On average, a 3-year-old boy weighs about 32 pounds on his third birthday and about 36 pounds at the end of the year, just before his 4th birthday. About 5 percent of boys weigh less than 26.5 pounds on their third birthday and about 5 percent of boys weigh more than 44.5 pounds on their 4th birthday.
The average girl weighs about 31 pounds on her 3rd birthday and grows to 35 pounds by her 4th birthday. However, 5 percent of girls weigh less than 25.5 pounds on their 3rd birthday and 5 percent weigh more than 45 pounds on their 4th birthday.
The average 3-year-old boy weighs 33.7 pounds and the average 3-year-old girl weighs 32.7 pounds, both measured at 42 months of age. Most children follow a growth curve established in infancy, and parents should not be concerned unless children deviate significantly from their weight percentile the previous year.