FFMI Calculator

Fat-free mass index divides estimated fat-free mass in kilograms by height in meters squared. Fat-free mass includes water, bone, organs and muscle. FFMI therefore describes estimated fat-free tissue relative to height; it is not a direct measurement of muscle.

The calculator needs total body weight, height and a body-fat percentage obtained elsewhere. It does not measure body fat. Two people of the same height and weight can have different FFMI values when their estimated fat proportions differ.

Estimated fat-free mass (kg) = weight (kg) × (1 − body fat (%) / 100).

Raw FFMI (kg/m²) = estimated fat-free mass (kg) / height (m)².

At 80 kg, 175 cm and 15% estimated body fat, fat-free mass is 80 × 0.85 = 68 kg. Raw FFMI is 68 / 1.75² = 22.2041 kg/m², displayed as 22.20. Calculations retain precision until display.

Metric height is entered in centimeters; imperial height is total inches. The tool converts inches using 0.0254 m/in and pounds using 0.45359237 kg/lb before calculating. Weight and height must be finite and positive, and body fat must be between 0 and 100%. Mathematical endpoint values are not evidence of a valid body-composition measurement.

The result is an index, not a fitness grade, population percentile or personal muscle target. This tool does not assign status bands or infer use of anabolic-androgenic steroids.

An error in body-fat percentage changes the estimated fat-free mass and FFMI. In the example above, changing body fat from 15% to 18% changes fat-free mass from 68 to 65.6 kg and raw FFMI from 22.20 to 21.42, even if weight and height stay the same.

A precise decimal display describes the arithmetic, not measurement accuracy. Consider the body-fat method and measurement conditions when comparing results.

Select the optional checkbox to display the height transformation reported in the abstract of Kouri et al., published in October 1995:

Kouri normalized FFMI = raw FFMI + 6.3 × (1.80 − height in meters).

For the worked example, this adds 6.3 × 0.05 = 0.315, producing 22.5191, displayed as 22.52 kg/m². At 1.80 m the correction is zero. The raw and normalized values remain separately labeled.

The source studied 157 male athletes: 83 AAS users and 74 nonusers. That population does not provide a female reference scale or a universal biological ceiling. An observation in that sample cannot establish an individual natural limit or determine drug use.

Record the input measurements and body-fat method with each result. Comparing estimates collected with consistent equipment and conditions is easier to interpret than comparing numbers from different measurement methods.

A change in FFMI may reflect changes in weight, estimated body fat or measurement error. It should not automatically be described as muscle gained or lost. The optional normalization is a specific published mathematical transformation; it does not remove differences in age, body composition or measurement quality.

How is this different from BMI?

BMI uses total body weight. FFMI uses the fat-free fraction estimated from the entered body-fat percentage. Both divide by height squared.

Can women calculate raw FFMI?

The arithmetic does not require sex. The optional 1995 height transformation came from male athletes and is shown with that limitation; the tool supplies no sex-specific score.

What if I do not know my body-fat percentage?

There is insufficient information to estimate fat-free mass from these inputs. Obtain a measurement or estimate with a documented method and retain its uncertainty.

Kouri, Pope, Katz and Oliva (October 1995), Fat-free mass index in users and nonusers of anabolic-androgenic steroids. The primary abstract states the raw formula, the +6.3 adjustment and the source population.

Sources and methods checked: 2026-09-30.