A weight and height percentile is a method of comparing children and young adults up to 20 years of age to others in their age group. This is done for the purpose of tracking growth rates and helping to indicate overall health. It also allows doctors to notice patterns in the child's growth; for instance, if he or she was growing normally for a time and then suddenly stops. There are two different weight and height percentile charts used to track this data. The first is for infants up to three years old, and a second chart for children two to twenty years old. The overlap helps to ensure the data is complete.
The information collected for weight and height percentile charts is relatively straightforward. In addition to the expected weight and height measurements at each doctor's visit, the doctor will also measure head circumference in kids up to three years old. Body mass index, or BMI, is included as well in the measurements, though this is simply calculated using the height and weight. The doctor then takes this information, and looks on the weight and height percentile chart to determine where the child falls compared to other children his or her age for each measurement.
A child in the fiftieth percentile, for example, can be considered exactly average compared to other children. A child in the seventieth percentile for height, for example, is taller than 70 percent of other kids his or her age. This type of information can be extrapolated for each measurement to see where the child falls in relation to others. There are different weight and height percentile charts used for male and female children, since their expected growth rates can be quite different, particularly in adolescence.
Based on this information, and the demonstrated growth rates tracked over time, a doctor will be able to make recommendations for the child's care and his or her regular diet and amount of exercise. If sudden changes in growth occur, the doctor will be able to track this information, and be better able to determine why it is happening. This information ceases at age 20 because by this time, most people have stopped growing. Though there are height and weight comparison charts for adults, which provide average information based on body type, and can provide general guidelines for fitness goals; there is no percentile information available for people above the age of 20, however.