How to Choose & Use a Backpack - Infographic from REI. Recommend you ignore the 4-6 pound recommendation for an expedition pack and get a 2-3 pound pack. If you're backpacking 5+ days, you'll want the least weight possible. Big packs also increase your desire to fill them which increases weight. Tip: buy all your gear first, then buy the smallest, lightest pack that will carry it.