Back length is the distance between your 7th cervical vertebra (the one that sticks out on your neck) and the line between your iliac crests (tops of your hip bones).

Vicki, I highly recommend going to an outdoors store to get "fitted" for a bag. If you measure yourself and buy a bag based on that, the bag could be the wrong fit and could compromise your musculoskeletal health and increase your chances for injury. Be careful!