A patient can find his body mass index, or BMI, on a chart by lining up his weight on the vertical axis and his height on the horizontal axis, according to WebMD. The body mass index is the ratio between height and weight, and it indicates whether someone is underweight, normal weight, overweight or obese, notes the Centers for Disease Control.
Technically, body mass index is weight in kilograms divided by the square of height in meters, states the Centers for Disease Control. In adults, the metric is a screening tool but is not diagnostic by itself. Medical professionals can use specific measures such as skinfold thickness or underwater weighing to assess body fat percentages if a body mass index screening warrants them.
People with a body mass index of 30 or higher may be obese, while those with indexes between 25 and 29.9 are overweight, according to the National Institutes of Health. People in the 18.5 to 24.9 range are in the normal weight range, and a body mass index of 18.5 or lower indicates underweight status.
Body mass index assesses large populations for potential weight problems, according to the Centers for Disease Control. The U.S. population with an index in excess of 30 increased from the 1970s to the 2010s before leveling. As of August 2015, the only group in which obesity continues to rise is adult women over the age of 60.