BMI is calculated by dividing a person’s weight in kilograms by height in meters squared. The mathematical formula is “weight (kg)/height (m²).”

TO DETERMINE BMI using pounds and inches, multiply weight in pounds by 704.5,* divide the result by height in inches, and then divide that result by height in inches a second time. (You can also use the BMI calculator at www.nhlbisupport.com/bmi) NOTE: * The multiplier 704.5 is used by the National Institutes of Health (NIH). Other organizations may use a slightly different multiplier; for example, the American Dietetic Association suggests multiplying by 700. The variation in outcome (a few tenths) is insignificant.

AN EXPERT panel convened by the National Heart, Lung, and Blood Institute (NHLBI) in cooperation with the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), both part of NIH, identified overweight as a BMI of 25 to 29.9 kg/m², and obesity as a BMI of 30 kg/m² or greater. However, overweight and obesity are not mutually exclusive, since people who are obese are also overweight. Defining overweight as a BMI of 25 or greater is consistent with the recommendations of the World Health Organization (WHO) and most other countries.

CALCULATING BMI is simple, quick, and inexpensive—but it does have limitations. One problem with using BMI as a measurement tool is that very muscular people may fall into the “overweight” category when they are actually healthy and fit. Another problem with using BMI is that people who have lost muscle mass, such as the elderly, may be in the “healthy weight” BMI category (BMI 18.5 to 24.9) when they actually have reduced nutritional reserves. BMI, therefore, is useful as a screening tool for individuals and as a general guideline to monitor trends in the population, but by itself is not diagnostic of an individual patient’s health status. Further assessment of patients should be performed to evaluate their weight status and associated health risks.

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