Body mass index ( BMI ) is an indirect measure of a person's body fat. The formula is the same for adults and children, as well as with different genders and ethnicity. Knowing your BMI status is a must for every body. If you are Obese you stand the risk of getting heart attacks, high blood pressure and various fat related ailments. You can calculate it on your own or go to a hospital where they will charge you just to do the same thing. However, the BMI formula does differ slightly depending on whether you are using the metric system or the English system. How to Calculate BMI Using the Metric System If you're using the metric system, you can learn how to calculate BMI by using the following formula