What Size Hiking Backpack Do I Need? | Fit By Trip

Choosing the right-size hiking backpack comes down to two questions: how long are you heading out for, and how much gear does that trip actually demand. The single biggest mistake is picking a pack based on height. Your torso length, measured from the base of your neck to the top of your hip bones, determines the frame fit, while the volume in liters dictates what you can carry. Get both right and the trail feels lighter.

Backpack Size By Trip Length

Volume is measured in liters, and the ranges below are the consensus across outfitters like REI. They reflect what most hikers carry for a given trip, not hard rules. Your personal gear bulk can shift you up or down a category. For day hikes, a 15–25L pack handles water, snacks, and a rain shell comfortably. Overnight and weekend trips generally call for 30–50L, while multi-day treks need 50–65L. Winter, expedition, or guided trips with shared group gear push you into 65L and up.

How To Measure Torso Length And Fit The Pack

Torso length is the distance from the C7 vertebra—the prominent bump at the base of your neck when you tilt your head forward—to the iliac crest, which is the top of your hip bones. REI recommends measuring between these two points and then matching the result to the manufacturer’s sizing chart. Osprey’s guidance mirrors this: locate the iliac crest, find the C7 vertebra, and measure the span between them. Torso-length categories vary by brand, so always check the specific chart for the pack you’re considering. Hip belt fit matters just as much—the padding should wrap your iliac crest, not sit above or below it.

Volume: Match It To Your Gear Bulk

Pack what you actually plan to carry, then choose a volume with a small margin. One common rule of thumb is to leave about 10% spare capacity in your pack. A 30L pack stuffed to the seams with a bulky sleeping bag, tent, and stove is a sign you need to size up. Consider the specific items that inflate volume: a bear canister, a winter sleeping bag, or a four-season tent each add significant space. For trips requiring those items, moving to 50–65L or even 65L+ is wise.

Choosing too small a pack forces external strapping and awkward loads. Choosing too large encourages overpacking, leaving your contents unstable inside the pack. REI’s backpack fit guide offers a deeper look at volume selection and torso measurement for your next purchase.

For a curated selection of well-fitting, lightweight options that won’t weigh you down, check out our roundup of the best women’s lightweight hiking backpacks.

When you land on a pack in the right volume range, remember the fit rules: the hip belt should sit on your iliac crest, the shoulder straps should wrap your shoulders without gaping, and the load lifters should pull the pack’s weight onto your hips. A well-fitted pack transfers weight to your hips, not your shoulders, which is the difference between a pleasant hike and a painful one.

Common Backpack Sizing Mistakes

Choosing by height instead of torso length is the most frequent error. Two people of the same height can have different torso lengths, and a pack that fits one will strain the other. Buying too much volume is the second; a 70L pack for a weekend trip invites you to fill it with things you don’t need. Using one brand’s sizing chart for another brand’s pack also fails, since manufacturers define small, medium, and large differently. Always measure your torso and consult the pack’s own size chart.