The Body Mass Index (BMI) is a method of comparing a child's build to those of other children with the same age and gender. It's widely used because it only requires height and weight measurements, which are easily obtained. The BMI should used by a doctor in conjunction with other factors to make decisions about a child's weight.
Formula
The formula for the BMI is w/h^2 where w is the child's weight and h is the child's height. BMI charts typically use BMI values in metric units where the height is in meters and the weight is in kilograms. A BMI with the height in inches and weight in pounds may be converted to metric units by multiplying the BMI by the conversion factor of 703.
Chart
Each gender has a separate BMI chart. The horizontal axis shows the child's age, usually from 2 to 20 years. The vertical axis shows the child's BMI, usually in metric units. The chart contains various curves that show BMI percentile rankings over age. The child's BMI and age are cross-referenced on the chart and that point is compared to the percentile curves to obtain the child's BMI percentile ranking.
Interpretation
The percentile ranking indicates the percentage of children with the same age and gender that have a lower BMI than the test subject. A ranking of 5 percent or less is underweight, a ranking of 85 to 95 percent is overweight and a ranking of 95 percent is obese.
Use
The primary use of the BMI in children is to serve as a screening method for children who may be underweight or obese. It's not a diagnostic tool and a health care provider will need to make the actual assessment. The BMI is designed for children in average physical condition and average frames.
Limitations
The BMI has a statistical correlation with body fat but doesn't actually measure the amount of body fat that a child has. A measurement of body fat would require tests such as underwater weighing, dual energy X-ray absorptiometry or skin-fold measurements with calipers.
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