The BMI, or body mass index, is a screening tool used to determine at a glance whether you may be overweight or obese. You can obtain your BMI by comparing your height and weight to a table to produce a number; that number is then used to classify you as underweight, normal, overweight or obese. Different tables are used for adults and children.
BMI Calculation in Adults
To calculate your BMI without a table, divide your weight in pounds by the square of your height in inches. Multiply the result by 703 (a conversion factor based on a mathematical formula). Or, if using the metric system, divide your weight in kilograms by your height in meters.
In adults aged 21 and over, a BMI below 18.5 is considered underweight. A BMI between 18.5 and 24.9 is considered normal. Overweight BMIs fall between 25.0 and 29.9, while a BMI over 30.0 is obese.
BMI Interpretation in Children
A child's BMI is calculated the same way as an adult's BMI, but is interpreted differently. While all adults use the same chart, children's BMI is interpreted according to statistical norms for both age and gender.
Underweight children fall below the fifth percentile. Healthy weight BMIs fall between the fifth and 85th percentiles. Children with a BMI between the 85th and 95th percentile are considered overweight, while those above the 95th percentile are considered obese.
Inaccurate BMI Measurements
Because the BMI is based on total weight, it may be inaccurate in athletes and others with a great deal of muscle mass. Likewise, the BMI may underreport body fat in elderly or inactive people who have lost muscle mass. If your BMI falls outside normal parameters, your doctor may want to perform other body fat tests such as a skin-fold thickness test using calipers. According to the Centers for Disease Control (CDC), some athletes with an overweight BMI may not have excess body fat, but most people in the obese range do.
BMI and Waist Circumference
The CDC cautions that BMI alone cannot determine current or future health. The BMI should be used in conjunction with waist circumference in order to more accurately predict health risks. According to the National Institutes of Health, a waist circumference above 40 inches for men or 35 inches for women increases the risk for heart disease and type 2 diabetes. To accurately measure your waist circumference, place a tape measure around the widest part of your waist, above your hip bones. Breathe in and out a few times, and then take the measurement just after exhaling.
Other Risk Factors
Besides BMI and waist circumference, there are numerous risk factors for cardiovascular disease, diabetes, and other weight-related medical conditions. These include, but are not limited to, smoking, uncontrolled high blood sugar, low HDL cholesterol and high LDL cholesterol. If your risk factors are low and your waist circumference is acceptable, then a higher BMI may not be of much concern. Likewise, increased risk factors may indicate the need for changes even if your BMI is within normal limits. Only your doctor can accurately make these determinations.
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