Measure of fatness or thinness of an adult person, computed by dividing his or her bodyweight (W) in kilograms by the square of the height (H) in meters: BMI = W ÷ H2. For imperial units (weight in pounds and height in inches) the formula is: BMI = 703.07 x W ÷ H2. In the US, a BMI of 19 or less is termed as underweight, 20 to 24 as average, 25 to 29 as overweight, and 30 or more as obese.