What is FFMI?
The Fat-Free Mass Index expresses how much muscle you carry relative to your height: lean body mass in kilograms divided by height in metres squared. Average untrained men sit around 18–19; well-trained natural lifters reach 22–23; about 25 is widely cited as the practical natural ceiling. FFMI fixes BMI's biggest flaw — it ignores fat, so muscle finally counts.
How the calculator works
Lean mass = weight × (1 − body fat %). FFMI = lean mass ÷ height². Because taller lifters have a mechanical advantage, we also show the normalized FFMI (adjusted to a 1.8 m reference) that makes cross-height comparisons fair, plus a category readout.
Common use cases
- Tracking muscle gain across a bulk without BMI panicking
- Comparing muscularity between lifters of different heights
- Reality-checking 'natural' claims — FFMI 25+ is rare without pharmacological help
Formula & caveats
FFMI inherits the error of your body-fat input — a ±3% body-fat mistake moves FFMI by roughly half a point. Track with the same method over time; the trend is the signal.
Frequently Asked Questions
What is a good FFMI for a natural lifter?
20–22 after a few years of training is a strong natural result; 23+ is excellent; numbers near 25 are the commonly cited natural limit.
Why is my FFMI different from my friend's at the same weight?
Height and body fat differ. Use normalized FFMI for fair comparisons.
Does FFMI work for women?
The scale shifts down by about 2–3 points; average untrained women sit near 15–16.
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