Going along with what everyone else has said.....The two thing that need to mate up are the sleeping bag and the pack. Of course, the pack needs to fit you, but the sleeping bag needs to fit the pack. If the pack is too small and the bag too large, then you have a big problem. I find this all too often with beginners and especially with my scouts. In fact, my dad went with me on a recent trip. He borrowed my sisters huge Deuter pack and brought his huge coleman sleeping bag. The pack was 70 liters and we barely got that bag into the lower compartment with enough room to fit the rest of his gear. Many here say buy your pack last. I like to say buy your sleeping bag first. If it doesn't fit inside the lower compartment of the pack you are looking at, then either buy a different bag, or buy an external frame pack.