rescales results to the height of a 1.8 m man, and it was set using a sample of 157 male athletes¹. It was not built for women, so treat the normalized number as approximate if you are a woman or much taller or shorter than 1.8 m. The calculator does not ask for sex and its labels are for men, so women should compare their result with the lower female reference values above² instead.

FFMI also needs a real body fat estimate. The Lean Body Mass Calculator works from sex, height and weight only, so it predicts typical muscularity and cannot show extra muscle from training. For FFMI, use a body fat percentage from a method that measures you, such as calipers or a scan.

When a Low FFMI Matters

At the low end, FFMI is also used in medicine. The European Society for Clinical Nutrition and Metabolism (ESPEN) treats an FFMI below 17 in men or 15 in women as low³. Under the newer GLIM criteria, which ESPEN and other nutrition societies now use, low muscle mass like this is one sign of malnutrition, alongside signs such as unintentional weight loss, and a diagnosis also needs a cause such as eating too little or an illness⁴. Compare these cut-offs with your raw FFMI, not the normalized one. A low FFMI on its own is not a diagnosis, but if you are also losing weight without trying, talk to a doctor.