Noobie here. I couldn’t find any guide to it, so I’m listing out what I went through. Hope it’s helpful to other newbies like myself. Here’s what I do. My process takes a long time, but I think it’s worth it. I initially went and just bought a $100 pack because it was on sale. I regret my purchase, because the pack does not fit me that well. spending another $100 would have been money really well spent over the life of use of the pack for more comfort/better fit.

-Determine the necessary pack size. Depending on where I’m going, weather,days out i have pack list. Then I pack it in to one of my existing bags to get an idea of how much volume and weight I typically use for the trips I like.

-go to REI. This is better done during camping season when they have a better selection of packs. Ask an associate to measure you with their tool. I try all the packs in the volume I’m looking for. different models from the same brand have different suspension systems, so I try different models of even the same maker.

-Take the pack over to the baffle bags they have. load a light baffle bag, then weight bags until i’m around my actual carry weight determined in step 1 above. usually i’ll be able to narrow it down to two packs. walk around the store and choose the one that fits best.

-when I had less $, I would then go online and buy the pack since it’s cheaper than REI. I feel it’s important to buy it from the store if you can since you just used their facilities. Or you can just keep in mind the two packs you liked the most, and wait for a sale on either on from REI.

-I take it on a test pack and hike with all the gear. I like to put some real trail miles on. Better to discover the problems now instead of later in backcountry

Generally recommended--get the rest of your gear first, the pack last. When shopping for a pack, pack up your gear in a box and take it to the store with you. If you can, make an advance appointment at a slack time and tell them you want their most experienced pack fitter. That's because the pack must fit your gear and be comfortable for you with your gear packed inside. There is considerable difference between real live backpacking gear and a bunch of weights or sandbags.

As soon as you get the pack, load it up with your gear, a liter or two of water and the equivalent in weight/bulk of a week's food, and hike around the house with it (don't remove the tags!) for a couple of hours. Boring, yes, but you need to determine if the pack is a keeper while you can still return it.

More on pack selection in the articles on the home page of this site, left hand column.

I would get fitted at a locally owned hiking/mountaineering store if you have one. My experience with REI clerks (most of whom appear to have little knowledge of backpacking) has not been good.

My personal experience has been that the best packs are made by the various "cottage" industries who sell on the internet. Examples of lightweight framed pack (what I prefer) makers include ULA-Equipment, Elemental Horizons, Gossamer Gear, Six Moon Designs, Hyperlite Mountain Gear, among others. That's what I ended up getting. I was prepared to pay return shipping charges on half a dozen packs, but it turned out that the very first pack I ordered fit me as though it had been custom made for me! I did go through the store bit (hoping to learn what a well-fitted pack feels like) but found nothing there that I wanted.

Thats good to know. I've never tried any of the cottage industry packs.

One thing I left out, probably because I live in the soggy Pacific NW where water sources are generally frequent: If you'll be doing desert hiking, test that pack with a gallon of water instead of a quart/liter!

I mentioned framed packs. Some folks do fine with a frameless pack (such as those made by Mountain Laurel Designs) for total pack weights up to 25 lbs. Others (like me) need a supportive frame and load lifters to transfer nearly all the weight to the hip belt for anything over 7-8 lbs. Certainly if your total pack weight (including pack) is much over 25 lbs., a frameless pack will be torture for most people. Individuals, of course, vary. Pack fit is almost as individual as shoe fit, and is just as important.