Choose a backpack by trip length and gear volume, not body height. Your torso length decides the fit.
Most new backpackers pick a pack by their height, and that is the mistake REI and Osprey both warn against. Whether you need a 30-liter or a 75-liter pack depends on how many nights you’re out, how much gear you carry, and one critical body measurement: your torso length, not your height. Here’s how to land on the right volume and get a fit that carries weight comfortably.
Backpack Volume Depends On Trip Length
Pack capacity is matched to how many nights you’ll be out, plus how much weight and bulk you want to carry. REI’s general ranges give a solid starting point, and they assume a typical load of shelter, sleep system, clothing, food, and cook gear.
These numbers shift with gear weight. Winter and shoulder-season trips need more volume than summer ultralight outings, because the sleep system, extra layers, and food take up far more space.
Day and Ultralight Packs (20–50 L)
Day trips and ultralight overnighters fit in this range. Many ultralight backpacking packs run 28 to 60 liters, and the lightweight school of thought says to start with gear first and build the pack around it.
Weekend Packs (50–60 L)
A 50 to 60-liter pack handles one to three nights for most people carrying a moderate load. If you pack lightly, this range can stretch to three or even four nights.
Multi-Day Packs (60–80 L)
Two to five nights with a full kit, including a bear canister, fits comfortably in 60 to 80 liters. This is the most common zone for standard three-season backpacking trips.
Extended and Winter Packs (80+ L)
Trips of five or more nights, or any winter expedition, call for more than 80 liters. A heavier sleep system, bulky layers, and more food force the bigger volume.
Measure Torso Length, Not Height
Torso length is the main sizing metric for a backpack, and both REI and Osprey use the same method to find it. This measurement decides which frame size fits you, not how tall you stand.
- Tilt your head forward and feel for the prominent bone at the base of your neck — that’s the C7 vertebra. Locate the C7 vertebra and mark the spot.
- Place your hands on your hip bones so your thumbs point backward. The line across the top of your hips is the iliac crest. Find the iliac crest line.
- Have someone measure between those two points. That distance, in inches or centimeters, is your torso length.
- Measure your hip circumference around the top of the hips, at the iliac crest, for the hip belt size.
Most brand size charts map a range of torso lengths to small, medium, and large frames. Try a pack on with realistic weight — 8 to 12 kilograms of books or water bottles is the standard fitting check — before you commit.
Choosing Volume By Gear, Not Guesswork
Packing order fixes the volume question faster than any chart. Lay out everything you plan to bring, load it into the pack, and see how it fits. A good rule of thumb is to choose a pack volume with about 10% extra space beyond your base gear, so food and water have a home.
Pick gear first, then weigh it, then size the pack. This sequence prevents the common mistake of choosing too little volume for food, layers, and shelter. MEC notes that a pack should be big enough to fit all gear inside, except items intentionally attached outside. Strapping a tent to the outside is fine; strapping half your kit there is a safety problem on rough terrain.
If you are evaluating models, start with this internal roundup of our top backpacking packs with detachable daypacks — a useful configuration for basecamp trips and summit pushes.
Fitting The Pack To Your Body
A correctly sized pack only works if you put it on right. The hip belt cinches first, and it must sit on the iliac crest — not above or below it. The belt carries the bulk of the load, so getting it level on the hip bones is the difference between a good trip and a sore one.
After the hip belt, snug the shoulder straps so they sit flat without pressure, then tighten the load lifters at a 45-degree angle. On a fitted pack, you should feel most of the weight on your hips, with your shoulders just steadying the load.
FAQs
Is a 65-liter backpack big enough for a week?
Usually yes, if you pack thoughtfully. A 65-liter pack in the 60–80 L range covers two to five nights under REI’s guidance, and a disciplined packer can stretch it to a week with careful food planning and compression. Winter trips with bulky gear are the exception.
Do I need a bigger pack if I’m tall?
No. Torso length determines frame size, not overall height. A tall person with a short torso takes a small frame, and a shorter person with a long torso takes a large one. Measure from the C7 vertebra to the iliac crest to find your fit.
What happens if my pack is too small?
You run out of room for food, layers, and shelter, and resort to strapping gear on the outside. That makes the load unstable, catches on branches, and leaves your kit exposed to weather. MEC recommends a pack big enough to hold everything inside except intentional exterior items.
References & Sources
- REI. “How to Choose a Backpack.” Provides volume ranges by trip length and the torso measurement method.
- MEC. “How to Choose and Fit a Backpack.” Explains gear-first packing and full interior carry guidance.
- Osprey. “Size & Fit.” Details the C7-to-iliac-crest measurement for torso sizing.