FFMI and FMI calculator

Calculate FFMI, height-normalized FFMI and FMI from height, weight and body-fat percentage.

Body measurements

Body-fat percentage is required to split body weight into lean and fat mass before calculating both indices.

Height

Prefer an estimate obtained with a consistent, repeatable measurement method.

Method & sources

FFMI and FMI divide fat-free mass and fat mass respectively by height squared. This decomposition was proposed as a way to complement BMI by separating the two components of body weight. The primary result here is normalized FFMI: the Kouri adjustment standardizes FFMI to a 1.80 m reference height. Raw FFMI remains visible so the adjustment is transparent.

Formula
Lean mass = weight × (1 − BF%/100). Fat mass = weight × BF%/100. FFMI = lean mass(kg) / height(m)². FMI = fat mass(kg) / height(m)². Normalized FFMI = FFMI + 6.1 × (1.80 − height(m)).

Limitations

  • FFMI and FMI accuracy depends directly on the accuracy of the body-fat percentage entered.
  • The 1.80 m FFMI adjustment comes from Kouri et al.’s study of male strength-trained athletes and is not a universal medical standard.
  • The often-quoted normalized FFMI value around 25 comes from a specific sample in that study. It cannot diagnose substance use or establish an individual genetic limit.
  • FFMI and FMI are descriptive indices, not health diagnoses or direct measurements of muscle.

Sources