Many people like to use this weight chart as an estimate....
Some feel like it is not real accurate though and usually the pups end up weighing more than what the chart says. A better estimate is to look at the weight of the mom and dad and grandparents and to know your lines.
By the time the puppy is 12 weeks, you can double it and add a pound. That gets it very close to adult weight. I have also heard you can triple the 8 week weight.
Of course all these are just guesses. Sometimes teensy puppies grow up to be larger than the parents, etc.