How do you calculate your fat free mass index FFMI ?

When we want to assess our fitness and health, we often think of weight and body mass index (BMI). However, these indicators do not distinguish between fat mass and muscle mass. The fat free mass index (FFMI) is a more precise measure that takes into account muscle mass, bones, skin and other non-fat components of the body.

BMI simply compares weight to height without considering body composition. So a very muscular person may have a high BMI but be in excellent shape. The BMI distinguishes between muscle mass and fat mass, providing a better understanding of your general state of health.

To help you calculate your MMI easily, we have included an online calculator on this page.

How do I use the IMM calculator?

Using our IMM calculator is quick and easy. Here are the steps to follow:

  • Enter your weight in kilograms in the field provided.
  • Indicate your size in centimetres.
  • Enter your body fat percentage (if you don't know it, use our body fat calculator available on the website).
  • Click on the "Calculate to obtain your IMM.

For example, if you weigh 70 kg, are 175 cm tall and have 20% of body fat, enter these values in the calculator. One click and you'll get your BMI and a better understanding of your body composition.

The main advantage of this online calculator is its precision and its ease of use. It allows you to track your progress and adjust your workouts effortlessly. Using this tool, you can get reliable results in seconds, helping you to make informed decisions about your health.

Lean Mass Index Calculator

Table of contents

Interpreting results: reference values

Once you have calculated your lean body mass index (LBMI)It is essential to understand what your results mean. The reference values vary according to age and sex. Here are some tables of results to help you interpret your MMI:

Reference table for men

AgeIdeal IMM
20-39 years old18,5 - 20,5
Age 40-5918,0 - 20,0
60-79 years17,5 - 19,5

Reference table for women

AgeIdeal IMM
20-39 years old15,5 - 17,5
Age 40-5915,0 - 17,0
60-79 years14,5 - 16,5

An MMI below these values may indicate insufficient muscle mass, while a higher MMI may reflect heavy muscling. For athletes, higher values are common, but extremely high values could indicate the use of performance-enhancing substances.

Interpreting your BMI gives you a better understanding of your body composition and enables you to adjust your health and performance goals.

  • If your MMI is too low, consider increasing your protein intake and strengthening your muscle training.
  • If your MMI is optimal, keep up your efforts to maintain good muscular health.

What is the Lean Mass Index (LMI)?

Lean mass index (LMI) is a measure of a person's non-fat body mass. It includes muscle massthe bonesthe skinand other non-fatty body tissues. This index is essential for obtaining an accurate picture of body composition, beyond just weight.

Unlike the BMI, which simply relates weight to height, the BMI distinguishes between fat mass and lean mass. For example, two people of the same height and weight may have identical BMIs, but very different body compositions: one may be muscular with little fat, while the other may have a high fat mass. The BMI therefore offers a finer, more relevant assessment of physical health.

Methods for calculating the MMI

To measure your lean body mass index (LBMI)The first step is to determine your body fat percentage. Here are the main methods:

  • MRI (Magnetic Resonance Imaging) Very accurate, but expensive and generally reserved for medical use.
  • Skin fold pliers This method measures the thickness of subcutaneous adipose tissue at several points on the body. The results are then converted into fat mass percentage using specific formulas.
  • Bioelectrical impedancemetry A weak electric current is sent through the body to estimate lean body mass and fat mass. This method is practical but can be less accurate.

Once the body fat percentage has been determined, here's how to calculate the MMI:

IMM formula :

IMM = (lean mass in kg / height in m²) + 6.3 × (1.8 - height in m)

Calculation example :

For a person weighing 70 kg with 20% of body fat and measuring 1.75 m :

  • Body fat: 70 kg × 0.20 = 14 kg
  • Lean mass: 70 kg - 14 kg = 56 kg
  • IMM: (56 kg / (1.75 m × 1.75 m)) + 6.3 × (1.8 - 1.75) = 18.29

By using these methods and formulas, you can accurately calculate your BMI and better understand your body composition.

Comparison between BMI and IMM

L'lean body mass index (LBMI) has a number of advantages over the BMI. Whereas BMI simply relates weight to height, MMI distinguishes between muscle mass and fat mass. This provides a more accurate assessment of body composition.

For example, an athlete with a high muscle mass may have a high BMI, suggesting overweight or obesity, when in fact he or she is in optimum health. Similarly, a person with low muscle mass but high body fat may have a normal BMI, masking potential health problems. By focusing on fat-free mass, the BMI overcomes these limitations.

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