Dogs are usually considered to be full grown between 1 to 2 years old, depending on their breed size. Smaller dogs typically reach their full size at much younger ages than larger dogs. Where a smaller dog may be full grown by 10 to 12 months of age, a giant breed often takes the full 2 years to mature.
Dogs finish growing taller several months before they stop adding muscle and body weight. For example, a medium-sized dog may reach its adult height around age 6 months but continue to fill out until 12 months of age. One way to roughly estimate a puppy's adult weight is to double the puppy's weight at age 4 months.