Method 1

Dogs are categorized as small, medium, and large. Based on your dog's breed, identify its size.

Calculate your puppy's age in weeks. Below is the general time frame when each size grows the fastest:

Small, toy, and teacup: birth to 3 months (12 weeks)
Medium: birth to 4 months (16 weeks)
Large: birth to 5 months (20 weeks)

Weigh your puppy on the scale.

Here is an equation to give you a general idea of your puppy's adult weight. Remember that males are often larger than females and depending on your puppy's age, he may be past his stage of rapid growth:

Growth = Current weight / Age in weeks
Estimated adult weight = Growth x 52

Example 1: A toy poodle is 3 pounds at 14 weeks. His projected adult weight is less than 9.75 pounds.
Growth = 3 lbs / 14 weeks = 0.214 lbs/week
Estimated adult weight = 0.214 lbs/week x 52 weeks = 9.75 lbs
The toy poodle has passed his rapid growth stage, so he should grow to less than the estimated adult weight.

Example 2: A beagle weighs 10 pounds at 4 months. His projected adult weight is 32.5 pounds.
Growth = 10 lbs / 16 weeks = 0.625 lbs/week
Estimated adult weight = 0.625 lbs/week x 52 weeks = 32.5 lbs
The beagle is at the end of his growth spurt, so the estimated adult weight can be considered fairly accurate.

Method 2

1.    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.

(adult weight_{small breed} = weight @ 6 wks X 4)

2.    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.

(adult weight_{medium/large breed}= weight @ 14 weeks X 2.5)

3.    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.

 (weight_{giant breed} = weight @ 6 mos X 2)

Method 3

To estimate the size of dog your puppy will grow up to be, use this chart. 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.