Fat Mass Index Calculator
FFMI describes how much muscle you carry for your height. FMI does the same for fat. Together they separate the two things BMI blends into one number.
Your results
Entirely dependent on the accuracy of your body-fat figure, which for most methods carries an error of several percentage points. Category boundaries vary between sources.
Why split BMI into two indices
BMI is weight divided by height squared, and it cannot distinguish a kilogram of muscle from a kilogram of fat. Splitting it fixes that:
- FMI = fat mass ÷ height² — how much fat you carry for your height.
- FFMI = lean mass ÷ height² — how much lean mass you carry for your height.
- FMI + FFMI = BMI. They add up exactly.
Which is why two people with an identical BMI of 25 can have completely different profiles: a lifter at FFMI 21 and FMI 4, and a sedentary person at FFMI 17 and FMI 8.
Reading the two together
The useful comparison is the pair, not either number alone.
- High FFMI, low FMI — muscular and lean. What most people training are aiming at.
- High FFMI, high FMI — muscular and carrying fat. Common mid-bulk.
- Low FFMI, low FMI — light in both. More muscle would help most goals.
- Low FFMI, high FMI — the profile where resistance training has most to offer.
And because both are height-normalised, they are comparable across people of different heights in a way that raw kilograms are not.
Common questions
What is a normal fat mass index?
Roughly 2–6 for men and 3.5–9 for women is the normal band. Above that is classed as excess, though sources differ.
How is FMI different from body fat percentage?
Percentage is relative to your total weight; FMI is relative to your height. FMI does not change when you gain muscle, whereas percentage does.
Do FMI and FFMI add up to BMI?
Yes, exactly. They are BMI split into its fat and lean components.
Which is more useful?
FFMI if you are training for muscle; FMI if you are tracking fat loss. Both beat BMI.
Related calculators
Track the numbers, not just calculate them
MyFitsy stores your weight, measurements and every session, so the figures above become a trend instead of a one-off.
