The body mass index measures normal height and weight by dividing weight in kilograms by height in meters squared before comparing the score to a range according to age and height, states the Centers for Disease Control. For adults age 20 and over, 18.5 to 24.9 is the normal range.
The BMI correlates with health issues and can be used as a screening tool along for health risks when used alongside other tests rather than a diagnostic approach to specific conditions, according to the CDC. Health professionals also conduct additional tests to determine if a high BMI score is associated with obesity as opposed to a muscular athlete with normal health. Tests such as skinfold thickness measurements, bioelectrical impedance, densitometry and dual energy X-ray absorptiometry measure body fat more directly than the BMI. As of 2015, the BMI is moderately correlated with these measures and is simpler and less expensive to perform than most other screenings.
While BMI is traditionally measured using the metric system for adults, the same approach can utilize pounds and inches if the result is multiplied by 703 to arrive at a score. BMI scores for children and teens are compared to scores that differ by sex and height in order to account for variations in body fat within these categories. Scores for children and teens are expressed as percentiles to demonstrate how each individual compares to others in their peer group, says the CDC.