For example, a puppy that weighs 45lbs at 20 weeks old, you would do the following: (45/20) x 52 which is 45/20 = 2.25 x 52 = Adult weight of 117. Method 2. A much simplier method for giant breeds, is to double the weight of your puppy at 6-months old to determine what their adult weight is likely to be. For example, a puppy that weighs 65 lbs ...