The body mass index, or BMI, is a ratio of your height and weight that is used to estimate body composition, or degree of fatness. It has the advantage of being simple and applying to a broad range of people, but it does have its limitations. Still, your BMI is a useful piece of information to have and can serve as a general guide to predicting your risk of obesity-related disease.
Formula
To find your BMI, divide your weight in pounds by your height in inches squared anad then multiply by 702. The resulting number is your BMI. This formula is only valid for adults older than the age of 20 years because children and teens are still growing. The formula used to calculate their BMIs take age into account because you cannot effectively compare the body composition of a toddler with that of an 18-year-old young adult. Toddlers naturally have a higher fat percentage that when compared with their height would make them register as overweight or obese on the teen scale. Instead, doctors use a percentile system to determine healthy weights and growth rates.
Interpretation
By the time you're an adult, your body has its fat-to-muscle ratio all figured out, so the same scale is used for everyone. If your BMI is 18.5 to 24.9, you're considered at normal weight for your height. If your BMI is below that, you are considered underweight. If your BMI is between 25 and 29.9, you are considered overweight, and anything over 30 is considered obese. The BMI formula makes it much easier for doctors to communicate a healthy weight to their patients rather than listing a range of weights for each height.
Limitations
The one area where the BMI formula falls short is that it doesn't consider muscle mass. A bodybuilder who is 5 feet, 9 inches tall can easily weigh 200 pounds, which would put his BMI in the obese category even though he may only have six percent body fat. A 2001 study in the "American Journal of Clinical Nutrition" tested the body composition of female military recruits before and after basic training and found that the BMI formula did not accurately reflect their changes in body composition. These women lost fat, but they gained muscle at the same time. Their weight didn't change, so their BMI didn't change -- but their body composition sure did.
Alternatives
Dual X-ray absorptiometry is considered the gold standard in measuring body composition, but it requires specialized machinery and trained technicians, so it's not widely available. Skin fold measurements are used at health clubs to estimate body fat percentage, but they must be done just the right way in just the right places to be accurate. When done right, they can provide a more accurate picture of your level of fatness. Waist circumference measurement is used to predict your risk of heart disease but can't say much about body composition because not everyone stores all of their fat around the waist.
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