The Body Mass Index is a calculation of a person's weight relative to his height. The BMI is calculated by dividing the weight in kilo-grams by the height in meters squared: the formula is Kg/M 2 where Kg is the weight in kilograms and M is the height in meters. It has been used extensively to determine if adults or children are overweight or obese. We will demonstrate several ways to determine your child's BMI. A BMI equal to or greater than 25 in adults indicates that the adult is overweight. A BMI equal to or greater than 30 indicates obesity. As the BMI increases to above 25, the risk of complications from overweight also increases.

In addition to determining whether a child is overweight or obese, the BMI is very useful in determining if a child is underweight or nor-mal weight. Since the weights and heights of children change with age, the BMI changes until growth is completed. In children the BMI changes as the child grows taller and gains weight. In adults, since height is fixed, the BMI changes only with weight.

The Centers for Disease Control has gathered data on children relative to height and weight (BMI). We rely on percentiles of data from the Centers for Disease Control to determine overweight and obesity. These percentiles rank children according to BMI and age: the greater the weight, the higher the percentile.

A child with a BMI equal to or greater than the 85 th percentile is considered to be overweight. Any child with a BMI equal to, or greater than, the 95 th percentile is considered to be obese, and this obesity is very likely to persist into adulthood. The percentage of children in the U.S. who are overweight is approximately 25%. This means that 25% of children will have a BMI that exceeds the 85 th percentile when matched for age and sex.

Although the BMI charts and percentiles are very useful for measuring body fat, some overweight children may be misclassified as nor-mal weight. Conversely, a small percentage of upper-limit normal weight children could be misclassified as overweight. For instance, some athletic, muscular children may have a normal amount of body fat, yet have a high BMI. It also has been noted that children who are very inactive may have a normal BMI, but an unhealthy amount of body fat. In spite of these limitations, the BMI is considered to be extremely useful in identifying weight problems in most children.

We suggest three ways to determine your child's BMI.

First, you can ask your pediatrician to determine your child's BMI. The pediatrician has access to tables to determine the BMI for all ages. Then you can refer to our tables to determine the percentile ranking.

Second, you can calculate the BMI. The formula is Weight/ Height/Height 3 703. Divide the weight in pounds by the height in inches and divide by the height in inches again. Multiply the results by 703. For example, if the weight is 140 pounds and the height is 60 inches, the calculation would be as follows: 140/60 5 2.33, 2.33/60 5.0388, .0388 3 703 5 27.2764. The BMI 5 27.

You can use the calculator at the Centers for Disease Control's Web site to calculate the BMI. This is found on the Internet at www.cdc.gov/nccdphp/dnpa/bmi/calc-bmi.htm.