Physiology. A ratio of body weight to body height as an indicator of nutritional status. BMI equals body weight in kilograms divided by the square of the body height in meters. BMI correlates with body fat. For adults, BMI falls into these categories: below 18.5 underweight; 18.5-24.9 normal; 25.0-29.9 overweight; 30.0 and above obese.