What 40 liters actually holds
For a sense of scale rather than a promise: a well-packed 40L carry-on comfortably takes a week of clothing for one adult, with packing cubes doing the sorting. In practice that is around five to six tops, two bottoms, a week of underwear and socks, a mid-layer, a packable jacket, a wash kit and one spare pair of shoes — with the bulkiest layer and the heaviest shoes worn rather than packed.
Stretch that to two weeks and it still works, because two weeks in a carry-on is a laundry problem rather than a capacity problem. We work that through in how to pack for two weeks in a carry-on.
The three numbers that separate 40L packs
External dimensions, not capacity
Two 40L packs can measure very differently on the outside, because liters describe usable internal volume and airlines measure the external box — including straps, handles, side pockets and whatever the bag is bulging into. A pack that states 20.5 x 13 x 9 in has real headroom against a 22 x 14 x 9 limit. A pack that states 21 x 15 x 10 in is already over on two dimensions before you pack anything.
This is the whole reason we print a “Not published” in the spec table rather than an estimate: on a 40L bag, the dimension line is the buying decision, and a maker who won’t state it has told you something. Measure any bag you already own the way a gate agent would — our how to measure a backpackguide walks through it, and the bag-by-bag version of this question — every 40L pack we cover against the box eight carriers publish — is is a 40L backpack a carry-on.
Empty weight
On US carriers this rarely matters; the major domestic airlines publish no cabin weight limit at all. On Emirates, Qantas and most of Asia and Europe it matters enormously, because a 7 kg allowance includes the bag. The spread across our 40L picks is real: 2.2 lb at the light end against 3.4 lb at the heavy end is a pound of clothing you either can or cannot bring. The airline-by-airline table lives in carry-on weight limits by airline, and every published empty weight we have sits in one table in how much does a travel backpack weigh.
Whether it expands
An expandable 40L is really a 40L pack with a 45–50L mode, and the expanded mode is not carry-on. That is fine if you understand it as “a carry-on that becomes a checked bag on the way home”, and a problem if you fill it on the way out. More on that trade in our expandable travel backpacks roundup.
Should you buy 40L at all?
An unpopular opinion for a page about 40L packs: for a lot of travelers, 35L is the better buy. A 35L pack clears strict sizers with room to spare, weighs less, and — because you cannot overfill what isn’t there — enforces the packing discipline that makes one-bag travel pleasant. The Cotopaxi Allpa 35L is our top pick on the carry-on backpacks hub for exactly this reason.
The same family shows the trade at its starkest. Cotopaxi’s bigger sibling adds seven liters for three ounces, but its published 22 x 14 x 11 in has no width margin at all and sits two inches over the box on depth — our Cotopaxi Allpa 42L review works through exactly who should take that deal.
If that argument lands, the whole class is ranked on best 35L travel backpacks— deliberately a different pick set from this page, because the 34–36L rule excludes every bag above. It also prints the published depth of each one, which is the figure that decides whether the extra sizer margin is real or assumed.
Buy 40L when:
- You fly mostly US carriers, where the limit is generous and rarely enforced hard.
- You need bulk volume for cold-weather layers or bulky gear.
- You are traveling for two weeks or more without checked luggage.
- You want the option of checking it on the return leg with souvenirs in it.
Buy 35L or under when:
- Your itinerary includes Ryanair, easyJet, Wizz or their equivalents.
- You are flying carriers that weigh cabin bags.
- You know you will fill whatever you buy — most people do.
- You want to walk with the bag rather than roll it between transfers.
Our what size travel backpack guide works the decision through by trip length and body size, which is a better starting point than a capacity number you saw in a listing.
The 40L mistakes worth avoiding
Buying on capacity alone. The liter figure is a manufacturer’s claim about internal volume with no agreed measurement standard behind it. The external dimensions are the checkable number, and they are what the sizer cares about.
Packing it full for the outbound leg. A 40L pack at 80% is a comfortable, compliant carry-on. The same bag at 100% is a bulging object that fails a sizer it would otherwise clear, because soft bags grow in the two dimensions you can’t control.
Ignoring torso length. A 40L pack carries roughly 20 lb, and 20 lb on a harness sized for someone else’s back is genuinely unpleasant. Torso length, not height and not gender, is the measurement that matters — see how to fit a travel backpack.
Assuming “flight approved” means anything. No airline or agency certifies backpacks. That phrase is listing copy. The only test is your carrier’s published numbers against the bag’s published numbers, which is what the size guide exists for.
