Definition of the fat-free mass index

The fat-free mass index, known as FFMI, indicates the body’s muscle development. It can therefore be used to monitor the success of strength training or of a change in diet. For ambitious or professional strength athletes, the FFMI is used as a replacement for the body mass index (BMI), which is no longer appropriate for muscular people in particular. BMI is designed more for untrained people and only produces an initial comparison. The fat-free mass index, by contrast, is more suitable for trained athletes and provides them with a significantly more accurate assessment of their physical condition.

FFMI: easy monitoring of training success

The higher a sportsperson’s FFMI, the better! A high FFMI value means more muscle (and less fat) in relation to height. Compared with BMI, FFMI evaluates the physical state much more accurately as it also takes into account the body fat percentage. 
The FFMI value therefore enables an assessment of the current fitness level.

This is how you calculate your fat-free mass index (FFMI):

Underlying data
Body weight (W), height (H) and body fat percentage (BFP)

Fat-free mass
First, calculate the fat-free mass (FFM) using the following formula:

FFM = W * (100 – BFP) / 100

FFMI formula

The fat-free mass index is the fat-free mass in relation to height plus a small adjustment factor (Kouri et al.):

 FFMI = FFM / (H * H) + 6.3 x (1.8 – H)

Studies show that the body has a natural upper limit in terms of FFMI: For men, this is an FFMI of 25 (Kouri et al.).
Consequently, it can be concluded that sportspeople with an FFMI of over 25 are using additional means (doping). That is why FFMI is also used in the world of natural bodybuilding to control the use of prohibited substances (GNBF).

At the same time, your own FFMI shows you the remaining potential for building up muscle mass. For ambitious fitness athletes, an FFMI close to 25 is the ultimate training goal! But it can also be a big motivation for amateur sportspeople to keep their FFMI as high as possible in order to increase their own health and well-being.

An FFMI higher than 17 (for women) or 20 (for men) is desirable.

The majority of the population has an FFMI of approx. 16 (women) or 19 (men) (Schutz et al.). A well-designed workout aims to build muscle whilst reducing fat. An increase in FFMI shows that the training has been successful.
 
Example
A woman who is 1.65m tall, weighs 65kg and has 30% BFP has an FFMI of 17.66 (BMI = 23.88).

Case 1
The woman trains with EGYM, loses two kilograms and reduces her BFP by 5%. Her FFMI is now 18.3 (BMI = 23.14). 

Case 2
The woman goes on a crash diet, loses five kilograms and maintains her BFP. Her FFMI is now 16.37 (BMI = 22.04).
 

You can clearly see that the FFMI gives an unambiguous assessment about muscle building and therefore the success of the training when compared with BMI. It can also monitor whether a special diet really is just reducing fat or whether the body is using muscles as a source of energy.

Athletic men should aim for a target FFMI of over 20; women for an FFMI of over 17. Since FFMI, in contrast to BMI, does not require adjustment for age, identical values can be used for all age groups.

Interpretation of FFMI for women

13-14 = weak

15-16 = normal

17-18 = good

19-20 = very good

22 = upper limit

Interpretation of FFMI for men

17-18 = weak

19-20 = average 

21-22 = good

23-24 = very good 

25 = upper muscle limit