Measure your torso and hip circumference
1. Bend your head forward to locate the C7 vertebra at the beginning of the neck. The C7 vertebra is the most protruding of the vertebrae.
2. Use a flexible measuring tape and have someone measure from the C7 vertebra down the spine.
3. Place your hands on top of the hip bone. Let the thumbs point back and towards the spine.
4. Have your friend finish the measurement where the tape measure crosses an imaginary line between the thumbs.
This distance from the C7 vertebra to the line between the thumbs is the length of your torso which in turn determines the back length of the backpack you need.
Measure the circumference around your hips slightly below the mark on the picture.