FFMI Calculator (Fat-Free Mass Index)

How the FFMI Calculator Works

This calculator finds your Fat-Free Mass Index (FFMI) — a measure of muscularity that, unlike BMI, excludes body fat. Enter your weight, height and body fat percentage in metric or imperial units. It returns your raw FFMI, your height-adjusted normalized FFMI, and your lean mass, then places you in a muscle category.

The Formula

First convert weight and fat into lean mass:

Lean mass = weight × (1 − body fat %)

Then compute the index, with lean mass in kilograms and height in meters:

FFMI = lean mass (kg) ÷ height (m)²

Finally, normalize to a 1.8 m reference height so people of different heights compare fairly:

Normalized FFMI = FFMI + 6.1 × (1.8 − height in meters)

Worked Example

For a man weighing 80 kg at 15% body fat and 1.8 m tall:

• Lean mass = 80 × (1 − 0.15) = 68 kg
• FFMI = 68 ÷ 1.8² = 68 ÷ 3.24 = ≈ 21.0
• Normalized FFMI = 21.0 + 6.1 × (1.8 − 1.8) = 21.0

A 1.70 m man with the same 68 kg of lean mass would have a raw FFMI of 23.5, but a normalized FFMI of 24.1 — showing how normalization keeps the comparison fair.

FFMI Categories (Men)

Normalized FFMI	Interpretation
Under 18	Below average muscle
18 – 20	Average
20 – 22	Above average
22 – 23	Well-muscled
23 – 26	Superior, near natural limit (~25)
Above 26	Rarely natural

Women typically run a few points lower across these bands.

Why FFMI Matters

FFMI gives lifters and athletes a far more useful body-composition number than BMI, which counts muscle as if it were fat. Track your normalized FFMI over a training block to see real lean-mass progress, set realistic natural goals around the ~25 ceiling, and compare yourself meaningfully with people of other heights. Remember that the accuracy of every result depends on an honest body fat measurement.