How to Choose the Right Size Backpack? | Fit Your Torso, Not Your Height

Choosing the right backpack size comes down to one measurement that most people skip: torso length. Your overall height is a poor guide because people with the same height have very different spine lengths. Measuring from the C7 vertebra at the base of your neck to your iliac crest (the top of your hipbones) gives you the number every manufacturer uses to size their packs. REI’s fit experts, along with Osprey and Arc’teryx, all size packs this way before considering anything else.

The Torso Measurement That Decides Your Size

Finding your torso length takes about a minute and needs just a flexible tape measure. Tilt your head forward and feel for the most prominent bone at the base of your neck — that’s the C7 vertebra. Place one hand on each hip so your thumbs point backward, and feel for the top edge of your hipbones, which is the iliac crest. Measure the distance between these two points in a straight line down your spine. This single number is what you’ll compare against a pack’s size chart.

For example, The numbers differ slightly by manufacturer, so always check the specific pack’s chart rather than assuming one brand’s medium matches another’s.

Fitting the Pack to Your Body

Once the torso length is right, the pack needs three adjustments to carry weight properly. The hipbelt should sit snugly on top of your hipbones, covering them fully — if it rides above or below, the pack will pull on your shoulders instead of your hips. The shoulder straps should lie flat against your chest without pressing against your neck, but they also shouldn’t leave a large gap between the strap and your body. The sternum strap should connect across your chest loosely enough that you can breathe naturally and expand your ribcage fully.

A common mistake is buying a pack that fits fine when empty but feels wrong once loaded.

Walking Through the Proper Adjustment Sequence

When you try on a pack, follow this order from the official fitting guidance used by REI and other outfitters:

1. Measure your torso length from the C7 vertebra to the top of your hipbone.
3. Tighten the hipbelt first so it feels snug and stable over your hipbones.
4. Tighten the shoulder straps until they’re snug against your shoulders without restricting movement.
5. Pull the load-lifter or stabilizer straps until the weight feels balanced between hips and shoulders.
6. Fasten the sternum strap last, adjusting it so your breathing stays unrestricted.

The order matters because the hipbelt should do most of the work. If you tighten the shoulder straps first, the pack’s weight drops onto your shoulders and you’ll carry the load incorrectly.

Capacity by Use Case, and Common Fit Mistakes

After the torso fit is dialed in, think about volume. A general-purpose or commuting backpack commonly falls in the 20–30 liter range, while hiking and longer-carry packs climb higher depending on what you need to bring. REI and similar guides emphasize choosing capacity based on what you actually carry rather than how big the pack looks on your back.

Two mistakes tend to ruin an otherwise good fit. The first is selecting a pack by height alone, which REI explicitly discourages. The second is ignoring how the pack behaves when loaded — a pack that’s too long forces you to lean forward, and one that’s too wide can catch on doorframes and branches. A properly sized pack should not extend above your shoulders or sit below your hips, and its width should stay within the frame of your shoulders.

If you’re shopping for a daily commuter pack and want a model specifically designed for cycling, our roundup of the best cycling commuter backpacks covers tested options for carrying gear on two wheels.