What is the FFMI Calculator?
Fat-free mass index relates estimated fat-free mass to squared height. It separates estimated fat mass from the rest of body weight, unlike BMI, which uses total weight. Fat-free mass includes water, bone and organs as well as muscle; FFMI is not a direct muscle-mass measurement.
This FFMI calculator with body-fat percentage is intended for adult educational use. You supply the body-fat estimate. The Body Fat Calculator offers a separate circumference-based estimate, with its own measurement limitations.
How to calculate FFMI from height, weight and body fat
Choose metric (kilograms and centimetres) or imperial (pounds and total inches). Enter weight, height and body-fat percentage, then Calculate. Changing the unit system converts populated weight and height fields; editing any input clears the previous answer.
- Measure height without shoes and record body weight consistently.
- Use the body-fat estimate from your chosen measurement method.
- Leave height correction off for the standard FFMI, or select the explicitly labeled research correction.
- Read the FFMI, body-composition chart and actual-input steps together.
Optional body weight scales (paid link) can help record weight. No purchase is necessary.
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FFMI formula and optional height normalization
Fat mass = weight kg × body-fat percentage / 100. Fat-free mass = weight kg − fat mass. FFMI = fat-free mass kg / height m². One pound is 0.45359237 kg and one inch is 0.0254 m.
The optional correction follows the 1995 Kouri study abstract: corrected FFMI = FFMI + 6.3 × (1.80 − height m). Some later sources use 6.1; this calculator deliberately labels its 6.3 convention rather than silently mixing them. The main result always remains uncorrected FFMI. The correction came from male-athlete research and is not a universal clinical standard.
How to read the body-composition diagram
The bar represents your entered total weight. Gold is estimated fat-free mass and cyan is estimated fat mass. Both share one scale and sum to total weight; the chart is not an anatomical body scan. A new calculation redraws the proportions.
The numeric FFMI has units kg/m², even when you enter pounds and inches. Optional height measuring tapes (paid link) may help record height. Compare repeated measurements using the same body-fat method and conditions rather than treating minor changes as proven muscle gain.
FFMI worked example: 80 kg, 180 cm, 20% body fat
Fat mass = 80 × 0.20 = 16 kg. Fat-free mass = 80 − 16 = 64 kg. Height is 1.80 m, so FFMI = 64 / 1.80² ≈ 19.7531 kg/m². At 1.80 m the optional height correction is zero.
If the body-fat estimate were 22% instead, fat-free mass would be 62.4 kg and FFMI about 19.2593 kg/m². This difference illustrates why body-fat measurement uncertainty matters. Optional fitness journals (paid link) can keep the original measurements beside later readings.
Accuracy, body-fat uncertainty and supported scope
Supported entries represent 20–500 kg, height 1.00–2.50 m and body fat 0–99.9%. These are software limits, not recommended or normal ranges. Do not use this adult tool to interpret children, pregnancy, edema or clinical nutritional status.
Calculations use browser floating-point arithmetic and unrounded values; displayed numbers have up to 10 significant digits. Body-fat devices and circumference methods can disagree. FFMI alone cannot diagnose health, establish steroid use, determine an ideal body or prescribe training. Optional body composition books (paid link) provides background on measurement methods.
FFMI Calculator FAQs
No. FFMI uses estimated fat-free mass, whereas BMI uses total weight.
No. It also includes water, organs, bone and other non-fat tissues.
Yes. Choose imperial units and enter height as total inches; calculations convert to kilograms and metres.
It follows the coefficient printed in the 1995 Kouri abstract. Other conventions use 6.1; the main result remains standard uncorrected FFMI.
No. A single FFMI value cannot establish either.
No. This calculator processes them in browser memory without adding them to URLs or persistent storage. Existing site analytics and ads are described in the privacy policy.
Sources and methodology
Equations independently implemented and checked against the sources below, November 4, 2026. This is not a clinical validation or endorsement by the linked organizations.