Torso length is the straight-line distance down the spine between two specific bony landmarks: the C7 vertebra at the base of the neck, and the iliac-crest line at the top of the hip bones. It is not the same as height, shirt length, or waist-to-shoulder measurements used in clothing.
Why backpacks use torso length instead of height
A hiking backpack's frame has to bridge the exact span between where the shoulder straps sit and where the hipbelt sits, and that span is set by torso length, not overall body height. Two hikers who are both 5'8" can have torso lengths that differ by two or three inches, because torso-to-leg ratio varies by individual. Sizing packs to height would fit neither hiker reliably.
How torso length is measured
A flexible tape measure is run from the C7 vertebra — found by tilting the head forward and feeling for the bony bump at the base of the neck — down the curve of the spine to the iliac-crest line, located by placing hands on the waist with thumbs pointing backward to the top ridge of the hip bones.
Typical adult range
Adult torso lengths generally fall between about 15 and 21 inches (38–53 cm), with brands publishing their own XS-to-Large bands within that range. A measured torso length is compared directly against a specific pack's torso-length chart, not against a generic clothing size or shoe size.