Is there a formula for telling a puppy's adult size?

Sophie is 12 wks old and at the vet on Monday she weighed 6 1/4 lbs. Her father weighed 12 lbs and her mom 15. I'm thinking Sophie may be bigger than 15 lbs, since she's already close to half that. The vet said she wanted to wait til her next visit to start her heartworm meds, so we'd have a better idea how big she'll be, since you have to get a 6 month supply at once.

Her parents are both beagles.

Our vet said double her weight at 18 weeks and that will be approximately how much she will weigh as an adult.

Do you know if Sophie's parents were 13 inch or 15 inch Beagles? Misty, my Beagle is slightly larger than most Beagles. She weighs 32 lbs and is 15 1/2" tall.

I can't remember how big she was at 12 wks though, but I think she was a little heavier then Sophie by then.

There was a post a few weeks ago by saturnine that had a website link that estimated the size your dog will be when an adult.

Mom is a 13 inch and dad wasn't even that big. He was like a miniature.

The link from the post mentioned above is
http://mixedbreeddog.info/wtchart.htm

I find it hard to guess a dogs weight, and the Vet wasn't able to advise of what Justice SHOULD weigh untill she was 18 weeks.. then he told me the ideal weight for her. Justice ended up smaller than both mom and dad.
Just wait till 18 weeks and see what the vet advises, she/he should be able to give you a pretty accurate answer.

Don't try to guess a big dog by that... The range is so big. Boo was 75 lbs at around 6 months but he only ended up 96 lbs -- the range here would have been 80 to 135 lbs... That's a big range... We thought he was going to be 120lbs (closer to newf weight) but he fell in between.

Dahlia, looks like the chart says yours will be around 20lbs.. :)

According to the research I had done on Beagles when I got Misty, the 13" Beagles usually weigh in at 18 - 20 lbs. IMO I think that Sophie will probably get close to that just as Prin said.

As Safyre said, it is difficult to guess what a puppy will weigh as an adult.

She may weigh a tiny bit more than her mom did, but she will most likely stay within the same height range.

I'm sure that by her next visit, your vet will be able to have a better idea of how big she will get.

Thanks. Sometimes it's hard to go by how much the parents weigh, too. Once my step mom had 2 rat terriers - about 12 lbs each, and they raised 2 litters of puppies, all of which came out about the same size, except for one female, who ended up more the size of a JRT. Maybe one of them had some JRT somewhere in the bloodlines, I don't know, but they were both bought, with papers, from rat terrier breeders.