The average height and weight of a 9-year-old boy in the United States is 52.5 inches and 63 pounds, according to MDHealth. It is important to remember that this average can be affected by many things, such as growth spurts that cause children to gain and lose weight rapidly. Height and weight can also be considered using a BMI calculator or chart.
According to the CDC, height and weight in relation to BMI is calculated differently for children than for adults, and it changes with age. Healthy weight range charts cannot be provided by the CDC for children because children's weight changes in relation to growth in height. What is considered a healthy weight also changes for children each month. The CDC also suggests that BMI does not directly measure body fat but is simply a tool used to compare the height and weight of children who are the same sex and age.
WebMD provides a tool that people can use to predict a child's height upon reaching adulthood that is based on the height of both parents. Talking to a pediatric physician is best if there are any concerns about height or weight. There are also multiple BMI calculators online that can be used for children.