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Body Composition Formula

FFMI Formula: Kouri-Adjusted Fat-Free Mass Index

Fat-Free Mass Index normalizes lean mass by height, like BMI does for total mass. Kouri's adjustment to 1.8 m makes values comparable across heights. Natural ceiling lands around 25 for men.

3 VARIABLESPublished Mar 12, 2026Updated May 10, 2026Live Content
By AI Fit Hub · AI Fit Hub Team
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Formula 3 variables Worked example Variations

Education · Not medical advice. Output is deterministic math from your inputs.Editorial standardsSponsor disclosureCorrections

Formula

Copy the exact expression or work through it step by step below.

FFMI = LBM_kg / (height_m)² + 6.1·(1.8 − height_m)

Variables

FFMI

Fat-Free Mass Index

Height-normalized lean mass score in kg/m². Reference: 18–20 average male, 22–23 advanced trained, 25 natural ceiling, >26 likely PED-assisted.

LBM_kg

Lean body mass

Kilograms of fat-free mass. From Boer equation, DXA, or LBM = mass × (1 − BF%/100).

height_m

Height

Meters (cm / 100). The Kouri term corrects for the fact that shorter people naturally score higher on raw FFMI.

Step By Step

  1. 1

    Compute lean body mass either from a circumference equation, DXA scan, or BF%-derived calculation.

    80 kg total, 22% body fat → LBM = 80 × 0.78 = 62.4 kg.

  2. 2

    Convert height to meters and square it.

    178 cm → 1.78 m → 3.1684 m².

  3. 3

    Divide LBM by squared height.

    62.4 / 3.1684 = 19.69 kg/m² raw FFMI.

  4. 4

    Apply the Kouri normalization: add 6.1 × (1.8 − height_m).

    6.1 × (1.8 − 1.78) = 0.122. Adjusted FFMI = 19.69 + 0.12 = 19.81.

Worked Example

Male, 80 kg, 22% body fat, 178 cm

Weight (kg)

80

Body fat (%)

22

Height (cm)

178

LBM = 62.4 kg. FFMI = 62.4 / 1.78² + 6.1·(1.8 − 1.78) = 19.69 + 0.12 = 19.81 kg/m²

FFMI 19.8 — average-to-recreational range. Trained natural lifter typically lands 22–24; 25+ warrants scrutiny.

Common Variations

Raw FFMI (LBM / height²) without the Kouri term is what most fitness articles cite — keep them straight.
Some sources use 1.83 m or 1.7 m as the normalization reference; the Kouri 1.8 m version is the academic standard.
For women, the same equation applies but reference ranges drop ~3 units (average ~15, ceiling ~22).

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FAQ

Questions people ask next

The short answers readers usually want after the first pass.

What is the FFMI formula?
FFMI (Fat-Free Mass Index) is lean body mass divided by height in meters squared, with Kouri's height normalization added: FFMI = LBM_kg / (height_m)² + 6.1·(1.8 − height_m). The base term mirrors how BMI divides total mass by height squared, and the +6.1·(1.8−height) term corrects for the fact that shorter people naturally score higher on raw FFMI.
Is FFMI lean mass divided by height squared?
Yes — raw FFMI is lean body mass (kg) divided by height (m) squared, the same structure BMI uses for total mass. The Kouri-adjusted version adds 6.1·(1.8 − height_m) on top so values are comparable across different heights. Most fitness articles cite the raw version, so it is worth keeping them straight.
What is a good FFMI and what is the natural ceiling?
Reference ranges for men are roughly 18-20 for an average male, 22-23 for an advanced trained lifter, and about 25 as the natural ceiling, with values above 26 likely PED-assisted. For women the same equation applies but ranges drop about 3 units, averaging around 15 with a ceiling near 22.
How do I calculate lean body mass for FFMI?
Lean body mass is the kilograms of fat-free mass, obtained from a circumference equation (such as Boer), a DXA scan, or from body fat percentage via LBM = mass × (1 − BF%/100). For example, an 80 kg person at 22% body fat has LBM = 80 × 0.78 = 62.4 kg.
Where does the FFMI formula come from?
The height-normalized version comes from Kouri et al. (1995), published in Clinical Journal of Sport Medicine, which studied fat-free mass index in users and nonusers of anabolic-androgenic steroids and established the ~25 natural ceiling for men. The 1.8 m normalization reference is the academic standard, though some sources use 1.83 m or 1.7 m.

Sources & References

General fitness estimates — not medical advice. Consult a healthcare professional for medical decisions.