The Centers for Disease Control and Prevention classify more than one-third of all adults and 17 percent of children in the United States as obese. Healthcare providers and public health organizations utilize the body mass index, also known as the BMI, as a screening tool to identify overweight and obese individuals. Although BMI does not directly measure the amount of body fat, it reliably correlates to direct measures of body fat.
BMI Calculation
Calculating your BMI requires your height measured in either inches or meters and weight measured in either pounds or kilograms. Using pounds and inches, divide the weight by the height squared and then multiply that result by the conversion factor 703. For measurements in kilograms and meters, divide the weight by the height squared.
BMI Interpretation
Adults can interpret their BMI result using standard weight status categories set for men and women of all ages. The category of underweight describes a person with a BMI less than 18.5. Normal weight ranges from 18.5 to 24.9. A BMI measurement of 25.0 to 29.9 classifies you as overweight and all those with a BMI over 30.0 as obese. Health officials interpret the BMI for children and teens differently since the body fat changes with age and differs between boys and girls. You calculate the BMI the same way as for adults, but you must then plot the BMI on a BMI-for age chart provided by the Centers for Disease Control and Prevention. Doing this provides a percentile ranking that can then be used to compare with other children of the same age. Children who rank under the 5th percentile are underweight while those between the 5th and less than 85th percentile fall into the healthy weight range. Children classified as overweight measure a BMI in the 85th to less than 95th percentile and those in the 95th percentile and higher are obese.
Body Fat Calculations
BMI is a useful screening tool and allows you to monitor your weight status and your risk for chronic disease; it is not however, a diagnostic tool. Those classified as overweight or obese should directly measure their body fat using a direct method. Doctors can measure body fat using a calipers to measure skinfold thickness. They can also take an underwater weight measurement, use bioelectrical impedance or perform dual-energy X-ray absorption methods.
Why Use BMI?
With ways to directly measure body fat, why do healthcare personnel and public health officials use BMI to classify weight issues? Mainly because you can measure BMI easily, it requires no special machinery and can be done anywhere by anyone at a low cost. In addition, the other body fat measurements require observers and machines making them difficult to standardize. The correlations between the BMI and body fatness is strong, according to the CDC, with only slight variations between men and women. One exception includes highly trained athletes who measure a high BMI due to increased muscle mass instead of increased body fat.
References
- Centers for Disease Control and Prevention: Obesity -- Halting the Epidemic; March 2011
- Centers for Disease Control and Prevention: About BMI for Adults; February 2011
- Centers for Disease Control and Prevention: About BMI for Children and Teens; February 2011
- National Heart Lung and Blood Institute: BMI Calculator
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