The BMI is a simple model that indicates whether a person is ideal weighted, over weighted or underweighted. A BMI between 20 and 25 is considered as an ideal weight person. If the BMI is more than 25, the person is considered as an overweight person. A BMI less than 20 is of course considered as underweight.
Nevertheless, the BMI is just a simple model that roughly indicates ideal weight, overweight and underweight, since no gender, age or body structure is taken into account. A measurement of a person's percentage of fat should be realized to obtain more precise knowledge of a person's body structure.
