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How do I Calculate Children's BMI?

By J. Beam
Updated Mar 03, 2024
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Calculating a person’s body mass index (BMI), which is a number attained from a mathematical equation factoring a person’s height and weight, has proven to be a reliable indicator for screening weight problems that could lead to health problems. When calculating an adult’s BMI, the number becomes an indicator of whether a person is overweight or obese. Similarly, calculating children’s BMI results in a similar indicator of whether a child is overweight or obese, but also serves as an indicator of whether a child is at risk for becoming overweight or obese.

To calculate children’s BMI, you must first have the variables to plug into the equation. You must have the weight and height of the child. There are two formulas to calculate children’s BMI – one using kilograms and meters and one using pounds and inches. They are not the same formula, but when worked correctly will calculate the same answer.

If you are calculating children’s BMI based on weight and height in kilograms and meters, the formula is BMI = weight (kg) divided by the child’s height in meters squared. As an example, take a child who weighs 45.5 kilograms and is 1.5 meters tall. To calculate the example child’s BMI, you would first multiply 1.5 times itself (1.5 X 1.5) to equal 2.25. Then you divide weight (45.5 kg) by 2.25 to equal a BMI of 20.2. (Round any numbers beyond the decimal in your calculation to the nearest tenth.)

When measuring weight and height in pounds and inches, the formula to calculate children’s BMI is weight in pounds times 703 divided by height in inches squared. Taking the aforementioned example, the child’s weight in pounds is 100 and his height in inches is 59” (or just under five feet). Taking the weight of 100 pounds and multiplying it times 703 gives us 70,300. Multiplying the example height times itself equals 3481. Dividing 70,300 by 3481 gives us a calculated BMI answer of 20.2 when rounded to the nearest tenth.

Calculating children’s BMI is much simpler than interpreting the answer because unlike adults, children are not done growing. This means their age must be taken into account as well as their growth chart history. A body mass index chart is provided to measure children’s BMI and supply percentile indicators that factor in age as well as gender. To help interpret children’s BMI, you can graph the information on a gender correct BMI chart that indicates average percentiles.

For example, an eighteen-year-old boy with a 20.2 BMI would fall just under the 25th percentile, meaning at least 75% of kids his age would have a BMI higher than his. Conversely, if a nine-year-old boy has the same BMI of 20.2, he would be in the 95th percentile, meaning that 95% of kids have a BMI lower than his. In the latter example, this child would most likely be considered overweight as a BMI that falls into higher than the 90th percentile is an overweight indicator. The exception to this could be if a child’s height, weight, and BMI have remained in the same percentile range (high or low) consistently since their growth began. This could be a case of a child with unusual muscle mass or above average height.

With or without the presence of a gender appropriate BMI chart, children’s BMI calculations can still be difficult to interpret. As a rule of thumb, children’s BMI should not be based on adult calculations since an ideal adult BMI is 19.1 to 25.8, which may or may not be ideal for a child depending on their age, weight, height, and growth history.

If you are concerned about your child’s weight, talk to your pediatrician or family physician. They can calculate and interpret your child’s overall physical development and assess risks and provide intervention if necessary.

The Health Board is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.

Discussion Comments

By Cageybird — On Feb 11, 2014

@Reminiscence, I agree that a child's BMI should mostly be viewed as part of his or her development. My youngest daughter got picked on a lot at school because of her size, and we thought we were doing the right things with her diet and activities. She remained heavy until she turned 11, then she had a growth spurt and became interested in sports. She decided on her own to take jujitsu classes and use their gym for workouts. I don't know if she would have done all that if we had shamed her with numbers back in the day.

By Reminiscence — On Feb 11, 2014

I suggest that parents be very sensitive when it comes to measuring a child's BMI or any other weight-related concern. It shouldn't be done to embarrass a child or shame him or her into making radical changes. A child may already be dealing with bullying or social ostracism at school because of his or her weight, so it may be traumatizing for a parent to add one more log to the emotional fire.

I'd say knowing a child's BMI would be useful information, especially if the results put him or her into a high risk category, but as the article points out, the BMI number itself is only a small part of the bigger picture. Children's bodies change from year to year, so a chubby eight year old may become a normal ten year old without making any adjustments in his or her lifestyle or diet.

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