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BMI Tests for Kids

Body mass index (BMI) describes the amount of body fat a person has by calculating height and weight measurements a specific way. While read the same basic way in children and adults, age and gender play a much larger role in deciphering a child's BMI results, as they're still growing. Finding your child's BMI helps determine whether they're overweight, in good health, underweight or obese.

  1. Calculating a Child's BMI

    • Before you measure your child's BMI, get an accurate measurement of both his height and weight. The formula for determining BMI is the weight (in lbs) divided by the height (in inches), squared. Multiply this result by 703 to get the BMI results. Next to the number, write the child's age and gender. If you're not interested in doing the math yourself, try an online children's BMI calculator.

    Deciphering the Results

    • Since children are still growing and young boys and girls are so different, the resulting BMI number may be identical but mean totally different things for each child. For example, if your 13 year-old daughter's BMI is 20, she's a healthy girl. However, if she's only six with a BMI of 20, it indicates obesity.If you've used a children's BMI calculator that included age and gender options, the results should accurately apply to your child's age and gender. Instead of a set BMI number, children BMI weight categories follow percentage, based on the BMI of other children their age. Children in the 5th percentile or lower are underweight. Those in the fifth to 85th percentile are considered healthy, while those between the 85th and 95th percentile are overweight. Finally, children with a BMI equal or greater than the 95th percentile are generally obese.

    Importance

    • According to the Centers for Disease Control and Prevention, your child's doctor begins testing and tracking BMI as early as two years old. While BMI isn't a sure-fire method of determining your child's weight category, it's a good indication of health in most cases. Over the course of your child's life, periodic BMI results can be used to track changes in health regarding weight. Knowing where your child is in relation to what is considered healthy also gives you and your child a clear idea of where he is versus where he should be.

    Considerations

    • While BMI is generally accurate in the majority of cases, certain factors could lead to varying results that weaken a BMI number's accuracy. Since BMI measures weight in general instead of determining muscle weight versus fat weight, the results can be skewed. A 12 year old boy could be in optimal health but have a BMI saying he's obese if he's got more muscle mass than most children his age. If you suspect your child's BMI results are off, ask his doctor to conduct skin fold thickness measurements and other tests to accurately determine whether he's overweight.

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