How Is the BMI Created?
Body mass index (BMI) is a common measure of obesity and body fat. It is a simple ratio of weight-to-height squared, in appropriate units.
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Definition
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BMI is defined as weight in kilograms divided by height in meters, squared. In Imperial units, it is weight in pounds times 703 divided by height squared in inches. So, a person who is 6 feet (72 inches) tall and weighs 200 pounds has a BMI of 200 times 703 divided by 72 squared = 27.1.
Function
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One role of the BMI is to measure fitness and obesity. To this end, there are standard classifications of BMI results: An index of 30 or more is classified as obese, 25 to 29.9 as overweight, 18.5 to 24.9 as normal and under 18.5 as underweight.
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Problems
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The BMI has a few limitations. First, it applies only to adults. Second, it does not account for body shape. Not all people who are 6 feet tall have the same proportions. A person with long legs, a short torso, long neck and small head will weigh less than a person in similar shape who has short legs, a long torso, short neck and large head. People with broad shoulders for their height will weigh more than those with narrow shoulders. Third, it does not account for muscularity. Felix Jones, a running back on the NFL Dallas Cowboys, is 5 feet 11 inches tall and weighs 207 pounds, which would classify him as overweight. In competition, Arnold Schwarznegger was 6 feet 2 inches tall and 230 pounds, which gave him a BMI of 29.5, or close to obese.
Alternatives
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An alternative to the BMI is the waist-to-hip ratio. Simply divide the circumference of your waist by the circumference of your hips. For men, a value of under 0.95 is low risk, 0.96 to 1 is moderate risk and over 1 is high risk, compared to values of under 0.8, 0.81 to 0.85 and over 0.85 for women, respectively.
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