So I originally went by weight, and realized it's best to physically measure them. I decided to keep the large that are too big for the possibility of having a larger dog. But the medium harnesses they ever so graciously expedited to me are incredible! I've had some bad luck with other harnesses, so I'm constantly vigilant over multiple things, and so far I'm very impressed. And my boys look so very handsome in their new gear. I also came back for a few other items, I'm still getting used to the distance, but we love our new gear. :)