Understanding body mass index

VOL 117 / NO 2 / FEBRUARY 2005 / POSTGRADUATE MEDICINE

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With all the talk about the growing problem of overweight and obesity in the United States, you've probably heard the term body mass index, or BMI. But what does it mean, and how is it calculated?

What does BMI mean?
BMI is better than weight at estimating body fat. Because excess weight can lead to heart disease, diabetes, cancer, and other diseases, BMI is one way of measuring your level of health risk. Other factors that also are important to your health include your daily diet, exercise level, waist measurement, blood pressure reading, family history of disease, smoking status, and levels of blood sugar and cholesterol.

Overweight and obesity are big public health concerns. About 64% of adults and 15% of children are overweight or obese. The reasons for these high percentages are complex, but genetics, culture, social status, and inactivity coupled with overeating all probably play a role.

How is BMI calculated?
BMI is calculated by dividing your body weight in kilograms by the square of your height (kg/m (2)). If you prefer to work in pounds and inches, multiply 700 by your weight in pounds and divide the result by your height in inches squared. For example, a 150-lb person who stands 5 ft 4 in tall has a BMI of 25.6 ([700 X 150] ÷ [64 X 64]). If this seems complicated, don't worry. A BMI calculator on the Internet can do the work for you. For example, the Centers for Disease Control and Prevention provides one at http://www.cdc.gov/nccdphp/dnpa/bmi/calc-bmi.shtml.

Once you have figured your BMI, you can find out which weight category you are in (see box). Remember that BMI is only a guideline.

What should I do if I'm overweight?
If you calculate your BMI and find that you are in the overweight or obese category, contact your doctor. If your doctor agrees, he or she can help you develop a plan that includes a balanced diet and increased activity. Studies have showed that a weight loss of even 10%--or 15 lb in a 150-lb person--can lower health risks.

It is important to note that BMI is used differently in children because they are still growing. BMI in kids is calculated by factoring in age and sex. Children should not try to lose weight without a doctor's input. For more information on the use of BMI in children, visit http://www.cdc.gov/nccdphp/dnpa/bmi/bmi-for-age.shtml.

How to read your BMI
BMI	Weight category
Less than 18.5	Underweight
18.5 to 24.9	Healthy
25 to 29.9	Overweight
30 or higher	Obese

This information is not a substitute for medical treatment.

Please send technical questions related to the Web site to Ann Harste