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

Choose a backpack by your torso length first, then match the liter capacity to your trip length and load.

Picking the right size backpack is one decision that decides whether every hike feels light or like a punishment. The reliable way to work out how to choose the right size backpack starts with one key measurement: your torso length, not your total height. REI’s expert guidance says the distance between your C7 vertebra and your iliac crest, plus where the hipbelt sits, determines whether a pack fits well.

Most people grab a pack by height or guess a liter size. That leads straight to the most common fitting mistakes: hipbelts resting too high or too low, and shoulder straps carrying weight they shouldn’t. Here’s how to measure, what size you actually need, and how to avoid the three fitting errors that ruin otherwise great packs.

Measuring Your Torso Length Correctly

Your torso length is the distance from your C7 vertebra (the bony bump at the base of your neck) to your iliac crest (the top of your hip bones). This single number is the backbone of every sizing chart from REI and Osprey.

To find the right size backpack, follow this simple process:

• Locate C7: Tilt your head forward. The prominent bone at the base of your neck is your C7 vertebra.
• Locate the iliac crest: Place your hands on the top of your hip bones. That’s your iliac crest.
• Measure: Have someone measure the distance between those two points in inches. That’s your torso length.

Common sizing mistake: choosing by overall height instead of torso length. Torso-length ranges also vary slightly by brand, so always check the pack’s specific sizing chart before buying. Once you have your torso measurement, it maps directly to a size band.

Matching Capacity (Liters) to Your Trip

With your torso size confirmed, capacity is the next question. Capacity is measured in liters, and it should match what you carry, not how tall you are. REI’s trip-based guidance is the benchmark: most backpacking packs start around 45 liters and can range up to 80 liters or beyond.

• Day hikes & school: 15–30 liters
• Weekend trips (1–2 nights): 20–50 liters
• Multi-day trips (2–3 nights): 50–60 liters
• Long trips (2–5 nights): 60–80 liters
• Expeditions (5+ nights): 80+ liters

For everyday use, a 20–30 liter pack around 18 × 12 × 6 inches covers most laptop and commuter needs. The volume you choose also depends on hip circumference, which REI notes should be matched to the pack’s hipbelt. Always check the brand’s volume-to-use mapping, since those ranges shift slightly between manufacturers.

The Three Fitting Mistakes That Ruin Comfort

Even a perfectly sized pack feels awful if it’s adjusted wrong or loaded poorly. The three errors that cause the most pain are all avoidable with attention to how the pack sits on your body.

• Hipbelt not on the iliac crest: The hipbelt should wrap snugly over the top of your hip bones. When it rides too high or too low, the weight shifts to your shoulders and lower back.
• Overloading the pack: Exceeding a pack’s comfortable carry weight strains your frame and spine. Keep the load proportional to the pack’s liter capacity.
• Relying on shoulder straps: Your hips should carry roughly 80% of the weight. If your shoulders feel the full load, the hipbelt is positioned wrong or the torso length is off.

A pack sized longer than your torso increases lower-back discomfort. Correct torso length distributes weight to your stronger hip and leg muscles, not your spine.

Checking Your Fit After You Load It

The measuring is the easy part. Once the pack is loaded, take 30 seconds to verify the fit before you hit the trail. If you’re carrying a laptop, check these tested backpack picks for fits that handle tech gear well.

REI’s adjustment method is straightforward: loosen all the straps, put the pack on, and tighten the hipbelt first so it sits squarely on your iliac crest. Then snug the shoulder straps so they follow your shoulders without gaping, and finish with the load-lifter straps at a 45-degree angle. Finally, check the pack for the best-balanced carry: REI’s pack-fitting guide walks through each strap’s exact job.

When it fits right, the pack feels like a natural extension of your body, and your shoulders barely notice the weight. When it doesn’t, you’ll feel it within the first mile. The one number worth measuring is your torso length; get that right, and every other sizing decision gets easier.