Puppies at different ages grow at different rates, and puppies of different breeds grow at different rates.
Generally, an adult dog will weigh about twice as much as he did when he was 4 months of age; giant breeds will double what they weighed at 5 months.
Studies done in the UK indicate that you can expect a small or medium breed puppy to reach 99% of it’s adult weight somewhere between 9 and 10 months of age.
Large or giant breed puppies take longer to mature, and don’t reach this stage until they’re at least 12 – 18 months old. Puppies from breeds at the top end of this scale (Great Danes, Mastiffs, Rottweilers etc.), are unlikely to really be ‘finished growing’ until they’re 2 (or even 3) years old.
THE EASY WAY: To estimate the size of dog your puppy will grow up to be, you can try using the chart above. Find your dog’s age, and then draw a line straight up until you find your dog’s weight. Then follow the nearest curve or make an imaginary one and follow it over to 30 months of age. Read the corresponding weight for that age. This will approximate the weight your dog will be at adulthood. Remember, this is only an estimate.
THE HARDER WAY: Or, you can try some math….. ( according to some ancient veterinarian, I’m sure )
Toy to Small Breeds – weight at 6 weeks x 4 = adult weight.
Medium to Large Breeds – weight at 14 weeks x 2.5 = adult weight
Large to Giant breeds – weight at 16 weeks x 2 = adult weight
Then, if we further believe these ancient veterinarians…
A pup will be between 2/3 – 3/4 of its adult weight at around 6 months of age.
True, guessing an adult dog’s weight by their puppy weight is a bit of a challenge… kind of like guessing an adult human’s weight by how large a child they were!
These guidelines, however, at least give us a start.