Why ultralight backpacks? Devotees of ultralight backpacking know you reduce your packweight most by concentrating on the "big three;" backpack, shelter, and sleeping bag. Forget those 6-pound backpacks, 9-pound tents and 5-pound sleeping bags. Reduce that 20 pounds to less than 5, keep the other gear to 5 or 6 pounds, and you can chuck the hot and heavy hiking boots, put on the running shoes, and start enjoying those trails.
Four Ounce Ultralight Backpacks
No, that's not a typo. One of the ultralight backpacking suppliers recently came out with a light pack with a capacity of 1800 cubic inches, plus more in an expandable rear pocket. Designed to carry just fifteen pounds, It may be more fragile than the average pack, but it weighs just 3.7 ounces! With a pack that light you're well on your way to keeping your total packweight to fifteen pounds.
I bought a six ounce pack at Walmart. Maybe you didn't know they sold ultralight backpacks, and niether did they. I'm sure it was intended as a daypack. However, it has 1200 cubic inches and weighs only 6 ounces, so I'll use it for summer overnighters. At $4, it was light on the budget too.
It easy to imagine that a decent-sized backpack can be made at home that weighs less than 8 ounces. If it's a simple one, without extra pockets, made from light nylon material, how much could it weigh? Making your own may be the way to get the lightest backpack.
Alternatives To Ultralight Backpacks
I've used my 8-ounce duffle bag for backpacking. With no pockets, it's tough to organize things, but it has lots of room, and it is very tough. In practice, the full-length zipper makes it so easy to see everything that organization isn't very important.
I also have tied the duffle bag to an an old aluminum pack-frame, to create a full-suspension backpack. It has a hip belt, comfortable padded straps, and weighs exactly two pounds. It worked fine on a trip above timberline in the mountains of Montana. If there are any lighter exterior-frame ultralight backpacks out there, I haven't seen them.
You can also go on overnight trips with just a bivy sack, a bottle of water and a few granola bars in the pockets of your jacket. I've done this in the summer. Of course, I suppose without ultralight backpacks, this can't even be called backpacking.
Steve Gillman is a long-time backpacker, and advocate of lightweight backpacking. His advice, stories and backpack recommendations can be found at http://www.TheUltralightBackpackingSite.com