Separate the components of body weight
Two people with the same height and weight can have different FFMI and FMI values when their body composition differs.
Health & wellness
FFMI and FMI calculator
Calculate raw FFMI, height-normalized FFMI and fat mass index, with the corresponding lean and fat mass.
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.
Your body composition indices
Normalized FFMI
kg/m²
FFMI adjusted to a reference height of 1.80 m using Kouri et al. (1995).
Raw FFMI
-
kg/m²
FMI
-
kg/m²
Lean body mass
-
Fat mass
-
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.
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How does this tool work?
FFMI (Fat-Free Mass Index) relates lean mass to height, while FMI (Fat Mass Index) relates fat mass to height. Together they split the idea behind BMI into two components and add context about body composition. Lean mass includes muscle, but also water, bone, organs and other non-fat tissues.
Enter height, weight and body-fat percentage. The calculator first splits body weight into lean mass and fat mass.
Each mass is divided by height squared: this produces FFMI for lean mass and FMI for fat mass.
Normalized FFMI then applies the Kouri adjustment to a 1.80 m reference height. The raw values remain visible.
How to use this tool
Enter your measurements
Provide height and weight using your preferred unit system.
Add body-fat percentage
This value is required to divide total weight into lean and fat mass.
Read the indices together
Compare raw FFMI, normalized FFMI and FMI alongside their corresponding masses rather than isolating one number.
Use cases
Track change over time
When the same body-fat measurement method is used consistently, FFMI and FMI can help track lean- and fat-mass trends separately.
Add context to BMI
FFMI and FMI provide extra context when BMI alone cannot distinguish the components of body weight.
Tips and best practices
- Use the same body-fat measurement method each time when tracking a trend.
- Changes in body-fat percentage directly change FFMI and FMI, so avoid comparing results based on very different measurement methods.
- Do not turn a high FFMI into a conclusion about pure muscle mass or substance use: the index cannot establish either.
Frequently asked questions
What is the difference between FFMI and FMI?
FFMI divides lean mass by height squared; FMI does the same with fat mass. Their sum mathematically equals BMI when both masses come from the same body weight.
Why is body-fat percentage required?
Because total weight must first be split into lean and fat mass. Without that information or direct measurements of those masses, FFMI and FMI cannot be calculated from height and weight alone.
What is normalized FFMI?
It is raw FFMI adjusted with the Kouri correction to a 1.80 m reference height, reducing residual height effects in comparisons.
Does FFMI above 25 prove steroid use?
No. The value around 25 comes from a specific male sample studied in 1995. It is not an anti-doping test, diagnostic threshold or individual genetic ceiling.
Does FFMI measure muscle mass?
No. It uses fat-free mass, which includes muscle but also water, bone, organs and other non-fat tissues.
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