BMI, or body mass index, is a number that attempts to provide a reliable indicator of the degree of body fat and is often used to screen for health problems. Too much body fat can lead to other illnesses and health problems, such as cardiovascular disease, stroke, diabetes and peripheral vascular disease. Other methods used to measure the amount of body fat include skin fold thickness measurements, underwater weighing, bioelectrical impedance and dual energy X-ray absorbtiometry. However, because these methods are not readily available, or are too expensive, body mass index is more commonly used.
BMI Calculations
Body mass index is calculated the same way for children, men and women. The formula uses a ratio between weight and height, according to the National Heart Lung and Blood Institute. When the formula uses pounds and inches it is equal to weight in pounds divided by height in inches squared times 703. For example, an individual weighing 150 pounds and 65 inches tall would have a calculation of [150/(65)(65)]x703=24.96. In the metric system weight in kilograms is divided by height in meters squared. Using the same example where the height and weight are converted to metrics the calculation is 68/(1.65)(1.65)=24.98.
Normal BMI Range
The normal range of body mass index for adults who are 20 years of age and older is interpreted using standard weight categories. Children and teenagers use categories that are both age and sex specific. This is because the amount of body fat changes as teenagers mature and the amount of body fat is different between young girls and young boys. Adult body mass index does not take into account sex or age differences. Also, healthy weight ranges are not provided by the Centers for Disease Control and Prevention for children and teens because they change each month for age and because healthy weight ranges will change as height increases. According to the CDC, normal range for body mass index is between 18.5 and 24.9. A BMI of 25 and over is considered overweight or obese and below 18.5 is considered underweight.
BMI Range the Same for Men and Women
As adults, the same body mass index recommendations are used for both men and women, even though fat content is different between the sexes. According to Weight Watchers, the same normal ranges are used for both sexes because doctors have found that as body mass index increases above 25 so does the risk for ill health. The average man with a body mass index of 28 has almost a 16-fold increase of risk for diabetes compared to men who have a BMI of 23 or less.
When BMI is not Accurate
Researchers at Baylor College of Medicine have found that body mass index indicators may not be accurate across all ethnic groups. In a study published in the "British Journal of Nutrition," researchers determined that BMI is inexact and should be tailored to the individual to help target those at risk. Researchers also found that athletes who were very muscular and weighed up to 300 pounds still had less body fat, and therefore less risk, than individuals at the same weight and height with less lean body mass.
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