Body mass index, or BMI, is a two-digit number calculated using your height and weight. According to the Centers for Disease Control and Prevention, your BMI provides a reliable indicator of your body fat and may be used as a screening tool to detect people who are in weight categories that are associated with health problems. Limitations exist, however, and BMI is less effective at estimating fat levels in the case of heavily muscular individuals. For this reason, waist size gives a more accurate predictor of the risk for certain diseases.
About BMI
BMI provides a reliable estimation of body fatness for most people, according to the CDC. Though more accurate measurements exist, such as underwater weighing, BMI is inexpensive, quick and easy to perform. For these reasons it is routinely used by health care professionals, in association with evaluations of your diet, activity level, family history and other screenings to assess your risk of a number of lifestyle-related diseases.
Calculating your BMI
A variety of methods exist for you to determine your BMI. For example, using the body mass index table available at the National Heart Lung and Blood Institute's website, linked to below, you can find your height in inches in the left column and move across until you locate your weight in pounds. The point at which your height intersects your weight is your BMI. Alternatively, the National Institutes of Health offers a BMI calculator that lets you compute your BMI without the aid of a chart, simply by entering your height and weight. Body mass index can be calculated by multiplying your weight in pounds by 703 and dividing the product by the square of your height in inches.
BMI Categories
There are four BMI categories. A BMI is considered underweight if it falls below 18.5. The normal range is between 18.5 and 24.9. Individuals with BMI from 25 to 29.9 are considered overweight. Obesity is defined as a BMI of 30 or more. These categories are the same for all adults, regardless of gender. For example, for a person standing 5'10'' tall, a body weight between 132 and 173 lbs. is considered normal. A weight between 174 and 208 lbs. is overweight. A 5'10'' person weighing more than 208 lb. is considered obese.
Considerations
Although BMI correlates fairly strongly with your degree of body fatness, according to the CDC, variations exist, based on age, gender and race. For example, for a given BMI, women tend to have more body fat than men. This is also true for older people, who in general have higher percentages of body fat than younger adults. Also, athletes may have higher BMIs than average persons, owing to increased muscularity. For an athlete, a BMI in the overweight range does not necessarily indicate excessive levels of body fatness.
Waist Size
To provide a more precise estimation of body fat, BMI is often used in conjunction with waist circumference. The risk for chronic diseases, such as heart disease and type 2 diabetes, goes up with a waist size larger than 35 inches for women. Other diseases more common in individuals with increased BMIs and waist sizes include stroke, high blood pressure and some cancers.
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