The Body Mass Index, or BMI, estimates the amount of body fat from a person's height and weight. For children and teenagers, BMI is also related to gender and age, since the amount of body fat differs between boys and girls and also changes with growth. To know the healthy BMI for a female child, you need a few statistics and charts.
BMI for Age and Gender
The Centers for Disease Control and Prevention (CDC) publishes two separate growth charts for children from ages 2 to 20 years, one for girls and one for boys, to plot a child's BMI based on age. This BMI-for-age growth chart interprets the calculated BMI as a percentile. A percentile shows how the individual child is doing in terms of her weight among children of the same age in the United States. The percentile also helps classify the child into a weight status category based on her BMI: underweight, healthy weight, overweight or obese.
Weight Status Categories
A BMI in a child less than the 5th percentile is considered underweight, and this may indicate malnutrition, an eating disorder or other health problems. Healthy weight is between the 5th and the 85th percentile. A BMI equal or greater than the 95th percentile is considered obese. Identifying the cause of obesity in children is important, because early intervention and losing weight can help prevent diseases that are associated with more body fat, like type 2 diabetes.
Calculating a child's BMI
Know your child's age in years as well as her height and weight. The height and weight should be measured in kilograms and centimeters or in pounds and inches.
You can use any of a number of online BMI calculators that allow you to enter the height and weight and then automatically give you the BMI number. However, you can calculate the BMI on your own using this formula: weight divided by height divided by height. In other words, to calculate BMI, you divide the weight by the height, and then divide that result by the height again.
If your measurements are in kilograms and centimeters, multiply the number by 10,000. If your measurements are in pounds and inches, multiply the number by 703. This result is the child's BMI.
Using the CDC growth chart for girls
Download a copy of the CDC growth chart for girls or obtain a copy from your child's doctor and plot the BMI based on age. Along the bottom of the chart, the ages range from 2 to 20. Find the vertical line that corresponds to your child's age, then travel up that line until you come to the right number for the BMI, which is listed along the side of the page, going up from 12 to 35.
The different percentiles are marked by the eight lines along the growth chart to show different percentiles: 5, 10, 25, 50, 75, 85, 90 and 95. Your child may fall on one of these lines, for example, on the 50th percentile, or between two of these lines, such as between the 75th and the 85th percentiles. Identify the percentile or percentile range for your child's BMI-for-age.
Figuring out if a female child has a Healthy BMI
If your child's weight falls between the 5th and 85th percentile, then she has a healthy weight, but don't forget that regardless of body weight, eating a well-balanced diet and doing regular physical activity are both important for your child's health and development.
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