BMI (from Body Mass Index) is a parameter for evaluating body mass. It is not applicable to children and elite athletes, since it is built on the average of the general population. The BMI is able to identify the subject's underweight or normal-weight or overweight condition, with the sole use of a formula that requires two data: the stature expressed in meters and raised squared, and the fasting weight. The result of the equation must then be contextualised in a specific and indicative evaluation scale.
In practice, the BMI is a value proportional to the body mass and inversely proportional to the square of the stature. So, if all the dimensions of the human body double, the BMI remains the same, but if it only doubles the mass (commonly identified as "weight") with a constant square of height, the BMI doubles; since the ratio is not between mass and height, but between mass and stature squared, and considering that the trend of mass and stature is not proportional, the BMI of people with a high stature indicates a higher level of body fat than the real condition.
In comparison, the Ponderal Index (Ponderal Index - PI) is based on the natural mass scale with the third power of height. However, not only the taller people are "overestimated", but it is necessary to keep in mind that low people tend to have smaller body structures in proportion to their height.
Nick Korevaar (professor of mathematics at the "University of Utah") suggests that instead of using the square of the stature (like the BMI) or the elevation to the cube (like the PI), it would be more appropriate to use an exponent of the power between 2.3 and 2.7 (as originally noted by Quetelet).
