It's best to keep the total carrying weight (including the pack and its contents) under 25% of your dog's body weight. First-time backpackers may want to go even lighter. And, as always, it's a great idea to check in with your vet.
Example: A 45-pound (20-kg) dog's pack should weigh no more than 11.25 pounds (5 kg) including food, water, gear, and the pack itself.