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Coach Sheet

Glossary entry — body-composition

Fat-Free Mass Index FFMI

A height-normalized measure of lean mass. BMI's pickier cousin, replaces total weight with lean body mass. Useful for tracking lean mass changes during a cut. 

FFMI = LBM_kg / height_m² + 6.1 × (1.8 − height_m)

What FFMI means

Fat-Free Mass Index normalizes lean body mass to height squared, the same way BMI normalizes total body weight. The result is a number that scales with how much muscle a body carries, adjusted for stature.

A 70 kg person at 8% body fat carries 64.4 kg of lean mass. A 70 kg person at 25% body fat carries 52.5 kg. Same BMI, different FFMI (and different physiques).

Worked example

A 180 cm, 80 kg male at 15% body fat:

LBM       = 80 × (1 − 0.15)             = 68 kg
Raw FFMI  = 68 / 1.80²                  = 68 / 3.24 = 21.0
Adjusted  = 21.0 + 6.1 × (1.8 − 1.8)    = 21.0

The height adjustment is from Kouri 1995. Without it, tall lifters score artificially low.

Suggested natural ceilings

Kouri 1995 suggested:

  • Male natural ceiling: ~25.0 (with documented outliers up to 26.5)
  • Female natural ceiling: ~21.5 (with outliers up to 23)

These are central tendencies of a 157-subject sample, not hard ceilings. Body composition measurement error alone causes ±0.5-0.8 swings in FFMI, which spans much of the supposed limit.

When FFMI is useful

For coaches:

  • Tracking lean mass during a cut: weight drops, FFMI ideally stays flat (lose fat, not muscle)
  • Reality-checking client goals: helps frame realistic timelines for lean mass gain
  • Quarterly check-in metric: not daily, not weekly

When FFMI lies

The biggest issue is body fat measurement quality. Bathroom-scale BIA is routinely 5-10 percentage points off, which moves FFMI 1.5-3 points. Use Navy method, DEXA, or skinfolds instead.

Athletes outside the average lean-mass population (genetically extreme, heavily trained) sit above the suggested ceiling without anything illicit; the ceiling is a population average, not a biological law.

In Coach Sheet

The Profile tab takes Weight, Height, Body Fat % once. The Quick Stats tab below auto-calculates both raw and adjusted FFMI. The trend over time (especially during cutting cycles) is the useful signal. A flat FFMI during a 4 kg weight loss confirms muscle preservation.

Sources

  1. [1]
  2. [2]