FFMI Calculator

How FFMI is calculated — and where implementations differ

FFMI starts from your fat-free mass: take your weight and remove the fat. At 80 kg with 15% body fat, fat-free mass is 80 × (1 − 0.15) = 68 kg. Then you scale it to height, exactly like BMI:

FFM        = weight × (1 − body-fat% / 100)
FFMI (raw) = FFM ÷ height(m)²
FFMI (norm)= FFMI + 6.1 × (1.8 − height(m))

That last line is the height normalization from Kouri et al. (1995). Because a taller person carries more lean mass at the same build, FFMI is adjusted to a reference height of 1.8 m so two people of different heights can be compared. At exactly 1.8 m the raw and normalized figures are identical; below it the normalized number is a little higher, above it a little lower.

This is the step calculators quietly disagree on. Some report only the raw FFMI, some only the normalized figure, and a few apply a different constant — so the same body can come out as two different “FFMI” numbers depending on the tool, with no label to say which is which. We show both, each clearly named, so there’s nothing to guess.