Normally, a 9-year-old boy weighs about 29kg, but his weight may vary slightly due to genetic, nutritional and physical factors. Weight growth is non-isotonic, from birth, within 1 year of the first 3 months of growth faster than the next 9 months, about 3 times the birth weight, 2 years old to pre-puberty weight growth is slow, 9-year-old boys should be about 29kg normal weight, the formula is (age (years) × 7-5)/2. As weight is affected by multiple factors such as genetics and nutrition, there are individual differences. Therefore, when evaluating children’s weight, should be individual children’s own weight changes as the basis, can not take the “formula” calculated weight as a standard for evaluation, should be analyzed on a case-by-case basis. Children’s weight is not limited to a certain value, but if the weight of a 9-year-old boy is significantly different from that of children of the same age, it is recommended to consult a doctor in time.