Contrary to natural thought, finding the right backpack size has no relation to how tall you are, but rather the length of your torso. Torso ranges for pack sizes vary between brands and models, so you will need to check the size chart for any pack you are considering. Bearing in mind the height range given for each back length is only a guideline and may not be right for your particular proportions.
In order to accurately determine your torso length you’ll need a friend with a flexible tape measure to help you. Start by tilting your head forward and feel for the bony bump where the slope of your shoulders meets your neck. This is your 7th cervical (or C7) vertebra—and the top of your torso length. On each side of your body, slide your hands down the ribcage to the top of your hip bones (aka the iliac crest). With index fingers pointing forward and thumbs pointing backward, draw an imaginary line between your thumbs. This spot on your lumbar is the bottom of your torso measurement. Stand up straight and have your friend measure the distance between the C7 and the imaginary line between your thumbs. This measurement will give you an idea of which pack back length to choose.
While it’s rare for a pack’s waist/hips sizing to be off if your pack size is correct for your torso length, it’s still important to check your hip measurement. Some backpacks come with the option of interchangeable hip belts. In this case, it’s a good idea to know your hip size.
To measure your hip size, wrap your tape measure around the top of your hips, hugging the iliac crest you found when measuring your torso length. This line is slightly higher than your beltline, so hipbelt size differs slightly from pant-waist size.
You’ll be carrying most of your pack weight on your hips, so a good hipbelt fit is critical.