Body mass index (BMI) uses height and weight measurements to estimate how much body fat a person has. To calculate BMI, divide weight in kg by height in meters squared; for pounds and inches, divide weight by height squared and multiply the result by the conversion factor 703.
An easier way to measure BMI is to use a BMI calculator. Once you know your child's BMI, it can be plotted on a standard BMI chart. Kids fall into one of four categories:
underweight: BMI below the 5th percentile
normal weight: BMI at the 5th and less than the 85th percentile
overweight: BMI at the 85th and below 95th percentiles
obese: BMI at or above 95th percentile
BMI is not a perfect measure of body fat and can be misleading in some situations. For example, a muscular person may have a high BMI without being overweight (because extra muscle adds to a body weight — but not fatness). In addition, BMI may be difficult to interpret during puberty when kids are experiencing periods of rapid growth. It's important to remember that BMI is usually a good indicator — but is not a direct measurement — of body fat.
Childhood obesity :)