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Do you wonder if your weight is normal, or if it's excessive for your height? Calculating your Body Mass Index (BMI) can help you find out.

Body Mass Index (BMI) is a number calculated from a person’s weight and height. BMI is a reliable indicator of body fatness for people. BMI does not measure body fat directly, but research has shown that BMI correlates to direct measures of body fat, such as underwater weighing and dual energy x-ray absorptiometry (DXA). BMI can be considered an alternative for direct measures of body fat. Additionally, BMI is an inexpensive and easy-to-perform method of screening for weight categories that may lead to health problems.

BMI is used as a screening tool to identify possible weight problems for adults. However, BMI is not a diagnostic tool. For example, a person may have a high BMI, but in order to determine if excess weight is a health risk, a healthcare provider would need to perform further assessments. These assessments might include skinfold thickness measurements, evaluations of diet, physical activity, family history, and other appropriate health screenings.

The correlation between the BMI number and body fatness is fairly strong; however the correlation varies by sex, race, and age. These variations include the following examples:

• At the same BMI, women tend to have more body fat than men.
• At the same BMI, older people, on average, tend to have more body fat than younger adults.
• Highly trained athletes may have a high BMI because of increased muscularity rather than increased body fatness.

It is also important to remember that BMI is only one factor related to risk for disease. For assessing someone’s likelihood of developing overweight- or obesity-related diseases, the National Heart, Lung, and Blood Institute guidelines recommend looking at two other predictors:

• The individual’s waist circumference (because abdominal fat is a predictor of risk for obesity-related diseases).
• Other risk factors the individual has for diseases and conditions associated with obesity (for example, high blood pressure or physical inactivity).

The following table, taken from the National Institute of Health Publication, The Practical Guide: Identification, Evaluation, and Treatment of Overweight and Obesity in Adults, shows the body weight categories based on BMI and waist circumference, and the indicated relative risk of disease associated to each category. Note that the risk increases considerably in people with a BMI above 30.

Classification of Overweight and Obesity by BMI, Waist Circumference, and Associated Disease Risk*

*Disease risk for type 2 diabetes, hypertension, and cardiovascular disease

**Increased waist circumference can also be a marker for increased risk even in people of normal weight

If you have calculated your BMI and have been classified as underweight, overweight or obese, you should consult with your health care provider for an additional assessment of your weight and health status.

Sources:

• Centers for Disease Control.  BMI - Body Mass Index.  http://www.cdc.gov/nccdphp/dnpa/bmi/adult_BMI/about_adult_BMI.htm
• National Institute of Health Publication, The Practical Guide: Identification, Evaluation, and Treatment of Overweight and Obesity in Adults.  http://www.nhlbi.nih.gov/guidelines/obesity/prctgd_c.pdf