Find your fat-free mass index (FFMI) — a muscularity index that, unlike BMI, adjusts lean body mass for height using your body fat percentage.
How it works
Fat-free mass is weight × (1 − body fat %), then FFMI is that mass divided by height squared. A normalized version adjusts to a reference height of 1.8 m: normalized FFMI = FFMI + 6.1 × (1.8 − height in meters), making FFMI comparable across different heights.
What this does not include
FFMI depends entirely on an accurate body fat percentage, which this calculator takes as a direct input rather than estimating — an inaccurate body fat reading (from any measurement method) will produce an inaccurate FFMI. This is an informational fitness metric, not a medical diagnostic tool.
How to use this calculator
- Enter your weight.
- Enter your body fat percentage.
- Enter your height.
Why does FFMI need a “normalized” version?
Raw FFMI is still somewhat height-dependent even after dividing by height squared, so the normalized adjustment (from the original research paper) corrects for that, making FFMI values more directly comparable between people of different heights.
What FFMI range is considered natural without performance-enhancing drugs?
The original research associated a normalized FFMI around 25 as roughly the upper natural limit for most men, though individual genetics can push this somewhat higher or lower — this is a population-level observation, not a rule for any specific individual.
Why does FFMI use body fat percentage while BMI doesn’t?
BMI can’t distinguish muscle from fat, since it only uses total weight — FFMI was specifically developed to isolate lean mass by using a body fat percentage, giving a more muscle-specific picture than BMI alone.
