"So, how big will he get?" is one of the most asked question about puppies. Purebred puppies are relatively easy to estimate an adult weight. You know if you purchase or adopt a Yorkie you will not end up with a 50 pound dog. Estimating the adult weight of mixed-breed puppies is a little more difficult. For a simple cross of say a Labrador and a Beagle you know that the resulting puppies will not weigh more than a Labrador or less than a Beagle. When mixes become more complex you will have to decide if your puppy is a toy/small breed, a medium/large breed or a giant breed. Have your veterinarian or a professional at a shelter classify your puppy.
Obtain the weight of the sire (father) and dam (mother) of purebred puppies. Female puppies will weigh about what their dam weighs. Male puppies will weigh about what their sire weighs.
Ask your veterinarian or shelter adoption counselor if your puppy is a toy/small, medium/large or giant breed. Toy/small breed puppies are pugs, poodles and most terriers, and will not exceed 20 pounds. Medium/large breed puppies tend to be working breeds like collies, shepherds and labs, and will not exceed 80 pounds. Giant breeds are mastiffs, and any breed with "great" like Great Danes or great Pyrenees, and can reach 200 pounds.
Take the weight of small or toy mixed-breed puppies and double their weight at 6 weeks. Take the total and double again. This is about what the puppy will weigh as an adult. For example, if your puppy weighs 2 pounds at six weeks, double it to get 4 pounds and then double again to get 8 pounds.
Take the weight of your medium to large mixed-breed puppy at 14 weeks and multiple it by 2. Halve the weight of your puppy at 14 weeks and add it to the total of the weight at 14 weeks multiplied by 2. For example, if your puppy weighs 20 pounds at 14 weeks multiple 20 by 2 to get 40 pounds. Add 10 pounds (1/2 of 20) to the 40 pounds. Your puppy's adult weight will be about 50 pounds.
Take the weight of your giant mixed-breed puppy at 6 months and double it. For example: if your puppy weighs 60 pounds at 6 months his adult weight will be about 120 pounds.