Backpack sizing is one of the most commonly misunderstood specs in outdoor gear — the number that actually matters isn't the one most people check first.
Fit is about torso length, not height
Two people of the same height can have noticeably different torso lengths — the actual measurement that matters is the distance from the bony vertebra at the base of your neck down to the top of your hip bones, not your overall height. Most quality backpacks come in multiple frame sizes based on this torso measurement, and many outdoor retailers will measure it for you in-store. Buying by height alone is one of the most common backpack-fitting mistakes, and it's a big part of why a "correctly sized" pack from a chart can still feel wrong on an actual body.
The hip belt should carry most of the weight
A properly fitted backpack transfers the majority of its load — often cited as roughly 80% — onto the hips through a padded hip belt, not the shoulders. Shoulder straps are mainly there to stabilize the pack and keep it close to your back, not to bear the bulk of the weight. If your shoulders are sore after carrying a loaded pack, that's often a sign the hip belt isn't sized or tightened correctly, not simply that the pack is too heavy.
Capacity in liters is a starting point, not the whole answer
Backpack capacity is measured in liters, but how much actually fits comfortably depends heavily on the pack's internal shape and how well it's organized, not just the raw number. As a rough starting point: daypacks and short trips are typically 20–35 liters, multi-day trips often 50–65 liters, and extended expedition trips 65 liters and up — but two packs with the same liter rating can pack very differently depending on their shape and compression design.
The one thing people forget
Load the pack with real weight and walk around in it for a while before committing, ideally in-store or during a return window — not just trying it on empty for thirty seconds. An empty pack tells you almost nothing about how the hip belt and shoulder straps will actually feel once it's carrying real gear.