FFMI Calculator
Work out your fat-free mass index and height-adjusted FFMI from weight, height and body fat, with the usual interpretation bands and their limits.
Interpretation
Lean mass 68.0 kg ÷ (1.80 m)² = 20.99 • adjusted = 20.99 + 6.1 × (1.8 − 1.80) = 20.99
Band reference
| Adjusted FFMI | Description |
|---|---|
| 0 – 18 | Below average |
| 18 – 20 | Average |
| 20 – 22 | Above average |
| 22 – 23 | Excellent |
| 23 – 26 | Superior |
| 26 – 28 | Very rare drug-free |
| 28+ | Above the drug-free range |
Estimates only, not medical advice. FFMI inherits every bit of error in your body fat number — a caliper reading three points off moves FFMI by roughly 0.7. The often-quoted “25 is the natural ceiling” line comes from a 1995 study of 157 male bodybuilders and is a population observation, not a test that can tell you anything about an individual. The height adjustment is a linear correction and stretches badly at the extremes of height.
What is the FFMI Calculator?
FFMI is your fat-free mass in kilograms divided by your height in metres squared. Lean mass is weight × (1 − body fat %), and the adjusted FFMI adds 6.1 × (1.8 − height in metres) so tall and short people can be compared on the same scale.
- Raw FFMI plus the height-normalised adjusted FFMI in one view
- Metric and imperial input, with feet-and-inches height entry
- Fat-free mass and fat mass broken out alongside the index
- Separate interpretation bands for men and women, with your band highlighted
- Shows the full working so you can check the arithmetic
- Runs entirely offline in your browser — no measurements leave your device
How to use the FFMI Calculator
- 1
Choose metric or imperial units and set your sex, which only selects the interpretation band.
- 2
Enter your height, either in centimetres or as feet plus inches.
- 3
Enter your body weight and your best body fat percentage — from calipers, a DEXA scan, a smart scale or a tape estimate.
- 4
Read the FFMI and adjusted FFMI tiles along with your fat-free and fat mass.
- 5
Check the band reference table to see where your adjusted FFMI sits and how wide each band is.
About the FFMI Calculator
The ByteTools FFMI Calculator turns three numbers — your weight, your height and your body fat percentage — into a fat-free mass index. Where BMI treats all body mass the same, FFMI strips the fat out first and asks how much lean mass you carry for your height, which makes it far more useful for anyone who lifts.
It reports both the raw FFMI and the height-adjusted version from Kouri and colleagues' 1995 study, which adds 6.1 × (1.8 − your height in metres) so that a 1.65 m lifter and a 1.95 m lifter can be read against the same bands. You can work in centimetres and kilograms or feet, inches and pounds, and the interpretation table highlights the band you land in.
Everything is computed in your browser with JavaScript — nothing you type is uploaded, logged or stored anywhere. These are estimates for training context only, not medical advice, and FFMI is only as good as the body fat number you feed it.
Frequently asked questions
What is a good FFMI?
For men, roughly 18–20 is average, 20–22 is above average and 22–25 is what a well-trained lifter typically reaches. For women the equivalent bands sit about four points lower. These are population descriptions rather than targets, and where you land depends heavily on your frame and how long you have trained.
What is the difference between FFMI and adjusted FFMI?
Raw FFMI divides lean mass by height squared, which quietly favours shorter people. Adjusted FFMI adds 6.1 × (1.8 − height in metres) to correct that, so it nudges tall lifters up and short lifters down. Most published FFMI bands assume the adjusted figure, which is why this calculator shows both.
Is an FFMI of 25 really the natural limit?
No — that number comes from a 1995 study of 157 male bodybuilders in which the drug-free group topped out near 25, and it describes a group, not an individual. Plenty of drug-free lifters with unusual genetics or long training histories sit above it, and plenty of enhanced lifters sit below it. It cannot be used as a test of anyone.
How accurate is an FFMI calculation?
The arithmetic is exact; the inputs are not. Body fat is the weak link — a reading three percentage points off shifts FFMI by roughly 0.7, which is enough to move you a whole band. Use the same measurement method every time and watch the trend rather than obsessing over one decimal place.
Do I need a body fat percentage to calculate FFMI?
Yes. FFMI is built on fat-free mass, so without a body fat figure there is nothing to subtract and the number would just be BMI again. If you do not have one, a tape-measure method such as the US Navy formula gets you close enough to track changes over time.
Is my data private?
Completely. Every calculation runs in JavaScript inside your browser and nothing is sent to a server, so your height, weight and body fat figures never leave your device.
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