My beautiful BorderJack (a border collie and jack russel cross) Dolly is now just over 12 weeks old and I am extremely curious as to how big she will be at full size. Recently (17th Feb) the vet weighed her and she was 2.7kg and had doubled in wait in 2 weeks. I have done a lot of research and cannot find anything, so can someone help? Thanks :)
There are a lot of variables in play. My suggestion is to take the average size of a border collie, add the average size of a JRT, then divide by two.
I'm not saying the following numbers are inclusively representative, but they were the easiest to get. According to the AKC standards, the height range for a female BC is 18-21 inches, and the ideal height for a female JRT (now known as a Parson Russel Terrier) is 13 inches. 19.5 (the midpoint of the BC range) + 13 = 32.5. The average of that is 16.25 inches.
The weight of a JRT is listed as 13-17 pounds, the midpoint being 15. No weight was mentioned for the BC, but in my experience, females are around 40#. The average of those two is 27.5.
My semi-scientific estimate :-D is that Dolly will be about 16" tall at the shoulders, and weigh about 28#. But genetics is a complex thing; I could be way off.
Without knowing anything about Dolly’s parents, it may be hard to get a general idea of how large she may become (especially considering she may have multiple parents). In my experience, I’ve never come across a Border Collie / Jack Russell mix that is large. They are usually small-medium.
Some pet owners look at the size of a pet’s paws to get a rough estimate. If Dolly has small paws, she’ll likely stay closer to the normal Jack Russell size. This definitely is no guarantee, however.
One thing to keep in mind, sprouting off of Shannon L’s comment, is the average weight of these breeds. Jack Russells are usually only about 15lbs. Female Border Collies are usually 30-35lbs. Odds are, she shouldn’t get over 40lbs.
If you want better information, you may try contacting your vet. They should be able to help you out! Either way, she’ll be happy to have you and you her, no matter the size! :-)