How to Choose the Right Size Backpack? | Fit First, Then Volume

The biggest mistake in buying a backpack is picking one by height or by liters alone. A 30-liter pack on a 6-foot frame can feel just as wrong as on a 5-foot frame if the torso length doesn’t match. The right process is three steps: measure your torso, confirm the hipbelt fits, then choose the volume that matches what you carry.

Measure Your Torso Length (The Only Measurement That Matters for Fit)

Your torso length—not your overall height—is the key measurement for backpack fit, per REI’s official fitting guide. Here is the at-home method that works every time:

• Find C7: Tilt your head forward. The prominent bony bump at the base of your neck where the slope changes is the C7 vertebra.
• Find your iliac crest: Place your hands on your hips with thumbs pointing back. You will feel the top ridge of your hip bones—that is the iliac crest.
• Measure between them: Have a friend measure from C7 down your spine to the line between your iliac crests (the imaginary horizontal line across your lower back).
• Average it: Repeat the measurement three times and use the average for most accurate results.

If you don’t have a helper, use a piece of string or cord, mark the points, and lay it against a ruler.

How Torso Length Translates to Pack Sizes

Torso-length ranges vary slightly by brand, but REI’s general sizing scale gives a reliable starting point. Extra small packs fit torso lengths up to about 15 inches, small fits 16 to 17 inches, medium or regular fits 18 to 19 inches, and large or tall fits 20 inches and up. When in doubt, try the pack with weight in it—the hipbelt should cover your hipbones entirely, and the shoulder straps should lie flat without touching your neck.

One common puzzle: your hip measurement for a backpack sits higher than your pants waistline. Wrap the tape measure around the top of your hips, following the iliac crest—that gives the correct hipbelt size, which is often one to two inches larger than your pant size.

Choose the Right Volume for What You Carry

Once the fit is dialed, the question shifts to capacity. A pack should be big enough to hold all intended gear inside, except items you deliberately carry externally. Here is a practical volume guide:

For most everyday carry and short travel, 20–30 liters is the sweet spot. That range accommodates a 15-inch laptop, books, lunch, and a jacket without being oversized. If you are choosing a pack for hiking or camping, consider how much weight you can manage comfortably—a bigger pack tempts you to carry more than you need, adding pounds you will feel by mile three.

If you need a pack that compresses small for travel or storage, check out our tested recommendations for the best collapsible backpacks—these fold flat when empty but expand to carry a full load when needed.

Adjust and Test Before You Commit

The final fit check requires weight. MEC’s fitting sequence is the industry standard: put about 8–10 kg (17–22 lbs) in the pack, loosen all straps, then tighten the hipbelt snugly—it should sit on the hipbones, not restrict breathing. Then adjust shoulder straps just enough to transfer weight to the hips, followed by the top stabilizer straps, sternum strap (allowing chest expansion), and hipbelt stabilizers. Walk around and check for shifting, rubbing, or any pressure on the shoulders. If the pack rides off your hips or loads your shoulders after adjustment, the size is wrong.