Body mass index, or BMI, is a value calculated from a person's height and weight that allows for a general assessment of the individual's body weight. BMI should be used as a screening tool rather than a diagnostic tool, as it has inherent limitations. The recommendations for healthy BMI are set by the U.S. National Institutes of Health and are used on an international level by the World Health Organization.
Calculation
To determine BMI, you must measure your height in meters or inches and your weight in kilograms or pounds. If you use metric measurements, the formula is BMI = weight in kilograms divided by height in meters squared. If you use non-metric measurements, it's BMI = weight in pounds x 703 divided by height in inches squared. As an alternative you can use an online calculator like the one on the American Heart Association website.
BMI Ranges
For an adult, a BMI below 18.5 is considered underweight; normal is 18.5 to 24.9; overweight is 25 to 29.9; and anything over 30 is considered obese. The ranges for children, although calculated in the same way, are defined differently. Children are compared to other children in same age range to determine percentiles. A child below the 5th percentile is considered underweight; one between the 85th and the 95th percentile is considered overweight; and one above the 95th percentile is considered obese.
Limitations
Factors that are not taken into consideration in the calculation of BMI are frame size, muscularity, gender, age and specific body type. BMI standards are based on a medium frame-sized individual with sedentary to average activity levels. BMI does not take into consideration decreased height measurement due to postural changes with aging. It may also classify an extremely muscular professional athlete as obese, not because he carries too much weight for his height, but because he has a large muscle mass.
Health Risk Factors
An increased BMI has been correlated with many risk factors, including high blood pressure, high cholesterol, gallbladder disease, heart disease, stroke, sleep apnea, respiratory problems and some cancers. A healthy BMI eliminates these risk factors.
Misconceptions
BMI is too often used as a one-stop assessment. If a person falls outside of healthy BMI parameters, it is recommended that a trained professional do further diagnostic testing to get a more accurate picture of the person's weight and potential risk factors.
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