FFMI Calculator
Measure muscularity with fat-free mass index.
Results update as you type.
How it works
FFMI, the fat-free mass index, is a way to gauge how muscular you are relative to your height, and unlike simple body mass index it ignores fat entirely, making it far more useful for athletes and lifters. This FFMI calculator takes your weight, height, and body fat percentage and returns your height-adjusted FFMI along with your lean body mass in kilograms. The index answers a question that bodyweight alone cannot: is that mass muscle or fat? Two people at the same BMI can have completely different physiques, so BMI often labels muscular athletes as overweight. By stripping out fat and scaling lean mass to height, FFMI produces a number that tracks genuine muscularity. Natural, drug-free trained men typically land somewhere in the low twenties, and values pushing past about 25 are often cited as difficult to reach without enhancement, which is why the metric appears in discussions of natural potential. The calculation first finds lean mass by removing the fat fraction from your bodyweight, then divides by height squared exactly as BMI does. A height-normalisation term is added so the score does not unfairly favour taller or shorter individuals. Because the result depends heavily on your body fat figure, the accuracy of that input matters: a rough visual estimate gives a rough FFMI, while a measured body fat percentage gives a dependable one. Lifters tracking progress, coaches assessing athletes, and anyone curious about their muscular development can use this tool to put a single meaningful number on their build.
FFMI = lean mass ÷ height² + 6.1 × (1.8 − height)
Fat-free mass index divides your lean body mass in kilograms by your height in metres squared, mirroring the structure of body mass index but counting only muscle, bone, and organs rather than fat. Lean mass is bodyweight multiplied by one minus your body-fat fraction. The extra term, 6.1 times the difference between 1.8 metres and your height, is a normalisation that adjusts the score so taller and shorter people can be compared on the same scale.
Source:Kouri et al. 1995 (fat-free mass index)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter body weight, height and body fat.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
FFMI (adjusted) updates instantly as you type — no button to press.
Frequently asked questions
What is a good FFMI?
For men, an FFMI around 18 to 20 is average, the low twenties indicate a well-developed, athletic build, and values approaching 25 represent the upper end of what is generally considered achievable naturally. Women's values run several points lower on the same scale. These are broad guides rather than strict limits, and genetics, training history, and measurement accuracy all influence where an individual realistically falls.
How is FFMI different from BMI?
BMI divides total bodyweight by height squared without distinguishing muscle from fat, so it frequently mislabels muscular people as overweight. FFMI first removes fat using your body fat percentage, then scales the remaining lean mass to height. The result reflects muscularity rather than mere size, which is why athletes and lifters prefer it. In short, BMI measures heaviness, while FFMI measures how much of that mass is lean tissue.
Why does body fat percentage matter so much here?
FFMI is built on lean mass, which is calculated by subtracting your fat from your total weight, so the body fat figure you enter directly shapes the result. An inaccurate body fat estimate produces an inaccurate FFMI. A rough eyeball guess can be several points off, while a measurement from calipers, a DEXA scan, or bioelectrical impedance gives a far more reliable index. Use the best body fat figure you can obtain.
What is the adjusted FFMI formula?
The adjusted, or normalised, FFMI adds a term, 6.1 multiplied by the difference between 1.8 metres and your height, to the basic lean-mass-over-height-squared calculation. This correction compensates for the fact that raw FFMI slightly favours taller individuals. The adjustment lets people of different heights be compared on a common scale, which is why the normalised version is the one most commonly quoted in fitness and research contexts.
Can FFMI prove whether someone is natural?
No. While FFMI values much above 25 are often cited as hard to reach without performance-enhancing drugs, the metric is a rough indicator, not proof. Genetics, exceptional training, favourable proportions, and measurement error all affect the number, and some naturally gifted athletes exceed the usual ceiling. Treat a high FFMI as noteworthy rather than conclusive, and remember the calculation is only as accurate as the body fat estimate behind it.