What Size Backpack Do You Need for Backpacking?

“What size backpack” is really two separate questions that get answered wrong together. One is capacity: how many liters your trip needs. The other is fit: which torso range on the maker's size chart is actually yours. Get the liters right and buy the wrong torso size and the pack still won't carry well. Get the fit right in a pack that's too small and you'll be strapping gear to the outside by night three.

Both questions have a real, published answer, and neither one is a guess on our part. REI puts a number on liters by trip length. Every maker we cite here puts a number on torso length, hip circumference, or both, on its own product page. This guide walks through both, then shows what four budget packs in our registry actually publish, side by side, so you can check your own trip and your own measurements against real specs instead of a marketing description.

Liters by trip length

REI's own sizing guide breaks backpacking packs into three bands by trip length, and gives a liter range for each:

(REI Expert Advice, “How to Choose a Backpack: Sizing & Fit Guide”, checked 2026-09-22.) These are bands, not a formula: REI frames the weekend range as something “efficient packers” can pull off, which is a polite way of saying it takes a genuinely light kit to fit a 3-night trip into 30–50 L.

How your base weight changes the math

REI's bands assume an average kit. Your actual number moves with your base weight: a lighter tent, quilt and pad pack down smaller as well as weigh less, so a light kit fits the same number of nights into fewer liters than a heavier one does. That is also why every pack in our budget roundups clusters in the middle of REI's multiday band rather than the extremes. The smallest here, the 3F UL Gear QiDian 40+16, lists 40 L + 16 L of capacity (the same page also lists “up to 55 L”), which sits right at the top of REI's weekend band (3F UL Gear). The largest, the Durston Kakwa 55, totals 70 L including its exterior pockets on a Medium — the floor of REI's extended-trip band, not deep into it (Durston Gear).

Worth flagging: 3F UL Gear's own QiDian 40+16 page lists two different capacity figures on the same page, “40L+16L” in the main spec and “up to 55 L” in a materials note further down. We print the maker's figure as they publish it, conflict and all, rather than picking whichever number makes the pack look better. A discrepancy like that is a reason to size a little conservatively when a pack's own listing disagrees with itself.

Volume isn't just the main compartment

Two packs both listed near 60 L don't necessarily hold your gear the same way. The Gossamer Gear Mariposa 60, at 968 g (2 lb 2.1 oz), splits its total as 36 L in the main body up to the extension collar, plus 24 L spread across seven exterior pockets (Gossamer Gear). The Durston Kakwa 55 splits its 70 L total (Medium) as 55 L internal plus 15 L across its exterior pockets (Durston Gear). Both are real, usable liters, but a pack that puts more of its total in outside pockets lets you reach snacks, a rain shell or a water filter without opening the main bag, while a pack with less exterior volume keeps more of your load centered and compact. Neither is wrong; it changes how you pack, not just how much you can carry.

Torso length, not height, decides fit

REI is direct about the most common sizing mistake: “It should be correctly sized for your torso length (not your overall height) and hip circumference,” and separately, “Torso length—not your height—matters most” (REI Expert Advice). Two hikers who are both 5'10" can need two different pack sizes if their torsos are built differently. REI also flags that “these ranges vary by manufacturer and by gender” and tells shoppers to “check the product specs tab for size details of a specific pack” — which is exactly why we only publish sizing a maker states on its own page, never an estimate.

Fit is really two measurements, not one. Torso length picks the frame size. Hip circumference picks the hipbelt, and REI says that range is usually wide: hipbelts “accommodate a wide range of hip circumferences, from mid-20 inches to mid-40 inches or so,” though “people with narrow waists sometimes find they cannot make a standard hipbelt tight enough and need to buy a smaller size” (REI Expert Advice). REI also notes that women-specific packs typically run “shorter and narrower” in the torso than men's versions of the same pack, with hipbelts and shoulder straps shaped differently — and several packs in our registry are sold that way: Granite Gear lists both the Virga3 55 and the Crown3 60 in women's versions alongside the unisex sizing we cite here. If the published unisex range doesn't fit, check whether the maker sells a women's or youth-specific version before assuming the pack itself is wrong for you.

An adjustable suspension is the other route some makers take instead of purely fixed sizes. REI notes it “can be modified to fit your torso, especially if you're between sizes,” which also makes a pack easier to share between people, but flags the trade-off plainly: “an adjustable harness adds weight to a pack” ( REI Expert Advice). Granite Gear's own spec sheets make the contrast explicit on two packs from the same maker: the Virga3 55 lists its torso as adjustable within its Regular and Long options, while the Crown3 60 lists its torso as fixed across its three sizes, Short, Regular and Long. Neither is wrong; it's the same trade-off REI describes, just on two packs you can compare side by side.

Published torso ranges, not a guess

Here is what four makers actually publish, side by side with the liters each pack offers:

Notice the QiDian 40+16 has a dash in the Fit row: 3F UL Gear sells it in one size, colors only, with no torso range published at all. That is a real trade-off for the lowest list price in the group, not an oversight on our part — if a dedicated torso fit matters to you, the other three all publish multi-size charts. Osprey does the same for its Exos 58, listing 17–20.5 in for its S/M and 19.5–23 in for its L/XL (Osprey); see how it stacks up against the Kakwa 55 in Osprey Exos vs Durston Kakwa 55.

Read the three sized packs side by side and a pattern shows up: none of them measure torso the same way. Virga3 states a range for each of its two lengths. Crown3 states a range for each of its three fixed sizes. Kakwa 55 does the same across its own three sizes, but with different inches at each step. There is no universal “Medium” across brands — a Medium Kakwa and a Regular Virga3 are not built to the same torso number, so a size that fits one maker's pack is only a starting guess for another's, never a substitute for checking the specific chart.

What this means for a budget buyer

Size a pack in this order, and you won't need to guess at any of it:

1. Start with your trip length and land on a liter band from REI's table above, not the biggest number on the shelf. A pack that's too large just gives you room to overpack, which works against everything a lighter base weight is trying to do.
2. Adjust for your current base weight. If your shelter, sleep system and cook kit are already light, you can usually go a size down from what REI's band suggests. If they're not yet, size up rather than fight to stuff a heavier kit into a small pack.
3. Measure your actual torso, not your height, and your hip circumference separately. Check both against the specific maker's chart, not a general rule, since REI says the ranges vary by brand and by gender.
4. Check whether a women's or youth version fits better if the unisex range you're looking at doesn't line up with your measurements. Several packs in our registry offer one.

Our best budget ultralight backpacks and best budget backpacking backpacks roundups list the published liters and torso range on every card, so you can filter by fit before you ever look at price. And once you've got a liter target and a torso size, how heavy should my backpack be covers the other half of the decision: whether that specific pack is rated to carry what you're planning to put in it.