FFMI Calculator
Calculate your Fat-Free Mass Index
FFMI Calculator
Calculate your Fat-Free Mass Index
How it works
FFMI (Fat-Free Mass Index) measures how much muscle you carry relative to your height — think of it as a BMI that ignores body fat. It is widely used to gauge muscular development and to discuss the natural muscle-building ceiling.
The calculator works out your fat-free mass from your weight and body-fat percentage, divides it by height squared, and normalizes it to a height of 1.8 m. If you don't know your body fat, the built-in US Navy estimator works it out from a few tape measurements.
As a rough guide: below 18 is below average, 18–20 average, 20–22 above average, 22–25 excellent, and above 25 is elite. Drug-free athletes rarely exceed an FFMI of around 25.
Formula
- Fat-free mass (kg) = weight × (1 − body fat % ÷ 100)
- FFMI = fat-free mass ÷ height (m)²; normalized FFMI = FFMI + 6.1 × (1.8 − height in m)
- US Navy body fat %, men: 495 ÷ (1.0324 − 0.19077 × log10(waist − neck) + 0.15456 × log10(height)) − 450
- US Navy body fat %, women: 495 ÷ (1.29579 − 0.35004 × log10(waist + hip − neck) + 0.221 × log10(height)) − 450 (all measurements in cm)
Related reading
- FFMI (Fat-Free Mass Index) – A measure of muscle mass relative to height.
- Body Fat Percentage – The proportion of your body weight that is fat.
- Lean Body Mass – Your total weight minus fat mass — muscle, bone, organs, and water.
- BMI (Body Mass Index) – A weight-to-height ratio that ignores body composition.
- Body Recomposition – Losing fat and building muscle at the same time.
Frequently Asked Questions
What is a good FFMI score?
Below 18 is below average, 18–20 is average, 20–22 is above average, 22–25 is excellent, and above 25 is elite. Natural athletes rarely exceed 25.
Do I need my body-fat percentage?
Yes — FFMI is based on fat-free mass. If you don't know it, use the built-in US Navy estimator, which calculates body fat from neck, waist (and hip) measurements plus height.
Results are estimates for general education, not medical advice. Talk to a doctor or registered dietitian before changing your diet or training, especially if you have a health condition.