The keys to buying a backpack are fit and capacity. While fit should be determined by your body type, capacity (the types and amount of gear a pack is capable of carrying) should depend on intended use and length of trip. Here is what to look for to determine what backpack is right for you.

Pack styles and uses

• The more weight you carry, the more supportive your pack needs to be.
• A waist or lumbar pack or small daypack is best if you are taking a short hike with little gear.
• If you’ll be carrying a bulky or heavy (16 pounds or more) load, or if you plan to be out for more than a day, consider an internal or external frame pack.
• Both internal and external frame styles have a harness system comprised of shoulder straps and a hip belt; compression straps pull the pack and load closer to the body.

Internal frame

• Refers to a support system that is built into the interior of a pack.
• Internal frames transfer a large percentage of the pack’s weight onto the hips, which can bear far heavier loads than the shoulders. This frame style is comprised of a hip belt that works with an internal suspension system.
• The internal frame suspension system usually consists of one or more aluminum or carbon fiber stays that curve to fit your spine.
• The stays extend from the top of the pack to the hip belt, and their job is to stabilize loads and transfer weight to the hips.
• Many models also include a framesheet, often made of high-density polyethylene, to stiffen the back of the pack and allow for better weight transfer.
• Internal frames offer better balance because of their low profile and close-to-the body fit.

External frame

• The first generation of framed packs.
• They feature a rigid support system, or framework (usually constructed of tubular aluminum), to which a pack and harness attach.
• External frame packs transfer weight and stabilize loads, but are much more rigid than internal packs.
• Usually have a wider profile than internal frame packs. On an open trail where balance isn’t a critical factor, this should present no major problems, but in the backcountry, the frame could snag on branches or get tangled in brush.
• Because the rigid frame keeps the pack away from your back, such models tend to be comfortable when used in hot weather.
• Usually less expensive than their internal frame counterparts because their design and production is less complicated.

Materials

• The most popular–and durable–technical pack materials are found in the nylon family: Cordura nylon, ballistics nylon, ripstop nylon, and nylon packcloth, which are all:
• Very durable
• Strong and abrasion resistant
• Many feature water-repellent or waterproof coatings or treatments

What to look for

• Backstitching and bar tacking in high-stress areas, such as around zippers, pockets, and external loops and webbing.
• High-abrasion areas, such as pack bottoms, should be reinforced with a strong material such as Kevlar, Hypalon, or heavy-weight Cordura.
• Back panels made of reticulated or compression-molded foam covered with a breathable, wicking fabric to disperse perspiration and enhance airflow.

Capacity

• The capacity of a backpack is measured in liters. The size you need depends on what you’ll be doing and the amount and type of gear you want to carry.
• 2-3 Days: For a warm-weather weekend trip (two or three days), look for a pack in the 35-50 liter range. A weekend backpack like a standard 40L backpack is big enough to carry a small backpacking sleeping bag, small tent, and pad, as well as extra clothing and layers.
• Multi-Day: For a week-long trip or more (Philmont) consider 50-75 liters. A light packer will have ample room for five nights or more with a multi-day backpack, which is typically a 50L backpack or larger. These are designed to carry more food and camping items than a weekend pack, as well as extra clothes and layers. Packs specifically for expedition or winter camping will be on the larger side too (over 75 liters) to accommodate a warmer sleeping setup, mountaineering equipment, and first aid and survival gear.
• Avoid using a pack that is too big. Most people tend to fill available space, which makes for a heavier than necessary load to haul.

Fitting tips

• Your height has little bearing on what size pack you should wear; it’s your torso length that matters.
• If the pack is too long, it will sag onto your rear end.
• If it’s too short, it won’t support your lower back.

Determining your proper pack size

• To determine your torso length, measure from the seventh vertebra (the bony protrusion at the base of your neck between your shoulders) to the small of your back (level with your hipbones).
• For torso length less than 18″ (45 cm.), your suspension size will likely be Small.
• For torso length between 18″ and 20″ (45-50 cm.), your suspension size will likely be Medium.
• Torso length over 21″ (52.5 cm.), your suspension size will likely be Large.

Determining your hip belt size

• The hip belt should cup your hips and when cinched tightly, the pads should not touch.
• Women (and men) with straight or narrow hips may prefer a standard hip belt.
• Women (and men) with more curve to their hips should choose a women’s-specific model.
• Shoulder straps should anchor to the backpack just below the seventh vertebra and the crest of your shoulders. They should wrap comfortably, yet securely, around the shoulders and should be at least 5″ below the armpit.