Riepilogo
Fat-Free Mass Index (FFMI) measures how much lean (non-fat) mass you carry relative to your height — like BMI but for muscle tissue only. It was introduced by VanItallie et al. (1990) and popularised by Kouri et al. (1995), whose landmark study found that an FFMI above 25 in men was not observed in drug-free athletes.
Come funziona
FFMI starts with your lean body mass — total weight minus fat. It then divides by height squared, just like BMI. The result tells you how much muscle per unit of height you carry.
Because taller people naturally spread the same muscle over a larger frame, a height-normalised version adjusts the score to what it would be at a standard height of 1.80 m. This lets you compare people of different heights fairly.
The normalised value is what most FFMI calculators report, and it’s what the Kouri study used to establish the natural limit of approximately 25 in men.
La formula
Where
Normalised FFMI (height-adjusted)
Where
At exactly 180 cm the adjustment is zero. Shorter people get a positive adjustment; taller people get a negative one.
Male FFMI categories
| FFMI range | Category |
|---|---|
| Below 18 | Below average |
| 18 – 19.9 | Average |
| 20 – 21.9 | Above average |
| 22 – 22.9 | Excellent |
| 23 – 24.9 | Superior |
| 25+ | Suspicious — above natural limit (Kouri et al.) |
Female FFMI categories
| FFMI range | Category |
|---|---|
| Below 14 | Below average |
| 14 – 15.9 | Average |
| 16 – 17.9 | Above average |
| 18 – 18.9 | Excellent |
| 19 – 20.9 | Superior |
| 21+ | Suspicious — above likely natural limit |
Female thresholds are approximately 4 units lower than male thresholds, reflecting sex differences in lean mass distribution.
Esempio pratico
Male, 90 kg, 175 cm, 10% body fat
Calculate lean mass
= 81 kg
Calculate FFMI
= 26.45
Normalise for height
= 26.76
Classify
= Suspicious
Result
FFMI = 26.4, Normalised FFMI = 26.8 — Suspicious (above natural limit)
Input spiegati
- Height — in cm (metric) or feet/inches (imperial). Converted to metres internally.
- Weight — in kg (metric) or lbs (imperial). Converted to kg internally.
- Body fat % — estimated body fat percentage. Can be measured via calipers, DEXA scan, or estimated from other calculators.
- Sex — determines which category thresholds are applied.
Output spiegati
- FFMI — your raw Fat-Free Mass Index score. This is the primary metric.
- Normalised FFMI — FFMI adjusted to a 180 cm reference height. Use this for comparing yourself against published benchmarks.
- Category — where you fall in the sex-specific classification range.
- Lean mass — your total weight minus fat mass.
- Fat mass — your total body fat weight.
Ipotesi e limitazioni
- Requires accurate body fat %. FFMI is only as accurate as your body fat estimate. DEXA scans are the gold standard; visual estimates or online calculators are rough approximations.
- The 25 limit is a guideline, not a law. Kouri’s study included 74 drug-free athletes. Some exceptionally gifted individuals may naturally exceed 25, though it’s rare.
- Female thresholds are less well-studied. The Kouri study focused on men. Female thresholds are extrapolated from limited data and general population studies.
- Does not account for frame size. A person with a naturally wider frame may carry more lean mass at the same FFMI.
- Normalisation assumes a linear height–FFMI relationship. The 6.3 correction is an empirical approximation, not a physiological law.
Verifica
| Test case | Weight (kg) | Height (cm) | BF% | Sex | Expected FFMI | Normalised FFMI | Category |
|---|---|---|---|---|---|---|---|
| Average male | 80 | 180 | 15 | Male | 20.99 | 20.99 | Above average |
| Muscular male | 90 | 175 | 10 | Male | 26.45 | 26.76 | Suspicious |
| Average female | 60 | 165 | 25 | Female | 16.53 | 17.47 | Above average |
| Light male | 60 | 180 | 20 | Male | 14.81 | 14.81 | Below average |
| Athletic female | 62.96 | 165 | 20 | Female | 18.50 | 19.42 | Excellent |
All FFMI values verified by manual calculation. Normalised values use 6.3 constant per Kouri et al. (1995).
Sources
Related calculators
Body Fat %
Estimate your body fat percentage using the US Navy method (Hodgdon & Beckett). See your category, fat/lean mass breakdown, and FFMI.
BMI
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Lean Body Mass
Calculate your lean body mass using Boer, James, and Hume formulas. See your body composition breakdown — lean mass vs fat mass — with optional body fat % input for higher accuracy.
Muscular Potential
Estimate your genetic muscular ceiling using Casey Butt frame analysis, Martin Berkhan height formula, FFMI natural limit, and Lyle McDonald rate-of-gain model.