Little Fido may be small now, weighing no more than a few ounces or pounds, but he won't stay that way forever. While predicting his weight in a few months or when he's an adult may be easier if he's a purebred, you can get an idea of his future size by looking at his parents and his general frame.
The Purebred or Crossbred Pooch
Because purebred pups are carefully bred from parents who are documented by their breeders, you'll definitely have an idea of Fido's future size by simply looking at either mom or dad. Generally, female pups will have similar stature to mom and male pups should resemble dad. Have your breeder list the weights of your pup's parents so you can prepare for the future, purchasing adult-appropriate carriers and crates for Fido to grow into. When it comes to crossbreeds, things are a little trickier. If you know who your pup's parents are, he likely will weigh as much as mom, dad or somewhere in between.
Mystery Mixed Breed
Mixed breeds are the hardest types of puppies to predict the weight of, although either of the parents can give you a general idea of what to expect. If you don't know who the parents are, you'll have to look at your pup's frame, paw size and features to get an idea of his breed and future size. Pups with large paws and loose skin usually will have bigger builds as adults. Small dog breeds tend to grow more quickly than large or giant breeds as well. Large dog breeds don't reach their adult size until they reach around 15 months old, while small breeds are at their adult size around 9 months old, according to a November 2004 article published in "The Journal of Nutrition."
General Size Calculations
If you have an idea of your pup's breed, you can multiply his weight at 4 months old by two to predict his weight as an adult if he's a small to large breed and at 5 months of age if he's a giant breed, according to PetEducation.com. Small breed puppies weigh about 1/4 of their adult weight at as young as 6 weeks of age, advises the Charlotte-Mecklenburg Police Department. A more specific calculation involves dividing your pup's current weight by his age in weeks, then multiplying it by 52, according to Dogster. Note that this formula works for pups younger than 1 year of age. While these aren't exact predictions, they can at least give you an idea of Fido's adult weight.
Healthy Weights Affecting Predictions
While you can get a general idea of your pup's weight from his breed and features, if he's getting improper nutrition, this could affect his eventual adult size, warns the People's Dispensary for Sick Animals. Obesity in puppies is caused by feeding your little one too much or continuing to feed him puppy food once he's reached adulthood. In the end, your pup could end up weighing much more than what is normal for his breed and size due to being overweight, which is unhealthy. An underfed pup could weigh much less than he's supposed to and that lack of nutrition could negatively impact his growth.