FFMI Calculator (Fat-Free Mass Index)

What is the FFMI?

The Fat-Free Mass Index (FFMI) measures how much lean tissue you carry relative to your height, similar to BMI but with the fat compartment removed. It was popularised by Kouri, Pope and Katz in 1995, who used it to study limits of natural muscularity. The math is simple: take your lean body mass (weight minus fat mass) and divide by height squared. Because raw FFMI rises with height, lifters compare a height-adjusted version that adds 6.1 × (1.8 − height in metres) so a 1.65 m and a 1.95 m athlete sit on the same scale. FFMI is the cleanest single number for tracking muscle gains over years: it is unaffected by water swings, cuts the noise that BMI suffers from, and is directly tied to the metric most strength athletes care about — pounds of meat on the frame. It is not a body-fat estimator and is only as good as the body-fat percentage you feed it.

How to use the FFMI calculator

1. Pick metric or imperial — Switch between centimetres/kilograms and inches/pounds. The calculator stores your choice locally so the next visit defaults to it.
2. Enter your numbers — Use a recent scale weight (morning, fasted, after the bathroom is the tradition) and your most accurate height. Don’t round — small differences move the index.
3. Estimate your body fat carefully — FFMI is sensitive to body fat. A 3-point error on a 80 kg lifter shifts lean mass by ~2.4 kg. Use the same method (DEXA, calipers, mirror calibration) every time.
4. Read the adjusted score — Compare the height-adjusted FFMI to the table. The percentile box tells you where you sit relative to the untrained adult-male baseline used in the original studies.

The formula

LBM = Weight × (1 − BodyFat%). Then FFMI = LBM ÷ height² with height in metres. The adjusted version is FFMI + 6.1 × (1.8 − height_m), which removes the height bias.

FFMI classification

Bands below are based on adjusted FFMI for adult males. Females typically score 3–4 points lower at the same training level — the qualitative ranks still apply but expect a parallel shifted curve.

Frequently asked questions