Learn how FFMI and FMI relate lean mass and fat mass to height, and how they complement BMI.
BMI relates total body weight to height without distinguishing fat mass from fat-free mass. FFMI and FMI follow a similar logic but separate these two components.
FFMI (Fat-Free Mass Index) relates fat-free mass to height. FMI (Fat Mass Index) does the same with fat mass. Together, they provide a more detailed view of body composition, provided that an estimate of body fat is available first.
FFMI is calculated as follows:
FFMI = fat-free mass in kg / height in m²
A person who is 1.80 m tall with 68 kg of fat-free mass therefore has an FFMI of approximately:
68 / 1.80² = 21.0 kg/m²
The index allows fat-free mass to be related to stature rather than comparing kilograms directly between people of different heights.
To understand exactly what lean or fat-free mass includes, read Lean body mass: definition, calculation and how it differs from muscle mass.
FMI uses the same structure:
FMI = fat mass in kg / height in m²
With 12 kg of fat mass at a height of 1.80 m:
12 / 1.80² = 3.7 kg/m²
FFMI and FMI have been proposed in scientific literature as height-normalized indices of fat-free mass and fat mass.
BMI is calculated using total body weight:
BMI = weight / height²
And:
weight = fat-free mass + fat mass
In a simple model, BMI can therefore be viewed as combining two height-adjusted components: FFMI and FMI.
Two people with the same BMI can consequently have different FFMI and FMI values. That is the descriptive value of these indices: they separate information that BMI combines.
This does not make BMI “useless.” BMI is simple, extensively documented and widely used in public health. FFMI and FMI answer a different question and require an additional estimate of body composition.
Weight and height alone are not enough to calculate FFMI and FMI separately. You first need an estimate of how much of the body weight is fat.
From body-fat percentage:
fat mass = weight × percentage / 100
fat-free mass = weight − fat mass
Both masses are then divided by height squared.
If your body-fat percentage is itself an estimate, that uncertainty carries into both indices. To understand where the value comes from, read How to measure body-fat percentage.
The body fat calculator can also provide an estimate from body measurements.
A 1995 publication by Kouri and colleagues proposed an FFMI correction intended to reduce its residual association with height and bring comparisons toward a reference height of 1.80 m. This value is commonly called normalized FFMI or height-adjusted FFMI.
Bethemesh uses the common variant:
normalized FFMI = FFMI + 6.1 × (1.80 − height in metres)
One detail is worth documenting: the PubMed abstract of the original Kouri et al. (1995) study reports a coefficient of 6.3, while later publications describe and use the equation attributed to Kouri with a coefficient of 6.1. Both versions therefore appear in secondary literature and calculators.
The practical difference is small. For someone who is 1.70 m or 1.90 m tall, changing the coefficient from 6.1 to 6.3 changes normalized FFMI by only 0.02 points. What matters most when comparing two results is knowing which convention was used.
It is therefore useful to distinguish:
The FFMI and FMI calculator displays both raw FFMI and normalized FFMI calculated with 6.1 so the correction is not confused with the base formula.
No. FFMI uses fat-free mass, not a direct measurement of muscle mass.
Fat-free mass includes muscle but also water and other non-fat tissues. Changes in hydration or errors in body-fat estimation can therefore affect the calculated FFMI.
This is an important limitation when the index is used in sports and bodybuilding contexts.
Publications have proposed distributions or reference points in different populations. Their interpretation depends on the population studied, sex, age and the method used to assess body composition.
The number 25 is particularly well known in discussions of FFMI. It comes from a specific 1995 study of male athletes who reported using or not using anabolic-androgenic steroids.
That number is often removed from its context. We therefore cover it separately in FFMI of 25: where does the number come from and what does it really mean?.
Consider a person weighing 80 kg, standing 1.80 m tall, with an estimated body-fat percentage of 15%.
Estimated fat mass:
80 × 0.15 = 12 kg
Estimated fat-free mass:
80 − 12 = 68 kg
FFMI is therefore approximately:
68 / 1.80² = 21.0
and FMI:
12 / 1.80² = 3.7
This example also shows why the quality of the initial body-fat estimate matters: it determines how the 80 kg is divided between the two indices.
FFMI and FMI describe body compartments relative to height. They do not summarize health status, fitness or athletic performance.
They are most useful for understanding body composition or tracking changes when the underlying measurements are sufficiently consistent.
To calculate your values, use the FFMI and FMI calculator. To start from the beginning, read Body composition: understanding what your weight doesn’t tell you.
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Explore the study behind the FFMI 25 threshold, its normalization to 1.80 m and why this often-cited number should not be treated as an absolute boundary.
Understand the difference between weight, fat mass, lean mass and muscle mass, and why the scale tells only part of the story.
Learn what body-fat percentage measures, how it differs from fat mass in kilograms and what to keep in mind when tracking changes over time.