Adolphe Quetelet invented the body mass index, or BMI, chart in the early 1900s. The purpose of the chart is to give a measurement of a person's weight based on their height and gender, determining whether the individual is underweight, healthy, overweight or obese.
A person's BMI is determined by converting their weight into kilograms and height into centimeters. The body weight is then divided by the height to yield the body mass index number.
A normal BMI falls between 18.5 and 24.9. If the number is below 18.5, the person is underweight. If the number is above 24.9, the individual is overweight. When the BMI is 30 or higher, the person is diagnosed as obese, according to WebMD.