The calculation of the BMI consists in the division of the weight of a subject, expressed in kg, by the square of its height expressed in meters. In an individual 175 cm tall and 70 kg heavy, the BMI calculation is therefore based on the following equation:
BMI * = 70 / (1.75) 2 = 22.9 kg / m2
The BMI calculation was first proposed by the Belgian scholar Adolphe Quelet (1796-1874). Today, the BMI has become a leading diagnostic tool for assessing the weight of a subject and its distance from the ideal one, considered as such because it is statistically associated with the lower risk of getting sick.
Depending on the BMI, the population is generally divided into five weight classes: normal weight, underweight, overweight and obese.
| Category | BMI range - kg / m2 |
| Severe underweight | <16.5 |
| Underweight | from 16.5 to 18.4 |
| Normal | from 18.5 to 24.9 |
| Overweight | from 25 to 30 |
| First degree obesity | from 30.1 to 34.9 |
| Second degree obesity | from 35 to 40 |
| Third degree obesity | > 40 |
The ideal values of BMI are around 22.5 kg / m2 in men and 21 kg / m2 in women (in one study, British men were more attracted to female models with BMI of 20.85). The normal range of BMI (20-25 kg / m2) is quite wide precisely as a function of the differences linked to sex and the physical structure of the population. The calculation of the BMI does not take into account the greater muscle mass of the man and the young person compared to the woman and the elderly, nor the differences regarding the bone mass.
