The body mass index (BMI) is the relationship established between the size and weight of a person between 18 and 65, and the amount of body fat, to calculate your ideal weight by the formula: weight (kg ) / height (m2). Normally, it is considered ideal BMI to be between 18 and 25 women, and 20 to 25 in men. If the resulting values are not in this range they are classified according to certain standards as underweight, normal weight, overweight (grade I, grade II or pre obesity), obesity (type I, type II obesity, obesity type III, or morbid obesity and type IV or extreme). In this classification all people equally are not covered because, for example, athletes with a lot of muscle have a higher BMI because muscle weight. Therefore, we should study each case in particular and for the elderly or children's formula does not apply. In any case, BMI helps realize weight control.