FFMI Calculator
Muscle mass relative to height — what BMI would be if it ignored fat.
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The index that separates muscle from mass
Fat-free mass index applies BMI's arithmetic to lean tissue alone: strip out the fat, then divide what remains by height squared. It answers the question BMI cannot — how much muscle are you carrying for your frame? — and it is why a lean athlete reading "obese" on BMI can read entirely normal here.
The figure is usually quoted normalised to a 1.8 m reference height, because raw FFMI still slightly favours taller people. The adjustment adds 6.1 × (1.8 − height in metres), so a 1.7 m lifter gains about 0.6 and a 1.9 m lifter loses about 0.6. When FFMI figures are compared, it is almost always the normalised one.
Rough interpretation: under 18 is below average, 20-22 is above average, and 22-25 is well-muscled. The much-repeated claim that 25 is a hard natural ceiling comes from a 1995 study of steroid users and non-users, and it is better read as a soft upper bound than a law — genuine outliers exist. The bigger caveat is input quality: FFMI is only as good as the body fat figure you feed it, and a tape estimate carrying ±4 points of error moves FFMI by roughly a full unit.
Medical disclaimer. This calculator is for general information and education only. It is not medical advice, not a diagnosis, and not a substitute for professional care. FFMI is a research and training metric rather than a health assessment, and it inherits all the error in whatever body fat measurement you supply.Speak to a doctor or another qualified clinician about any health decision.Last reviewed .
Frequently asked questions
What is a good FFMI?
Below 18 is below average, 18-20 is average, 20-22 is above average, and 22-25 indicates a well-muscled physique. Most untrained men sit around 18-19 and most untrained women around 14-15.
Is FFMI over 25 impossible naturally?
Not impossible, but unusual. The figure comes from a 1995 study comparing steroid users with non-users, where few drug-free subjects exceeded 25. Genuine natural outliers exist, particularly among very tall or heavily built people, so treat it as a soft ceiling rather than proof of anything.
Why normalise to 1.8 metres?
Because raw FFMI still carries a slight height bias, as BMI does. The normalisation adds 6.1 × (1.8 − height in metres), raising short lifters and lowering tall ones so the figures compare fairly.
How accurate is FFMI?
Only as accurate as the body fat measurement behind it. A tape-based estimate with ±4 percentage points of error shifts FFMI by roughly a full unit, which is the difference between two categories. DEXA input gives a far more meaningful figure.
How is FFMI different from BMI?
BMI divides total weight by height squared and cannot tell muscle from fat. FFMI removes fat first, so it measures lean mass relative to height. A muscular athlete often reads overweight on BMI and entirely normal on FFMI.