Capacity ratings can be used to gauge the pack's target usage. Our testing shows that the accuracy of capacity specs vary greatly from manufacturer to manufacturer. You can get around this inconsistency by bringing all your gear (in a duffel bag) with you to the store when you shop for a new pack. Below are general guidelines for evaluating capacity usages.
2,500 to 3,000 cubic inches. This size works as a high-volume daypack or an ultralight overnight bag. These packs will hold a light sleeping bag and the minimum of camp essentials (light stove, pot, small tent) for short, warm-weather trips.
3,000 to 4,500 cubic inches. You'll find this size ideal for three-season weekend trips. They'll hold all the camp essentials as well as some warm clothes and enough food for a couple of days.
4,500 to 6,000 cubic inches. This size is the mainstay among the backpacking community. Perfect for long weekends or weeklong outings. These packs comfortably hold all the essentials, additional warm clothes, plus any extra camp comforts you might want to bring along.
6,000-plus cubic inches. Whether planning a month-long expedition or just a week of snowshoeing, you'll need this size to swallow all the extra gear and clothing for the adventure. They're also useful for folks who end up with the bulk of their family's communal gear (e.g., tent, stoves, food) during family trips.