How to Choose a Backpacking Backpack Size (2026)
When I bought my first backpacking pack, I picked the wrong size because I trusted my height instead of my actual torso measurement. That mistake cost me about 12 miles of sore shoulders before I returned the pack and got one that fit my back. Knowing how to choose a backpacking backpack size saves you that same pain, and this 2026 walkthrough shows you exactly how to measure, fit, and pick a pack that carries weight where your body is strongest.
Backpack sizing is a two-part problem. First, you match the frame to your torso length so the weight sits on your hips. Second, you match the volume in liters to the trip length so the pack is big enough for your gear without forcing you to overpack. I have helped friends troubleshoot both halves for years, and the rules below cover both the measuring steps and the capacity math that no single competitor lays out together.
By the end of this guide, you will know how to measure your torso, which liter range fits your trip type, how to adjust every strap, and what to do if you sit between two sizes on a sizing chart. You will also see why height is a poor proxy for pack size, plus answers to the most-asked questions about 30L, 35L, and 70L packs.
Why Torso Length Matters More Than Height
Backpacks are sized to your torso length, not your height, because the load needs to transfer to your hips through the lumbar panel. A taller person with a short torso and a shorter person with a long torso need completely different packs, even if they are the same height standing up.
Torso length is the distance from the C7 vertebra at the base of your neck to the iliac crest at the top of your hip bones. Most adults fall somewhere between 15 and 22 inches. Manufacturers build packs in size ranges that match this measurement, often labeled XS, S/M, M/L, or L/XL on the back panel.
When the torso length is right, the hip belt wraps the top of your pelvis, the shoulder straps curve over your collarbone without pinching, and the load lifters sit at a 45-degree angle above your shoulders. When it is wrong, the pack sags, pulls on your shoulders, and creates hot spots that turn a 5-mile loop into an endurance test.
REI's official fit guide agrees with what I have seen on the trail: most backpackers end up in the medium range, but you cannot assume that without measuring. Two hikers of identical height can stand a full pack size apart.
How to Measure Your Torso Length Step by Step
Measuring torso length is the single most important step in learning how to choose a backpacking backpack size. The process takes about 5 minutes and only requires a soft measuring tape, though a friend helps because the landmarks sit on your back.
Step 1: Find Your C7 Vertebra
Tilt your head forward and feel for the bony bump where your neck meets your shoulders. That is the C7 vertebra, the most prominent spinous process in your upper back. Press a finger on it so you do not lose the spot.
If you cannot feel an obvious bump, slouch forward and have someone press lightly along your spine until the most protruding point is found. This landmark does not move, so once you find it you can use it for every future fitting.
Step 2: Find Your Iliac Crest
Place your thumbs on your hips and slide them upward until you feel the top edge of your hip bones. That ridge is the iliac crest, and it is the lower anchor point for the measurement. Mark it with a second finger.
The iliac crest is what the hip belt should sit on once the pack is loaded. If you aim too high or too low, the belt will slide off or dig in.
Step 3: Measure the Distance
With a soft tape, measure from the C7 down the spine to the level of the iliac crest. Keep the tape following the natural curve of your back, do not pull it straight across. Record the number in inches.
Most men measure between 17 and 21 inches, and most women between 15 and 19 inches. Compare that number to the manufacturer's size chart for any pack you are considering. If you are shopping online and there is no chart, that is a red flag to look at a different brand.
Step 4: Recheck Twice
Repeat the measurement two or three times. If your numbers differ by more than half an inch, you may have shifted the landmark. Take the average and use that to choose your size. Many outdoor stores will also remeasure you in person for free, and that is worth the trip.
Understanding Backpack Capacity by Trip Length
Torso length fits the pack to your body. Liter capacity fits the pack to your trip. Both matter, and conflating them is one of the most common sizing mistakes I see. A perfect-fitting 50L pack is the wrong pack if you only do day hikes, and a great-fitting 30L is wrong if you are carrying a tent and a three-season sleeping bag for a week.
| Trip Type | Recommended Capacity | Typical Packed Weight |
|---|---|---|
| Day hike | 18 to 35L | 5 to 15 lbs |
| Overnight backpacking | 35 to 50L | 20 to 30 lbs |
| 2 to 3 day trip | 45 to 60L | 25 to 35 lbs |
| 4 to 5 day trip | 55 to 70L | 30 to 45 lbs |
| Week or longer | 65 to 80L | 40 to 55 lbs |
| Ultralight 3 day | 35 to 45L | 15 to 20 lbs |
These ranges overlap for a reason. Ultralight gear shrinks the volume you need even on longer trips, and heavy traditional gear pushes you into the larger end for shorter trips. According to forum posts on r/Ultralight, many experienced hikers carry 2 to 3 day loads in 40L packs by trimming weight from consumables and shelter systems.
If your trip length sits between two ranges, plan for the larger size and pack compression straps. They will pull the volume down and stabilize the load when your pack is not full.
Day Pack vs Backpacking Pack: Key Differences
A day pack and a backpacking pack look similar at first glance, but they are built around very different loads. Day packs top out around 35L, have minimal hip belt padding, and rely on your shoulders to carry everything. Backpacking packs start around 40L, have rigid frames, padded hip belts, and load lifters to shift weight to your hips.
The biggest difference is the hip belt. On a day pack it is often a thin webbing strap, which works fine for 10 pounds of water, snacks, and a rain layer. On a backpacking pack it is a structural component that bears most of the load. If your day hikes ever cross into carrying a tent or group shelter, you have already left day-pack territory.
A 35L backpack can count as a carry-on for flights because most airlines allow bags up to about 45 liters in linear dimensions. That is why minimalist travelers like 35 to 40L packs. The trade-off is that 35L is too small for most multi-day backpacking trips unless you are running an ultralight kit.
If your style sits between day hikes and overnight trips, look at fastpacking packs in the 25 to 35L range. They have running-vest-style harnesses and can comfortably carry 10 to 20 pounds. We have covered some of those options on our best backpacking towels guide as part of a layered kit, but the same ultralight logic applies to your pack.
How to Adjust the Hip Belt and Shoulder Straps
Once you have the right torso size, you still need to adjust every strap to match your body. A correctly sized pack with the straps left loose will still hurt, and most new backpackers stop tightening before the load actually transfers to their hips.
Tighten the Hip Belt First
The hip belt should carry about 80 percent of the pack's weight. Buckle it centered on your hip bones, then pull the side straps until the padding wraps your iliac crest snugly. The belt should not slide down when you walk.
Set the Shoulder Straps
Pull the shoulder straps just tight enough to remove the gap between the pack and your back. They should curve over your shoulders without digging into your armpits or trapezius. If you feel pressure on your shoulders, the hip belt is too loose.
Angle the Load Lifters
Load lifters are the small straps that connect the top of the shoulder harness to the top of the pack frame. Pull them forward and down until they sit at about a 45-degree angle to your shoulders. They pull the load inward and upward into your upper back.
Close the Sternum Strap
The sternum strap slides up and down the shoulder harness. Position it roughly two inches below your collarbones and tighten until the shoulder straps do not slip outward. If your pack has a whistle built in, even better. If yours is missing or broken, our guide on backpack sternum strap replacements walks through the swap.
Use Compression Straps Last
Compression straps on the sides of the pack pull the volume inward as your gear runs out. Use them on every trip, even when the pack is full, because a tight load shifts weight closer to your back and improves stability.
When You Are Between Two Sizes
Sizing charts put you in a size range like 18 to 20 inches or 20 to 22 inches. If your torso measures exactly 20, you are sitting on a fence. Most backpackers lean one way or the other based on three questions.
First, how much weight will you carry? Heavier loads favor the larger size, because a longer frame lets the load ride higher on your back and keeps weight closer to your center of gravity. Second, how compressible is your gear? Down sleeping bags and ultralight shelters shrink a lot, which means you can usually go smaller. Third, what do experienced hikers on forums recommend? The most common answer I have read on r/backpacking is to go smaller with good compression straps, because a tight pack performs better than a loose one.
A practical test: load both sizes with 30 pounds of gear and walk around the store for 10 minutes. Whichever version keeps the hip belt in place without riding up and lets you turn your head without the pack following is your size. If you cannot test in person, order from a retailer with a generous return policy.
Common Fitting Mistakes to Avoid
Most backpack fit problems come from a handful of repeat mistakes. I have made three of them myself. Knowing what they look like makes them easier to spot on the trail before they turn into a pain problem.
Treating Height as a Size Proxy
Two hikers of identical height almost never share a pack size. If a store employee asks your height instead of your torso, ask them to measure instead.
Loosening the Hip Belt Because It Feels Tight
A snug hip belt feels restrictive the first time you wear it. That pressure is the load transferring off your shoulders. Loosening it puts that weight right back where you do not want it.
Sizing Up for Extra Space
A bigger pack tempts you to overpack. Every pound of unnecessary gear is harder to carry. Buy the size your trip needs, not the size you wish you needed.
Skipping the Loaded Test
An empty pack on the showroom floor feels great. A 30-pound load feels very different. Always try a pack loaded with weight before committing.
Ignoring Hot Spots on a Short Hike
Pressure points that show up on a 1-mile loop will only get worse at mile 10. Do not dismiss discomfort just because the trail is short.
Frequently Asked Questions
What is the ideal size for a backpacking backpack?
For most 2 to 3 day trips, 45 to 60 liters is the sweet spot. Shorter overnight trips fit in 35 to 50 liters, while week-long expeditions usually need 65 to 80 liters. Ultralight hikers can drop about 10 liters off every category because their gear compresses more.
Does a 35L backpack count as a carry-on?
Most 35L backpacks meet airline carry-on size limits, which usually allow bags up to about 45 liters in total linear dimensions. Check your airline's specific dimensions before flying, and remember that a fully loaded 35L pack can bulge past the limits if overpacked.
Is a 70L backpack too big for backpacking?
A 70L backpack is not too big for week-long trips with traditional gear, but it is more volume than most 2 to 3 day trips need. If you are not carrying a tent, sleep system, and food for 5+ days, a 50 to 60L pack will be easier to manage and will keep you from overpacking.
Is a 30L backpack too big for a day hike?
A 30L backpack is on the larger end for a typical day hike but it is fine if you carry bulky items like a winter layer, a small stove, photography gear, or extra food and water. For short summer hikes with just water and snacks, a 15 to 25L day pack is plenty.
How to know what size backpack to get for backpacking?
Measure your torso from the C7 vertebra to the iliac crest, match it to the manufacturer's size chart, and pick the pack whose volume fits your trip length. Then load it with 25 to 30 pounds of gear and walk to confirm fit before buying.
Final Thoughts on Picking Your Pack
If you only do short trips with a streamlined kit, a 35 to 50L pack in your measured torso size will keep you light and fast. If you carry full traditional gear for 3 to 5 days, a 55 to 70L pack matched to your torso gives you room for food and shelter without leaving empty space. For women looking at gender-specific fit, our roundup of the best women's backpacking backpacks compares packs built around shorter torso ranges.
The single best piece of advice on how to choose a backpacking backpack size is to measure twice, fit with weight, and trust the chart over your height. Get those three steps right and the rest of the buying process becomes much simpler in 2026.