The Body Mass Index (BMI) - Italianized with the initials IMC (Body Mass Index) - is obtained by dividing the body weight (expressed in kg) by the height (in meters) squared. With a simple calculation, a value is thus obtained, expressed in kg / m2, which correlates very well with the fat mass of the subject; in general, the higher this number corresponds to BMI, the greater the lipid deposits. Ultimately, the evaluation of the degree of adiposity is obtained by comparing this data with the average values of the population.
| BMI (adults) | CONDITION |
| <16.5 | SERIOUS MAGREE |
| 16-18.5 | UNDERWEIGHT |
| 18.5-25 | normal weight |
| 25-30 | OVERWEIGHT |
| 30-40 | OBESITY OF MEDIUM DEGREE |
| > 40 | HIGH DEGREE OBESITY |
Since in childhood and adolescence the BMI is characterized by a remarkable variability, linked above all to sex and age, it is appropriate to take as reference - rather than an absolute value as is normally done with adults - the tables of percentiles.
In creating these graphs, normality limits are obtained by dividing the range of data collected into 100 parts, called percentiles. This division is made in such a way that a given proportion of the sample children is found above and under particular measures at particular ages. The 50th percentile represents, for example, the average BMI value, while another curve (corresponding for example to the 30th percentile) informs us that a certain percentage of children has lower values (in this case 30%) and a equally determined part (in this case 70%) has higher values. We could then compare the growth percentiles to the normality tracks.
The percentages of BMI in childhood and adolescence available in the literature refer to different populations, because this parameter can have a significant influence on the normal ranges; in Italy, for example, the BMI tables of Cacciari and coll., which take into account the considerable variations between the various Italian geographical areas, proposing different standard values.
At the international level, instead, the percentiles proposed by Cole and coll. (reported at the bottom of the page), whose interpretation is based on the following reference values:
| Underweight | Less than the 5th Percentile |
| Normal | From 5th to 85th Percentile |
| At risk of being overweight | From 85th to 95th Percentile |
| Overweight | Higher than the 95th percentile |
| BMI children | BMI girls | Interpretation graphics |
Click on the image to enlarge; source: www.cdc.gov/growthcharts/
