Sizing and fitting backpacks
Consumers often buy goods based upon two things: brand and price. However, the backpack for your section hike or thru-hike should be based upon properly sizing and fitting your backpack. Backpacks typically come in small, medium, and large. The size is not related as much to height or build as it is the length of your torso.
The length of your torso is important in properly sizing and fitting backpacks because the hip pads need to hug the top of the hip bones aka the iliac crest, while the shoulder straps should wrap closely around the shoulders. The majority of the weight should sit on your hips or you could start experiencing pain in the shoulders, neck, or upper back.
This graphic by the backpacking gear maker, Gregory will give you a better visual on how to measure your torso. For this, you will need to find a friend to help you.
Most backpacks are designed to fit most hip sizes within the torso size but sometimes these may fall outside of certain body sizes. Therefore, you should then get a hip measurement or you can try on bags if in store. Hip measurement is not the same of your waist size as the pads will be going across the gut. Here you will measure around the top of your hips around the iliac crest in the same area you measured for your torso length. The padded part of the hip pads should not connect around the belly and should have a space of 3-6 inches.
Adjusting your backpack
Backpacks have four areas of adjustment: hip belt, load-lifter, shoulder, and sternum. Add 15-20 pounds to your pack before adjusting as an empty pack will not sit the same as a weighted pack.
Assure that the hip belt falls on the iliac crest. If your hip belt goes above or below the hip bones, you can loosen or tighten the shoulder straps to lower or raise the hip belt. The belt should rest comfortably and snug without pinching. Remember that you should have 3-6 inches of space between the two pads. Many backpack manufactures have interchangeable hip belts in the event that the size that comes with the pack does not fit.
While you may have just adjusted these to properly seat your hip belt, assure that you are not tightening them so much that you are placing too much weight on your shoulders. The harness should stop 2-3 inches under the armpits. As you hike, you may have pain and pressure points. Adjust as needed to relieve the pain.
These two straps connect at each side of the harness to the back (see photo below of the Gregory backpack). There should be an approximate 45-degree angle when under tension. If your bag feels like it is pulling backwards, you can try tightening these. However, assure that your bag is properly balanced. Do not over tighten these as it can cause pain.
Note: Some smaller or lightweight packs may not have load lifters.
This strap should help keep your shoulder straps in place and allow free arm movement. It should sit an inch or so below the collar-bone. If your chest feels compressed, the strap is probably too tight. You want it snug but not too tight.