Credit: Matt Honan
Size: Shop at a quality outdoor store and ask the staff to measure your torso length—the distance from the top of your hip bones to the bump at the base of your neck—to figure out what size you need.
Fit: Load some weight in the pack and put it on. Now tighten all the straps: hipbelt, shoulder, load lifters (from the shoulder straps to the pack), sternum and finally load stabilizers (between the hipbelt and pack). The shoulder straps should attach to the pack just below your shoulders. The load lifters should drop at a 45-degree angle from the pack to just in front of the shoulders. All straps should have room for adjustment and most of the weight should rest on your hips.
Volume: If you’re only going to own one backpacking pack, pick one in the 65- to 85-litre range. A 50- to 65-litre pack is fine for long-weekend trips. For serious trekking, go 85 litres or more.
Access: Top loaders are light and carry maximum gear for their size, but digging stuff out can be a chore. Multiple pockets make organization easy, but add weight. Clamshells and back-access packs open wide, but don’t fit as much gear.
Other factors Swivelling hipbelts make hiking more comfortable, and adjustable torso lengths help dial in the fit. Mesh back panels will keep you cooler. A rain cover is nice.