FFMI Calculator
Calculate your Fat-Free Mass Index to assess your muscular development relative to height.
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How this calculator works
Relative Fat Mass (RFM)v1.0.0
Body fat estimate from height and waist circumference (Woolcott & Bergman 2018).
- Applies to
- Adults of various ethnicities (validated vs DXA)
- Assumptions
- Waist measured at the navel level
- Limitations
- Estimate error of several percentage points vs DXA
- Output
- % body fat
- Last reviewed
- 2026-06-01
Evidence Sources & Literature
- Scientific Reports. Relative fat mass (RFM) as a new estimator of whole-body fat percentage. Sci Rep, 2018. doi:10.1038/s41598-018-29362-1.
About the FFMI Calculator
FFMI (fat-free mass index) is like BMI, but for muscle instead of total weight. It measures how much lean mass you carry relative to your height, which makes it a favorite among lifters and athletes who want a number that reflects muscularity rather than just bodyweight. This calculator works out your FFMI from your weight, height and body-fat percentage.
I find FFMI a refreshing counterpart to BMI, because it answers a question BMI cannot: how muscular am I for my frame? For anyone strength training, that is often the more motivating and meaningful figure to track.
What FFMI captures
FFMI looks only at your fat-free mass (everything that is not fat: muscle, bone, water and organs) relative to your height. Because it strips out body fat, it specifically reflects how much lean tissue you carry. That is why a lean, muscular athlete who reads as overweight on BMI will score high and favorably on FFMI, which tells the truer story for them.
How it is calculated
The calculator first uses your body-fat percentage to work out your fat-free mass, then divides that by your height squared, the same structure as BMI. It also reports an adjusted FFMI that normalizes to a standard height, so people of different heights can be compared fairly. You will need a reasonable body-fat estimate for it to be meaningful, so pair it with the body fat calculator if you do not have one.
Reading your score
As a rough guide for men, an FFMI around 18 to 20 is average, 20 to 22 is fit, and 22 to 25 is very muscular. For women the scale runs lower, roughly 14 to 17 on average. An adjusted FFMI near 25 is often cited as the approximate ceiling for natural, drug-free trainees, though genetics mean a few people sit a little above or below that. Use it to track your own progress more than to compare yourself to others.
Where it is useful (and where it is not)
FFMI shines for people who lift and want a muscularity benchmark that bodyweight cannot give. It is less useful for the general population, who are usually better served by body fat percentage or waist measurement. And because it depends on your body-fat estimate, an inaccurate body-fat reading will throw it off, so treat it as a guide and focus on the trend as you train.
Dietitian Insight: Fat-Free Mass Index normalizes lean body mass relative to stature, offering a standardized index of muscularity independent of body fat. Explore our analysis of body composition and muscular development, or compute your starting lean weight with our lean body mass calculator.
Sources & method
This calculator uses established, peer-reviewed formulas and reference ranges from recognized health and nutrition authorities. Results are estimates for general education, not a medical diagnosis. For decisions about your health, consult a qualified clinician. Reviewed by Macro & Meals Editorial Team, Content editor.
Frequently Asked Questions
What is a good FFMI?
For men, roughly 18 to 20 is average, 20 to 22 is fit, and 22 to 25 is very muscular. For women the scale runs lower, around 14 to 17. An adjusted FFMI near 25 is often cited as the approximate natural ceiling, though genetics create some variation.
How is FFMI different from BMI?
BMI uses total weight, so it cannot tell muscle from fat. FFMI uses only fat-free mass, so it specifically reflects muscularity. A lean, muscular athlete who reads as overweight on BMI will score high and favorably on FFMI, which is the more useful number for trainees.
What do I need to calculate FFMI?
You need your weight, height and a body-fat percentage estimate. The calculator uses your body fat to work out fat-free mass, then divides by height squared. Because it relies on your body-fat figure, an accurate estimate matters; pair it with the body fat calculator if needed.
Can FFMI tell if someone uses steroids?
Not definitively. An adjusted FFMI above about 25 is sometimes cited as beyond what most people reach naturally, but genetics, measurement error and training history mean it is not proof of anything. It is best used to track your own progress rather than to judge others.
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