Staying fit isn't just a matter of watching your weight. Body mass is just as important, if not more so, in determining your risk for certain health problems related to being overweight--such as heart disease, high blood pressure and Type 2 diabetes. Body mass index is a way to determine how much fat you're carrying based on your ratio of weight to height.
Benefits
Body mass index is an indirect measure of fat used in place of direct methods such as underwater weighing and dual energy X-ray absorptiometry, according to the U.S. Centers for Disease Control and Prevention. It's less expensive and convenient than such techniques. It allows people to "compare their own weight status to that of the general population," according to the CDC.
Calculation
To determine your body mass index, multiply your weight in lbs. by 703, according to Medline Plus. Then divide that by your height in inches. Finally, divide that total again by your height in inches. Medline provides an example: A woman weighing 270 lbs. with a height of 68 inches has a body mass index of 41.
Interpretation
A normal, healthy body weight for adults ages 20 and older, both men and woman, is 18.5 to 24.9, according to the Centers for Disease Control and Prevention. A measurement below 18.5 means you're underweight. A number between 25 and 29.9 indicates that you are overweight. A body mass index of 30 or higher means you are obese. Thus for a person who is 5 feet 9 inches tall, a weight of between 125 lbs. and 168 pounds is healthy. A weight of 203 pounds or more indicates obesity.
Exceptions
In some situations, body mass index is a less-than-ideal way to determine whether you need to lose weight. Body builders and other highly trained athletes are likely to have a high body mass index, but that's because they have packed on muscle, not fat. In people older than 65, a higher body mass--between 25 and 27--may be healthier than the standard "normal" range, according to Medline, because it puts them at lower risk for osteoporosis.
Children
Body mass index can be useful in children, but interpretation is different than for adults, according to KidsHealth.org. It involves comparing the body mass index calculation with a chart that indicates percentiles for the age of the child. In other words, the chart allows you to compare your child to other children of the same age, A child whose body mass index falls below the fifth percentile is underweight. Figures between the fifth and 85th percentile are considered normal. The 85th to 94th percentiles are the "overweight" range. And a body mass index at or above the 95th percentile indicates obesity. KidsHealth.org recommends seeing a pediatrician for measurement and interpretation of body mass index for a child.
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