Actually, BMI is a pretty unreliable way to assess an individual's health, and for health professionals using it on an individual basis, it can do more harm than good.
The BMI is a measure of height and weight - specifically weight divided by height squared. It is the main measure used by dietitians, health professionals and governments to determine what is and is not a “healthy weight” for an individual.
As a student dietitian I was taught to use BMI to inform patients if they are “at risk” for morbidity and premature mortality. In reality, however, BMI is not only not a good measure of health, it is actually not a measure of health at all.