So for a puppy that weighs 15 lbs at 5-months (22-weeks), his growth rate would be 15/22 = 0.68 To get his adult weight you'd multiply the growth rate by 52, i.e. 0.68 x 52 = 35.36. Based on this method, a puppy that weighs 15lbs at 5-months old would weigh about 35.36 lbs as an adult.