BMI, or body mass index, is the ratio of one's weight to the square of one's height and is used to get an idea of the fat content of a person's body. BMI has different ranges where less than 18.5 is underweight, between 18.5 and 24.9 is normal, 25.0 to 29.9 is overweight, and then after this, the classes of obesity begin. In William's case, his BMI will be classified as overweight. However, BMI is not always reliable, such as in the case of bodybuilders. William may have a muscle mass, instead of fat. But by standard definitions, he falls in the overweight category.