How FFMI Calculator works
Methodology for the FFMI Calculator: formulas, coefficients, data sources, assumptions, and known limitations.
Scope
The FFMI Calculator estimates Fat-Free Mass Index from bodyweight, height, and body-fat percentage, and reports a height-normalized muscularity figure together with a height-adjusted FFMI variant.
FFMI is descriptive, not diagnostic: it approximates how much lean tissue a body carries per square meter of height. It is most useful as a year-over-year self-comparison metric and as a loose check against published natural-lifter benchmarks.
This page describes only the math the calculator runs. It does not diagnose muscle dysmorphia, steroid use, or anything about an individual's training history.
Formula
FFMI = LBM_kg / (height_m)^2, where LBM_kg = weight_kg x (1 - bodyfat_fraction). Adjusted FFMI adds 6.1 x (1.8 - height_m) to normalize taller and shorter lifters.
lbm_kg = weight_kg * (1 - bodyfat_pct / 100)
ffmi = lbm_kg / (height_m ** 2)
adjusted_ffmi = ffmi + 6.1 * (1.8 - height_m)Coefficients
| Parameter | Value | Note |
|---|---|---|
| Height normalization constant | 6.1 | Kouri et al. 1995 adjustment term. |
| Reference height | 1.80 m | Height at which adjusted FFMI equals raw FFMI. |
| Common natural ceiling | ~25.0 adjusted | Upper edge of the natural-lifter range reported by Kouri. |
Data sources
- Kouri EM, Pope HG Jr, Katz DL, Oliva P. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clin J Sport Med. 1995;5(4):223-228. — PMID 8580417. Origin of the adjusted-FFMI formula and the ~25 natural ceiling observation.
Assumptions
- Body-fat percentage is measured or estimated accurately; FFMI inherits its largest source of error from that input.
- Height is measured without shoes and rounded to the nearest half-centimeter.
- Lean body mass is defined as total mass minus fat mass; it does not separate bone, water, and muscle.
Approximation range
Raw FFMI values cluster in 18–22 for trained natural men and 14–18 for trained natural women. Adjusted FFMI above ~25.0 for men was the cohort boundary Kouri found useful for flagging likely steroid users, but it is not a test.
Error is dominated by the body-fat input: a 4-percentage-point error on body-fat percentage typically moves FFMI by 1.0–1.5 units.
Limitations
- FFMI does not distinguish muscle, organ, bone, or water mass.
- The adjustment term is calibrated on adult men; applying it to adolescents, very tall or very short adults, or athletes with unusual limb proportions degrades accuracy.
- The original ~25 ceiling was observational, not a statistical proof of natural impossibility. Treat it as a range indicator, not a verdict.
- Hydration state and glycogen loading can shift measured lean mass by 1–3 kg day to day.
Reproducibility
Given weight = 85 kg, height = 1.80 m, bodyfat = 15%: LBM = 85 * 0.85 = 72.25 kg. FFMI = 72.25 / 1.8^2 = 22.30. Adjusted FFMI = 22.30 + 6.1 * (1.8 - 1.8) = 22.30.
Change log
- 2026-04-24: methodology page first published.
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