One of the keys to a comfortable backpack is to get a pack that is the right size for you. Your torso length (not height) is the key measurement; hip size can also be helpful.

Finding your torso length

To do so, you’ll need a friend and a flexible tape measure.

Have your friend locate the bony bump at the base of your neck, where the slope of your shoulder meets your neck. Tilt your head forward to locate it more easily. This is the top of your torso length.
Place your hands on top of your hip bones, with fingers pointing forward, thumbs in back. This is the “shelf” on which your pack will rest. The middle of an imaginary line drawn between your thumbs is the bottom of your torso length.
Using the tape measure, your friend should measure the distance between base of your neck and the imaginary line between your thumbs. Be sure you stand up straight when being measured. You now should have your torso length.
Use your torso length measurement to find your best pack size. Generally, manufacturers size their pack frames something like this:

Extra Small: Fits torsos up to 40cm
Small: Fits torsos 40cm to 45cm
Medium/Regular: Fits torsos 45cm to 50cm
Large/Tall: Fits torsos 50cm and up

Determine your hip size

When fitting a pack, your hip measurement is also useful to know. It’s especially helpful if you are considering a pack that offers interchangeable hip belts.

Take your tape measure and wrap it around the top of your hips. The correct measurement is along the “latitude line” that radiates out from your belly button to your sides and the high points of your hip bones. This is slightly higher than your waist, so your hip belt measurement may differ from your pants waist size.
A properly positioned hip belt will straddle your hips about an inch above and below that latitude line, wrapping around the 2 pointy pelvic bones on the front of your body.

How to fit a pack for women

Women have physiological differences from men that need to be considered when choosing a backpack. Those differences affect the required size, shape and position of shoulder straps, hip belts and dimension of the pack. To find a suitable backpack, women can look for packs with either: