This is a very common method of evaluating individual people to see if they are under or overweight. It involves comparing their weight to their height by dividing the weight measurement (expressed in kilograms) by the square of the height (expressed in meters). A BMI of below 18.5 is underweight, between 18.5 and 25 is an indication of healthy weight, 25 to 30 is overweight, a BMI of over 30 is referred to as obese, over 35 is known as morbid obesity, and over 40 indicates extreme obesity.