Different breeds of dogs develop at different rates. Most dogs have reached full size by 18 months of age. Some smaller dogs finish growing sooner.
Dogs grow very quickly in their first 12 weeks of life, after which growth gradually levels off. Large dog breeds have the most dramatic size increases, with some giant breed puppies gaining more than 30 pounds during this time. In general, a puppy attains roughly half its adult weight at 4 months of age (5 months for large and giant breeds dogs). A good estimate of the full-grown size can therefore be made by weighing the puppy at 4 or 5 months.