Guide
What Is FFMI?
Quick answer
Fat-free mass index is lean mass, in kilograms, divided by height in meters squared. Lean mass here is body weight minus the fat you estimated. The number is only as good as the body-fat estimate, which is the weak input.
Kouri, Pope, Katz, and Oliva published the index in 1995 in the Clinical Journal of Sport Medicine. They also described a height-normalized FFMI: FFMI plus 6.3 times (1.80 minus height in meters). In that paper, non-users in the male sample reached about 25. The authors treated the finding as preliminary. So do we. It is not a natural limit we will print as a rule.
Estimates only. Body-fat inputs are rough. This is not a medical assessment.
FFMI calculatorHeight, weight, and body fat, with the 6.3 normalization.Open →On this page
- How is FFMI calculated?
- What about the number 25?
- What should you use it for?
- What will this site not say?
- Key takeaways
- FAQ
How is FFMI calculated?
Convert body fat to a fraction. Fat mass is body weight times that fraction. Fat-free mass is body weight minus fat mass. FFMI is fat-free mass in kilograms divided by height in meters squared. The calculator does this and also shows the normalized figure with the 6.3 factor from the paper, not 6.1.
Some write-ups use 6.1. The 1995 abstract uses 6.3. We follow the paper and say so on the calculator page. If you compare numbers with another site, check which constant they used before you decide you gained muscle overnight.
What about the number 25?
It is what that study reported for non-users in a male sample, described as an approximate upper level in that group. It is not a measurement of you, and it is not a women's standard. A higher number does not prove drug use. A lower number does not prove anything is wrong.
Body-fat methods disagree. Calipers, a home scale, and a DEXA can give three fats for one person. The FFMI will move with the method even if your body did not. Pick one method if you are tracking.
What should you use it for?
A slow check on whether lean mass is changing, beside the mirror and the logbook. It is a poor daily metric. Water and food move body weight enough to annoy the formula.
It is not a training program. Progressive overload still decides whether you get stronger. FFMI does not tell you what to bench.
What will this site not say?
We will not estimate your chances of being 'natural,' and we will not rank your physique. The calculator returns the index. The paper is linked. The interpretation is yours, and it should stay modest.
Do not enter a guessed body fat to hit a number you want. The arithmetic will obey you and the result will be fiction.
DOTS and Wilks also use body weight, but they score a total, not muscle. A higher index is not a strength rank.
Key takeaways
- FFMI is fat-free mass divided by height squared.
- Kouri's normalized version adds 6.3 times (1.80 minus height in meters).
- The paper's figure near 25 was a finding in that male sample, not a law.
- A bathroom estimate of body fat moves the index a lot.
FAQ
Is FFMI a body-fat percentage?
No. Body fat is an input. FFMI is lean mass scaled by height.
Why does height change the normalized score?
The 1995 paper adjusted toward a height of 1.80 m so shorter and taller people could be compared inside their sample. The adjustment is linear and simple. It is not a full model of frame size.
Can women use the same calculator?
The arithmetic works. The 'about 25' comment in that paper was about men. Do not apply it to women as a limit.
How often should I recalculate?
Every month or two, with the same body-fat method. Daily weigh-ins are noise.
Does a higher FFMI mean I am stronger?
Not by itself. Strength is what you lift. The index does not know your squat.