Start with the calculation
| Step | Calculation | What it means |
|---|---|---|
| 1 | Fat-free mass = body weight × (1 − body fat / 100) | Everything in body weight that is not estimated fat mass |
| 2 | FFMI = fat-free mass ÷ height² | Fat-free mass adjusted for height |
| 3 | FMI = fat mass ÷ height² | Fat mass adjusted for height |
FFMI and fat mass index, or FMI, divide the two estimated parts of body weight by the same height-squared value. Before rounding, FFMI plus FMI equals BMI. The split depends on the body-fat percentage you enter, so a different tape, scan, or prediction method can change FFMI even when weight and height stay the same.
Fat-free mass is not muscle mass
Fat-free mass includes more than skeletal muscle. It includes water, bone, organs, and other non-fat tissue. A high or low FFMI result does not show how much skeletal muscle you have. It also does not show strength, training response, health, or a change in muscle tissue.
BodyNumeral calculates FFMI from weight, height, and a body-fat percentage that you provide. Keep the name of the body-fat method beside the result. Compare FFMI only when the body-fat method, height unit, and height-adjustment convention are the same.
Keep the height adjustment visible
Some calculators show an adjusted FFMI. The original 1995 abstract added 6.3 × (1.80 m − height in m) to set results against a height of 1.80 m. Other calculators use 6.1 instead. The two conventions can give different displayed values, especially when height differs from 1.80 m. Record the selected convention before comparing results from different calculators.
Use reference values with their population
Reference values depend on how fat-free mass was measured and who was studied. A 2002 cross-sectional study reported FFMI and FMI percentiles in 5,635 apparently healthy Caucasian adults in Switzerland, ages 24 to 98, using bioelectrical impedance with formulas cross-validated against dual-energy X-ray absorptiometry. Those percentiles do not become universal targets or diagnostic cutoffs.
Use a qualified professional when a body-composition estimate affects nutrition, training, or medical decisions. A calculator result can organize a method and an input set. It cannot replace a clinical assessment.
Make a later comparison repeatable
- Save the body-fat percentage and the method that produced it.
- Save body weight, height, units, date, and the height-adjustment convention.
- Compare results only after checking that the same calculation and body-fat method were used.
- Keep a scan, tape estimate, and prediction equation in separate series.
Open the FFMI calculator to inspect the arithmetic. Use the muscle-mass guide when you need to separate a skeletal-muscle estimate from fat-free mass.
Common questions
Is FFMI the same as muscle mass?
No. FFMI uses fat-free mass, which includes skeletal muscle, water, bone, organs, and other non-fat tissue. It cannot measure skeletal muscle directly.
Does FFMI prove that someone uses steroids?
No. The often-cited 25 comparison point came from one preliminary study of male athletes. It is not a test for drug use or a personal natural limit.
Why can two FFMI calculators disagree?
They can use different body-fat inputs, height-adjustment coefficients, units, or rounding rules. Keep the method and selected convention with the result.
CHECK THE EVIDENCESources & limitations3 sources
Sources checked September 20, 2026. These notes explain methods and assumptions. They have not received an independent clinical review.
- VanItallie and colleagues define FFMI as fat-free mass in kilograms divided by height in metres squared, and define a separate fat mass index.pubmed.ncbi.nlm.nih.gov
- Kouri and colleagues used a 6.3 height adjustment in a preliminary 1995 study of 157 male athletes that included steroid users and nonusers.pubmed.ncbi.nlm.nih.gov
- A 2002 Swiss cross-sectional study reported FFMI and FMI percentiles in 5,635 apparently healthy Caucasian adults aged 24 to 98, using bioelectrical impedance with formulas cross-validated against dual-energy X-ray absorptiometry.pubmed.ncbi.nlm.nih.gov
