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

Picking the wrong size backpack is the fastest way to turn a great hike into a painful one. Too big, and you carry extra weight up every climb. Too small, and your gear won’t fit. The real trick is ignoring height charts entirely and measuring your torso length instead. A pack that matches your body’s dimensions lets you walk all day without shoulder soreness, and choosing the right capacity means every item has a home. Here is how to nail both measurements so your next pack is your last one.

Why Torso Length Matters More Than Your Height

Your height tells you almost nothing about how a backpack fits. Two people who are both 5’10” can have entirely different torso lengths, and a pack sized for one will ride too high or too low on the other. The backpack’s frame and hipbelt are built around the distance between your neck’s base bump and the top of your hipbones. Get that wrong, and the weight sits on your shoulders instead of your hips.

How to Measure Your Torso for a Backpack

You need a flexible tape measure and a friend. Tilt your head forward and feel for the prominent bony bump at the base of your neck — that is the seventh cervical vertebra or C7. Then place your hands on your hips with thumbs pointing back, and feel for the top edge of your hipbones (the iliac crest). Have your friend measure straight down your spine from C7 to an imaginary line between your thumbs. That number in inches is your torso length.

If your measurement lands exactly on 15, 17, 19, or 21 inches, size up for a comfortable fit.

Matching Pack Capacity to Trip Length

Backpack capacity is measured in liters (one liter equals about 61 cubic inches). The right size depends on how many days you will be out and what gear you carry. A good rule is to pack all your gear first, then pick a pack volume that leaves roughly 10% spare room.

For food, plan on 2 to 2.5 liters of space per person per day. If your gear (excluding the pack, food, and water) weighs more than 10 pounds, you need a framed pack with a thick padded hipbelt to stay comfortable on the trail.

Hipbelt Fit: The Make-or-Break Adjustment

Wrap a tape measure around the top of your hips at the iliac crest — slightly higher than your regular beltline. That measurement tells you which hipbelt size to look for. When you put the pack on, the padded part of the hipbelt must sit squarely on top of your hip bones. If it rides too low, tighten the shoulder straps to raise the belt; if it sits too high, loosen them. The belt should feel snug and secure without pinching. A correct hipbelt fit means the load stays on your hips, not your shoulders.

Step-by-Step Fit Test (The REI Method)

Load your pack with about 15 pounds of weight to simulate trail conditions. Put it on and slide the hipbelt so it rests on your iliac crest, then fasten and tighten it. Pull the shoulder strap ends down and back to take up slack — the straps should wrap closely around your shoulders but carry almost no weight. At least 80% of the load should sit on your hips.

Tighten the load lifter straps (the small straps near your collarbone) gently to bring the pack weight forward against your back. Do not overtighten them, or you will lean backward to compensate. Slide the sternum strap to about one inch below your collarbones, buckle it, and adjust the tension just enough to pull some weight off your shoulders while still letting your arms move freely.

Walk around for a few minutes. If the pack shifts side to side or feels heavy on your shoulders, tweak the adjustments again. A good fit feels balanced and almost disappears after the first mile. If you are planning a trip that involves exploring a city, you can browse our top picks for city backpacks that balance comfort and everyday utility.

Stay within that range to avoid strain and injury.

Three Common Sizing Mistakes to Avoid

The biggest error is choosing a pack based on height instead of torso length. Height charts are a shortcut that almost never works. The second is buying the pack before you own your gear. Buy the pack last — pile all your sleeping bag, tent, clothes, and cookware together and measure the volume you actually need. The third is oversizing. A 70-liter pack might hold everything, but every liter of empty space is extra weight you haul up the mountain for no reason.

Trip-Specific Recommendations

• Day hikes and city walks: 15–25 liters is plenty for a jacket, water bottle, snacks, and a small first-aid kit.
• Weekend trips: 30–40 liters handles a sleeping bag, pad, change of clothes, and food for two days.
• Multi-day treks: 50–65 liters gives you room for a tent, stove, and extra layers without going overboard.
• Thru-hikes and expeditions: 65 liters or more for long resupply stretches and bulky cold-weather gear.
• General travel: 30–40 liters is the sweet spot — large enough for a week of clothes, small enough to stay as carry-on.

When your torso measurement falls on a border between sizes, always size up for the most comfortable fit. A slightly larger pack can be cinched down with compression straps, but a too-small pack cannot be stretched.

If you are a first-time buyer, visit an outfitter like REI where a specialist can measure your torso and hip circumference and load a test pack with weight. Trying it on in the store (or ordering from a shop with a generous return policy) is the best way to ensure your next adventure starts comfortably.