Body Mass Index calculators work by multiplying the patient's weight in pounds by 703, then dividing that figure by the square of the patient's height in inches. BMI is a measure of the patient's overall body fat.
BMI calculators use the same formula for both men and women. Additionally, the BMI range that constitutes a healthy weight is the same for both genders.
Some BMI calculators take factors such as age and sex into account to determine the percentile that a person falls into. A person's weight percentile determines how close they are to the national average. In the United States, a person with a healthy weight is defined as having a BMI of between 18.5 and 24.9 as of 2015. This means that a five-foot, nine-inch tall woman should weigh between 125 and 168 pounds. A shorter woman should weigh proportionally less.
While BMI may be a useful tool in some instances, it is not an accurate measure of a person's overall health because it does not take body composition into account. For example, lean muscle weighs more than fat, therefore a very muscular person may have a BMI that erroneously indicates that they are overweight. An elite athlete may have a higher BMI than an inactive person, yet the inactive person may be more overweight and unhealthy than the athlete.