Your Body Mass Index (BMI) is an estimate of your body fat, based on your height and weight. The higher your BMI, the higher your risk of developing such conditions as heart disease, high blood pressure, sleep apnea, and type 2 diabetes. While it is generally accurate, the BMI can read too high for athletes or others with large, heavy muscles. Likewise, it can exaggerate low readings for frail older people who have lost muscle mass. Start by entering your information into the tool, located on the upper right of this page. After entering your height and weight, click the Calculate button to get your BMI number.