While BMI does not measure body fat directly, it has been shown to correlate to direct measurements for most children and teens. Once BMIs are calculated, the values are plotted on BMI charts, which will give meaning to individual values by classifying children as either underweight, at a healthy weight, overweight or obese. For children, BMIs values are often referred to as BMI-for-age values, and they are both age- and gender-specific.
BMI Charts
Typically, only one BMI chart is used for both men and women. However, for children aged 2 to 20, there are two gender-specific charts which take into account the fact that the amount of overall body fat differs between boys and girls. The amount of body fat also changes with age and stage of development. Beginning at age 2, health care professionals will track children's BMI values as part of their routine physical examinations. This information is used to help identify those children who are significantly overweight or are at risk for becoming so.
Calculating BMI
BMI values are calculated using the same standard formula as is used for adults. Using imperial measurements, the child's weight in pounds is multiplied by 703, and the product is divided by his height in inches squared. This can also be written as: BMI = (weight in pounds) x 703 / (height in inches squared).
BMI-for-age Percentiles
Children's BMI values alone are not used to classify weight categories, as body fat distribution is known to change throughout development. Therefore, when calculating kids' BMI values, the numbers are further manipulated to produce a BMI-for-age percentile. According to Kids Health, the percentiles will indicate not only a child's weight category, but also how his BMI compares to other kids of the same age and gender. Children with values under the 5th percentile are deemed underweight, those falling between the 5th and 85th percentile are in the healthy weight range, values between the 85th and 95th percentile are overweight and those above the 95th percentile are considered obese.
Limitations of BMI values
While knowing a child's BMI can be useful in treating both under- and overweight children, it is not in itself a diagnostic assessment tool, but rather merely an estimate of body fat. According to the CDC, children with larger or smaller frames, or those with higher amounts of lean muscle mass due to athleticism, may find themselves falling outside of the healthy weight range when BMIs are calculated, when clearly this may not be the case. Children who are considered overweight will likely be recommended to undergo testing that directly measures body fat in order to get an accurate assessment.
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