Having the right backpack harness size that fit your body frame is vital especially for larger backpacks meant for heavy loads. A harness with the right fit will allow you to distribute the load properly between the shoulders and the hip and that the lumbar pad sits nicely on your iliac crest shelf.
Determining Torso Length
The torso length is the distance along your contour of your back between the 7th vertebra and the iliac crest. The 7th vertebra is the prominent bony bump at the base of your neck where it meets the shoulder. Look down such that the bony bump can be located more easily.
To locate the iliac crest, place your hand on the side of your hips with your thumbs pointing to the back and the other fingers pointing forward. Find the highest point of your iliac crest by running your hand down your waist. That is the point of the iliac crest which marks the horizontal plane where the torso length ends. Position your thumbs at your back such that it is at about the same level as the top of the iliac crest and slide it to your spine while maintaining the level.
Ask your friend or shop assistant to use a flexible measuring tape to measure the torso length by following the contour of your back from the bony bump at the base of your neck to the horizontal position indicated by your thumbs.
The shoulder straps should sit comfortable on your shoulders without sliding off with the sternum strap unbuckled. On the other hand, the straps should not be so close as to rub against your neck. When it is tightened, the strap should still contour round your shoulder without any gap between the top of your shoulder and the strap.
The hip belt of the backpack is designed to wrap round your hip bone or iliac crest so that it is able to carry the weight of the backpack. In the proper position, the lumbar pad is pressed against the lower back and sitting on top of the pelvis. Make sure that the hip belt can be tightened sufficiently to hug your hip bone.
The most important part to finding the backpack with right fit is to test it. If you are in the store, load two tents into the backpack so that it weighs about 10 kg or more. Tighten the shoulder straps and hip belt. Check the positioning of the shoulder straps and hip belt against your body. You should be able to feel the weight on your hip instead of your shoulder. Loosen the shoulder straps. The backpack should be supported by the hip belt and not slide down.
Walk around the shop and swing your arms. The shoulder straps should not be so wide as to obstruct the natural swing of your arms. The shoulder straps also should not start sliding off your shoulders even when the sternum strap is not buckled.
Walk up some steps if possible or lift up your legs as if you are walking up some steps. The hip belt should not be obstructing your thigh.
Twist your body from side to side. The backpack should move in unison with the harness and the harness should not be sliding from your body.
Try different bags. You will only be able to tell the differences when you put on different bags.