What this tool does
FFMI is lean mass divided by height squared — BMI that tries to ignore fat. You provide body-fat percent; lean mass is weight × (1 − fat%). A normalized FFMI adjusts toward a 1.8 m reference using a published linear correction so taller and shorter people are less distorted.
How it works
Kouri and colleagues (1995) popularized FFMI in a steroid-user versus non-user sample. That paper is why internet culture treats ~25 as a magic fence. It was not a courtroom. Untrained men often sit near 18–20; untrained women near 14–16. Your tape error becomes FFMI error immediately.
Formula and methodology
Lean kg = weight kg × (1 − bodyFat%/100). FFMI = lean kg / height(m)². Normalized FFMI ≈ FFMI + 6.1 × (1.8 − height_m) (Kouri-style height correction). Limits: the 25 “ceiling” is not a law of nature, and body-fat input quality dominates.
How to interpret the result
Use FFMI to see whether a bulk added lean mass relative to height, not to accuse strangers. A jump in FFMI with a sloppy 6% body-fat guess is fiction. Pair with photos, lifts, and waist. Sex on the form only changes the context sentence, not the index math.
Example
90 kg at 180 cm and 15% fat → lean 76.5 kg → FFMI ≈ 23.6, normalized similar near 1.8 m. At 12% fat the same weight prints a higher FFMI without any new muscle — that is the percent talking. Measure fat the same way each time.
Common mistakes
- Claiming “unnatural” from a website FFMI.
- Mixing a DEXA fat% from last year with today’s weight.
- Using BMI in place of body fat.
- Chasing 25 instead of chasing training and protein.
When this estimate may be off
- Body-fat percent is a guess from a cheap scale.
- Edema or glycogen supercompensation.
- Heights far from the 1.8 m normalization center — the correction is itself approximate.
- Youth athletes; the discussion sample was adults.