A child's BMI is determined by weight and divided by the his height, squared. The child's weight is then converted into metric and measured against the national percentile.
To calculate a child's BMI, first measure the child's size and weight using a tape measure and weight scale. Take the height results, and multiply it into itself. For example, if your child is 38 inches tall, multiply by 38. Then, take the child's weight, and divide it into his squared height. Finally, convert the resulting number into metric by multiplying it by 703, the standard number for converting non-metric measurements. The final number is the child's BMI.
There are two national percentiles to measure a child's BMI, based on gender. After calculating the BMI, find a Body Mass Index chart online and compare the results to the correct chart, measured by the child's age. If the child's BMI is less than the 5th percentile, the child is underweight. A healthy weight is generally regarded as falling within the 5th and 85th percentiles. If the BMI is greater than the 85th percentile, the child is at risk of being overweight. A child between the 85th and 95th percentile is typically considered overweight.