Calculating BMI in children

The body mass index (BMI) uses height and weight in a mathematical formula to determine a child's weight status compared with other children. BMI does not measure body fat.

To calculate your child's BMI, use one of the following equations:

If you know your child's weight and height in kilograms and meters

BMI = weight in kilograms divided by (height in meters times itself). Follow these steps to calculate your child's BMI.

1. Multiply your child's height in meters by his or her height in meters. For example, if your child is 1.24m tall, 1.24 x 1.24 = 1.5376.
2. Divide your child's weight in kilograms by the answer you got in step 1. For example, if your child weighs 25kg, divide 25 by 1.5376. The answer is 16.259105
3. The number you get is your child's BMI. Round it off to one decimal place. In the example, the child's BMI would be 16.3.

If you know your child's weight and height in pounds and inches

BMI = (weight in pounds times 703) divided by (height in inches times itself). Follow these steps to calculate your child's BMI. (Fractions and ounces must be entered as decimal values. See the table below for conversions to decimals.)

1. Convert ounces of weight and fractions of inches into decimals. For example, if your child weighs 35lb and 8oz, 8oz would be 0.5lb and you would use 35.5lb as your child's weight.
2. Multiply your child's height in inches by his or her height in inches. For example, if your child is 39.75in., 39.75 x 39.75 = 1,580.0625
3. Multiply your child's weight by 703. For example, if your child weighs 35.5lb, 35.5 x 703 = 24,956.5.
4. Divide the answer in step 3 by the answer in step 2. In the example, divide 24,956.5 by 1,580.0625 = 15.794628.
5. The number you get is your child's BMI. Round it off to one decimal place. In the example, the child's BMI would be 15.8.

See the following table to convert height fractions and weight ounces to decimals:

Child's BMI conversion table

Fraction	Ounces	Decimals
1/8	2	0.13
1/4	4	0.25
3/8	6	0.38
1/2	8	0.50
5/8	10	0.63
3/4	12	0.75
7/8	14	0.88

After calculating your child's BMI, you can use the BMI chart for boys or the BMI chart for girls to find your child's BMI percentile according to his or her age. Young people between the ages of 2 and 20 with a BMI:

• Below the 5th percentile for their age are considered underweight.
• Equal to or above the 85th percentile for their age are at risk for being overweight.
• Equal to or above the 95th percentile for their age are considered overweight.

A child who has continuously been in the 85th, 90th, or 95th percentile range is probably growing normally (homeostasis) for his or her body type. A child who has a large upward change in BMI percentile, even if not considered overweight, should be evaluated to determine the cause.

Some experts describe children at or above the 95th percentile as overweight, while other experts describe these children as obese. Similarly, children between the 85th and 94th percentiles may be described as "at risk for overweight" or "at risk for obesity."