The different types of hiking backpacks are distinguished by their intended use, size, materials and design. Very small, lightweight packs are usually made to carry hydration bladders and small pieces of personal gear or clothing. They are often simply called hydration packs. Slightly larger packs are designed to carry enough gear for a day on the trail and are known as day packs. Overnight packs allow just enough room for a night or weekend camping trip, while full-size or expedition packs provide ample room for a week or more in the backcountry. Technical packs are highly specialized for activities such as rock or ice climbing and skiing.
Hydration packs and many day packs are frameless or contain soft foam supports to cushion a load. These packs are usually designed for short distances and light loads under 20 lb. A heavier day pack might incorporate a frame sheet-a stiff piece of foam or plastic sewn into the back of the pack. These packs can take slightly heavier loads.
The most common frame seen in large day, overnight and expedition packs is internal. These packs have rigid plastic or aluminum tubing sewn into the backpack to evenly distribute weight. These are the most comfortable packs to wear with loads over 20 lb.
External frames were the predecessors to internal frames and can still be found, but they are bulkier, heavier and less comfortable to wear under a heavy load.
Because frameless packs are made of lightweight materials and don't hold much equipment, they are well-suited to activities like running, cycling and climbing. The load distribution in these packs is largely due to strap design and can vary widely between hikers.
Nearly all framed backs work on the same principles. When a frameless pack is heavily loaded, the weight pulls down on the wearer's shoulders and becomes uncomfortable. A framed backpack takes the load from the upper part of the pack and transfers it to the lower part, where it is centered on the hiker's hips.
Backpack capacity is measured by cubic inches. Day packs range in size from 500 to 2,000 cubic inches, with the largest often capable of handling an ultralight overnight trip. True overnight and weekend packs fall into the range of 2,000 to 4,000 cubic inches, while expedition packs hold 4,000 cubic inches and up.
Backpack straps should be measured by your torso length. While most packs are adjustable, every brand and style is different. Be sure to try on a pack before you buy it.
While load capacity and support are the most important aspects of a backpack, pocket and strap design are also big factors in comfort on the trail. Taking off your pack in the middle of a hike can be a major inconvenience. Your heart slows down, your muscles start to tense and, on windy days, you may even catch a chill. It is important to be able to reach small items like sunblock, lip balm and energy bars without taking off your pack. Pockets on the hip or shoulder straps solve this problem. A compromise may also be accessible pockets on the outside of the pack that a hiking partner can easily open.