The formula for determining BMI, or Body Mass Index, was designed by a Belgian statistician in the mid-1800s. It was originally called the Quetelet Index after its originator, and wasn't referred to as BMI until the early 1970s. A measurement of "fatness" and obesity, BMI is just one tool used to indicate whether a person's weight may contribute to health problems.
Calculations
BMI uses just two measurements -- height and weight -- to calculate body mass. The formula divides your weight in pounds by your height squared. This quotient is multiplied by a constant of 703. The product is your BMI. For example, a 72-inch-tall individual who weighs 190 lbs. would calculate BMI as 190 divided by 72 squared, or 5184, times 703, for a BMI of 25.8.
The next step is to compare the result to a standardized chart showing BMIs for ideal weights. A BMI below 18.5 is considered underweight, 18.5 to 24.9 normal, 25.0 to 29.9 overweight and 30 or over obese.
Assessment Tool
BMI is not a direct measurement of body fat, but it is considered to be an alternative because body mass correlates well to body fat. The Centers for Disease Control and Prevention (CDC) caution that BMI calculations should not be used as a diagnostic tool, but as a screening tool. A high BMI should encourage people to consult a healthcare provider to discuss other factors, like family history, diet and physical activity and to take other screening tests to assess whether increased risks of weight-related health problems exist.
Adults and Children
While the calculation is the same for adults and for children, interpreting the results is much different. Most adults over the age of 20 can compare their BMI calculations to a standardized chart, but for children and teens, you need to factor in age and sex to determine if their body mass indicates whether they are overweight. During the growth years, body fat and body mass vary over time, and it is different for girls than for boys.
Men, Women, Age and Athletes
The CDC answers the question of whether BMI is an accurate measurement of body fat by saying the evidence is "fairly strong." But the agency cautions that "the correlation varies by sex, race and age." Even at the same BMI, a woman tends to have more body fat than a man, and older people usually have more body fat than younger ones. Additionally, since this body mass measurement includes both fat and muscle, an athlete may have very little body fat, but still discover she has a high BMI.
The CDC concludes by saying that some people in the "overweight" range may not have excessive body fat, but most people with a BMI over 30 are likely to have increased levels of "body fatness."
Additional Measurements
The National Heart, Lung and Blood Institute recommends augmenting BMI calculations by measuring waistlines. More fat around the waist instead of the hips places the person at a higher risk for heart disease and type 2 diabetes. That risk increases with waist sizes over 35 inches for women and 40 inches for men. The key is to combine BMI with other measurements, tests and a discussion with the family physician to determine how at risk a person may be for weight-related diseases.
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