FFMI Calculator

The Fat-Free Mass Index (FFMI) is like BMI for muscle. Instead of dividing total body weight by height squared, it divides your fat-free mass by height squared — so it rewards lean tissue rather than penalising it. It is the standard way to gauge how muscular someone is relative to their frame, and to put a rough ceiling on what is achievable naturally.

The Formula

First strip out body fat to get fat-free mass, then index it to height. A normalization term adjusts everyone to a common 1.8 m reference so taller and shorter lifters can be compared fairly.

Worked Example

An 80 kg man at 15% body fat, 1.80 m tall:

• FFM = 80 × (1 − 0.15) = 68 kg
• FFMI = 68 / 1.80² = 68 / 3.24 = 21.0
• Normalized = 21.0 + 6.1 × (1.8 − 1.8) = 21.0 (no adjustment at reference height)

An FFMI of 21 sits in the "above average" band — clearly trained but well within natural range.

Interpretation Bands

Why Normalize to 1.8 m?

Raw FFMI slightly favours shorter people because the height-squared denominator does not perfectly capture how muscle scales with frame size. The normalization adds about 6.1 points per metre of height difference, so a 1.65 m and a 1.95 m lifter with the same build land on comparable numbers.

Accuracy Caveats

FFMI is only as good as the body-fat figure you feed it. A 5% error in body fat shifts FFMI by roughly a full point. Use a consistent method (calipers, DEXA, or a validated formula) and track the trend rather than a single reading.

Note: FFMI is a fitness benchmark, not a diagnosis or a hard limit. The natural "ceiling" near 25 is a population observation, not a rule. Consult a qualified professional for individual assessment.