The formula to calculate BMI in adults is weight (lb) / [height (in)] 2 x 703, states the Centers for Disease Control. For instance, for a person who weighs 150 pounds and is 65 inches tall the formula is [150 ÷ (65)2] x 703 = 24.96.
BMI for adults and children is calculated in the same way, but the criteria used to interpret BMI for children, teens and adults is different, according to the CDC. BMI can vary according to age, race and sex. For instance, women generally have more body fat than men, and very athletic people can have a high BMI because they have more muscle than fat.