BMI is short for body mass index, a numerical value of weight relative to height. BMIs are good indicators of healthy or unhealthy weights for adults, regardless of body frame size. A BMI of less than 25 indicates a healthy weight. A BMI between 25 and 29.9 is considered overweight. A BMI of 30 or higher indicates obesity. People with a BMI in the overweight or obese range are at increased risk for diabetes, high blood pressure, heart disease and stroke. Calculate your BMI using the American Heart Association's online BMI calculator.