Depending on pack volume and intended use the suspension system of a backpack becomes an essential feature. Defining torso length is not necessarily critical on a daypack but crucial on backpacking or alpine touring packs, as only perfect fit can warrant transfer of a bulk of the pack weight to the hips.
The center of the hipbelt ideally rests on the hip bone crest (pelvis) in order to bear the bulk of the load (refer to diagram) especially on large internal frame backpacks. But no rule without exceptions and sometimes a shorter torso length is better, for example in alpine or mountaineering situations where a short back allows easy access to the climbing harness.
How is torso length defined?
Use a measuring tape to measure the distance between the top edge of the shoulders or C7 vertebrae (the one that protrudes when facing down) to the top crest of the hip bone (pelvis).
A rough guide to pack sizing and torso lengths:
medium (M) = 48 - 52 cm / 19 - 20.5 inch
long (L) = 53 - 58 cm / 21 - 23 inch
Even the most elaborate suspension system can only work perfectly when fitted correctly! And as no two backs are identical, we recommend testing backpacks with sample weights in an outdoor speciality shop.