The most common types of hiking backpacks are day packs, internal frame packs, and external frame packs. Each of these hiking backpacks are designed for different purposes and generally carry a different amount of cargo. Day packs are designed for short day hikes or even short overnight trips; they generally do not have any solid frame because they are not large enough to require one. External frame packs are hiking backpacks that are affixed to a solid metal or plastic frame to increase support when carrying heavy loads. An internal frame pack is also a larger backpack, though the frame stays are built inside the pack rather than outside of it.
The sizes and shapes of hiking backpacks vary significantly. They are usually measured in cubic centimeters or cubic inches, and the number is representative of how much space is available for holding cargo. Day packs can range anywhere from a simple hip pack to hold a jacket and a sandwich or water bottle, to larger bags that can hold enough clothing and gear for an overnight trip into the woods. These larger packs often feature more than one compartment and external straps for compression as well as for securing items to the outside of the pack. Day packs usually do not have any sort of frame for support, though some designs do include internal frame stays.
Hiking backpacks designed for backpacking and trekking are designed to carry more cargo for longer periods of time. External frame packs are a more antiquated design, though they are still used because they are less expensive than internal frame packs. An external frame pack features a metal frame — usually aluminum to save on weight — to which the backpack itself is affixed. On the other side of the frame, shoulder straps and a hip strap are affixed, allowing a hiker to secure the pack to himself.
An internal frame pack also features shoulder straps and hip straps, and these straps are generally quite adjustable to accommodate hikers of different sizes. The straps are affixed directly to the backpack rather than to a frame. An internal frame pack features frame stays, or long metal slats or rods that slip into specially designed pockets within the backpack. These stays provide rigidity and support when carrying heavier loads without adding the weight and cumbersome design of external frame packs. Internal frame packs are more commonly used hiking backpacks because of the sleek design and light weight. Like some day packs, an internal frame pack may feature more than one compartment as well as straps for compression of the pack and for securing items to the outside of the pack.