Your Pack Requirements…
1) WEIGHT: Aim for 2 lbs or less.
A true ultralight backpacking pack will weigh under 2 lbs… and you don’t want anything above 3 lbs. There is a clear cutoff on style and design for packs above 3 lbs.
Rarely would I ever recommend getting one over 3 lbs. They have lot more padding and more framework. These extras are only worth it if your are carrying an extremely heavy load.
I’ll define “extremely heavy load” as anything above 45 lbs (ex: maybe you need to carry all your family’s gear). Otherwise, even with a week’s worth of food, your load should never exceed 35 lbs – ideally more like 20 or 25 lbs.
2) VOLUME: Keep it between 40 and 65 liters.
“If you have it, you will fill it.” Meaning if you carry a large pack, you will assume that you have the extra space and will bring more stuff. Carrying a smaller pack can be somewhat advantageous because it forces you to prioritize your item selection to be lighter and more compact.
I recommend a carrying capacity of no less than 40 liters and no more than 65 liters. 50 to 55 liters is a good middle ground. Mine was 58 liters on the Appalachian Trail and I would have liked just a tiny bit less space.
45 liters would be my ideal volume. I’ll admit though it can be a challenge to fit everything into 45 liters, especially with any extra winter gear and several days of food.
Note many manufacturers include the external pockets as part of their measured carrying capacity. You will pack items on the outside of your pack – water bottle, rain shell, etc.