There's no one-size-fits-all answer to when puppies stop growing -- "size" is the key word, as larger dogs tend to take much longer to physically mature than smaller dogs. All puppies have a fast growth spurt at birth, but the growth slows down after a few months. Proper feeding and exercising can keep your pup on the path to proper growth speed, for as long as his size requires.
All puppies grow very quickly from birth, but the growth rate tends to slow down once they hit 5 to 6 months old. After that time, the bones lengthen slowly and harden, then the muscles finish filling out. This second stage varies, with little dogs finishing not long after their spurt, while big dogs might keep growing slowly for years. Physical maturity isn't the same as sexual maturity; some dogs reach sexual maturity as early as 8 months old, but that could be well before the dog has finished growing.
Little dogs, often considered as weighing 20 pounds or less when fully grown, reach their adult size much faster than larger dogs. After his newborn growth spurt, a little dog will continue to develop for another few months. Some reach their adult size and weight by 9 months old, while others take a bit longer, reaching full physical maturity at 12 months old. Little dogs usually end up weighing up to 20 times what they did when they were born.
After big dogs finish their initial growth spurt, they settle into a slow increase in height and weight for at least another year. Depending on the final adult size of his breed, your large dog might reach his adult height and weight at 2 to 3 years old. It's typical for large breeds to reach their final height first, then spend several months packing on pounds as their muscle tone increases.
Food and Exercise
Fast-growing young pups need plenty of puppy food to keep their energy levels high and their growth on track. The PetMD website recommends you feed your puppy three times a day, but talk with your vet to determine how big the portion sizes should be. The ASPCA warns that overfeeding your puppy can lead to growth that is too rapid, causing bone problems that can follow your puppy throughout his life. Careful exercising is also important for ideal puppy growth. While the bones are growing and hardening, it's important to keep your dog from jumping or running on hard surfaces, such as concrete. Any type of high-impact exercise can damage the bones' growth plates. This can cause the bones to stop growing before they should or grow crooked. For small dogs, wait until they're at least 9 months old for high-impact exercise. Bigger dogs need at least 14 months of growth before leaps are safe.