Being overweight or obese increases your risk of heart disease, type 2 diabetes, cancer, hypertension, stroke, sleep apnea and other health problems. The BMI or body mass index test is a quick, almost no-cost way of estimating whether you're at, above or below your ideal weight. It's so quick and easy that it can be used to test large numbers of people at health fairs, and administered repeatedly to track progress over time.
BMI Formula
To calculate your body mass index, you need to know your height in inches and your weight in pounds. Square your height in inches -- that is, multiply it by itself -- then divide your weight by that number. Multiply the result by 703. For example: If you weigh 200 lbs. and are 5 feet 6 inches, or 66 inches tall, your BMI is [200 / (66 x 66)] x 703 = 32.28.
Standardized BMI charts save time and effort, and reduce errors. Instead of performing the full calculation described above, you cross-reference your height and weight in the chart. The number located where the two measurements intersect is your BMI.
Interpreting the BMI
For adults 20 and older, interpreting the BMI is straight-forward. If your BMI is below 18.5, you're considered underweight. If your BMI is from 18.5 to 24.9, you're considered normal. If your BMI is between 25 and 29.9 you're considered overweight and a BMI of 30 or more is considered obese.
BMI for Children
For children -- in this case, defined as anybody under 20 -- an age- and gender-specific percentile curve is overlaid on the BMI chart. This takes into account the natural changes in body composition as children develop. You cross-reference the child's height and weight, note the percentile number where they meet, then interpret that percentile to reflect the child's weight status. A child below the fifth percentile is considered underweight. From fifth to less than 85th percentile is considered healthy, between 85th and 95th percentile is overweight and 95th and higher percentiles are considered obese.
Disadvantages
Although the BMI is a quick, easy test, it does have a few flaws. It doesn't account for the differences between a small and large body frame. But more importantly, it doesn't account for the difference between weight from muscle or weight from fat. An extremely fit, lean individual might still return a BMI in the overweight or obese category because of extra muscle mass, not fat.
Alternatives
Other quick, reasonably effective ways of measuring your body fat include measuring waist circumference or estimating your ideal weight based on height. According to Medline Plus, women have a desirable body weight of 100 lbs. for the first 5 feet of height, then 5 lbs. for each extra inch of height. Men have an ideal body weight of 106 lbs. for the first 5 feet of height, plus 6 lbs. for each extra inch of height. Medline Plus also recommends subtracting 10 percent of the total to account for a small body frame, or adding 10 percent to account for a large body frame.
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