Backpacks are like hiking boots - very personal and meant to be as comfortable as possible. I typically spend an hour or more with a potential buyer just to make sure the pack fits well, feels good and is within their budget. Always try on several different packs with at least 25 lbs in it and then you'll be able to establish a standard as to 'what feels right' for your body. Below is a great article on 'How to Fit a Backpack'

If you routinely experience back pain, shoulder pain, or hip pain when you carry a backpack, there’s a good chance that your backpack doesn’t fit you or that you’re not wearing it properly. This is surprisingly common among backpackers, and one of the main reasons why people don’t go hiking more often.

The Biggest Mistakes that People Make

The number #1 reason why backpacks don’t fit is because people buy packs with the wrong torso size. Before you buy a pack, measure your torso (click for directions). Backpacks with adjustable torso lengths can ameliorate this problem, but they are heavier.

The number #2 reason why backpacks don’t fit is because the hip belt is too big or too small. Many manufacturers only make hipbelts in one size for their backpacks, and that size might not fit you. Come to terms with this and find a backpack that fits your waist, as it is today. Some manufacturers also offer replaceable hipbelts that can be switched out on backpacks, ensuring a perfect fit.

Don’t be tempted to buy a pack because it’s on sale, unless it fits your torso and hip size. Doing that is a mistake and you’ll suffer needlessly for it.

Here are a few things to check to make sure your pack is properly fitted:

Hip Belt

1. Does the hip belt cover your hip bones (illiac crest) or has it slid below them?
2. If you can’t tighten your hip belt enough and it keeps falling below your hips, it may be too long.
3. If your hip belt rests on your lower back and not the sides of your hips, it may be too short.
4. Is your hip belt snug, but not so tight that it causes back pain?
5. If you loosen your shoulder straps completely, can you feel the pack resting on your hips?
6. Drop your arms so they hang along your sides. If your hip belts pocket are behind your arms, your hip belt is probably too short.

Torso Length

1. Do you know the length of your torso and does it match the torso size of your backpack?
2. If there is space between the tops of your shoulders and your shoulder straps, your pack’s torso length is too long. If your pack is adjustable, make the torso length shorter. Otherwise, return your backpack and get one with a shorter torso length.
3. If you are carrying all of the weight of your pack on your shoulders, your torso length is too short. If your pack is adjustable, make the torso length longer, so most of the weight rests on your hip belt. Otherwise, return your backpack and get one with a longer torso length.

1. If the front of your shoulders get very sore when your backpack, make sure that most of the weight is on your hips and not your shoulders. If the cause of the pain is because your pack is too heavy, get a new pack with wider or more padded shoulder straps, or lighten your load.
2. If the shoulder straps on your pack rub against your neck, loosen the sternum strap. If this doesn’t work, your pack may have a harness that is too narrow for you and you should exchange it.

Sternum Strap

1. If you can feel your sternum strap on your neck, try lowering it. It is too high. If that still doesn’t work, try getting a pack with a larger torso size.
2. If tightening the sternum strap doesn’t keep the shoulder pads on your shoulders, you probably need a backpack with a narrower shoulder harness.

Load Lifters

If your backpack doesn’t come with load lifters, don’t panic. They are often provided on higher volume packs where you need to carry heavy loads, but not on smaller volume or ultralight backpacks.

1. If your backpack has load lifters and the back of your head hits the top of your pack, try loosening the load lifters.
2. If there is a gap between the tops of your shoulders and your load lifters, loosen them.
3. If you can feel your pack pulling you backwards, tighten them. This will narrow the gap between your back and the pack and tilt the pack forward, so more of the load is carried by your hips. Also make sure that the heaviest items in your pack, such as water, are located as close to your back or sides as possible and not in the back of your pack.

Try on Lots of Backpacks

Buying a backpack should never be an impulse decision. Try on lots of different packs and test them fully loaded on a long day hike before you commit to keep them. Come to grips with the fact that some packs will never fit you because you are too tall, too short, too round, too skinny, or you have no hips. Backpacks are just like business suits (men’s and women’s): some fit and some don’t.

Return policies are also flexible enough these days, that you can try lots of backpacks before committing to one, guilt free. Backpack makers want you to enjoy your backpack and tell your friends how much you love it. Buying a backpack is one of the most important decisions you can make if you like hiking, so take your time in making a decision.