A first indication of body composition is provided by weight. This, in fact, when combined with height, can provide the body mass index (BMI), also known as the Quetelet index. To calculate the body mass index, one proceeds as follows:

It is good practice to measure body weight early in the morning, on an empty stomach, without wearing clothes and after emptying the rectum and bladder. In the context of repeated measurements over time, it is also good that these always take place under the same conditions and on the same day of the week. For the evaluation of the result, the number obtained should be compared with the data below (Bellisle and Rolland-Cachera):

Severe thinness	Moderate thinness	Underweight	Normal weight	Overweight	Moderate obesity	Severe obesity
Less than 16	16 – 17	17 – 18,5	18,5 – 25	25 – 30	30 – 40	Greater than 40

Example: Assume you want to calculate the body mass index of a person who is 1.70 m tall and weighs 70 kg.

BMI = 70 : 1.70 ²

Or

BMI = 70 : 2.89 ≈ 24

The body mass index is about 24, which means, for a comparison of this value with the reference data, that the analysed person is normal weight. It should be noted that although the body mass index, as we have seen, is extremely easy to obtain (all you need is a bathroom scale and a tape measure), it has a serious flaw, namely that of not distinguishing between cellular mass, fat mass and water mass. If one were to try to calculate this for a bodybuilder who is 1.70m tall, weighs a quintal and has 6 per cent fat mass, the athlete would be wrongly obese. Similarly, a person who suffers from water retention, but who is not particularly fat, could be very overweight due to the body mass index. It is therefore a method for assessing body composition that, in many cases, is ill-suited to athletes. It can be used, especially given the ease of calculation to obtain it, by those who are beginners in training and whose lifestyle has never been particularly active. For everyone else, there are more reliable methods.