Is a 40 litre backpack carry-on size?

A 40 litre travel backpack, yes: packs sold that way are cut as a 55 x 35 x 20 cm box, inside the 56 x 35 x 23 cm limit that Delta, United and American publish. A 40 litre backpacking pack, usually not when full. The same volume is built 74 to 80 cm tall, and airlines measure height, not litres. Rolled down with about 28 litres of kit or less, a roll-top one can fit.

Why the litre number does not answer the question#

Search the question and two answers come back. Travel pack sellers say yes: 40 litres is "maximum carry-on size". Hikers on forums say their 40 litre pack was sent to the hold. Both are telling the truth about different packs.

No airline in this article states a limit in litres. Every one gives three outside dimensions, and most say that handles and wheels count. American adds that the bag "must fit in the sizer at the airport". Delta adds a total: 45 linear inches, which is 114 cm of length plus width plus height. A sizer is a box, and a box does not care how many litres you carry. It cares whether the longest side of your bag is longer than its longest side.

The litre figure on a pack also measures something different from the box. Pack capacity has no standard test, and what counts varies by maker, as how many litres of pack you need explains. Two published specifications show the gap:

• Tortuga lists its Pro Backpack 40L at 55 x 35 x 20 cm and 40 litres. Multiply the dimensions and the box is 38.5 litres, smaller than the stated capacity. Soft walls bulge, and pockets that stand proud of the main body add volume the box does not account for.
• ULA lists the Circuit at 68 litres total, and breaks it down: 39.3 litres of main body, an 8.19 litre extension collar, three 6.6 litre mesh pockets and two 1.5 litre hip belt pockets. The "68" includes nearly 29 litres that are pockets and collar.

So the circulating rule "40 litres is the carry-on maximum" is a rule about one shape of pack. It holds for travel packs designed around the box. It is the wrong rule for packs designed around a spine.

Carry-on limits, converted to litres#

Each row is the airline's own page, live or as captured by the Internet Archive, read in October 2026. The litre column is plain multiplication of the three published centimetre figures, and it is the largest rigid box that fits. A soft pack fits only if its packed shape fits too.

Three things in that table matter more than the litres.

Height is 55 or 56 cm everywhere. The width and depth vary by up to 10 cm between airlines; the long side barely moves. That is the dimension a backpacking pack fails on.

Depth is 23 cm almost everywhere, and 25 at easyJet. A pack stuffed round can exceed it while every other measurement is fine.

The fare can remove the overhead bag entirely. United's page grants a carry-on bag to Basic Economy only to South America, across the Atlantic or across the Pacific; for most trips it is one personal item. Lufthansa's Economy Basic on short and medium haul includes only the 40 x 30 x 15 cm personal item. easyJet's free allowance is the under-seat bag; the overhead locker costs extra. On those fares the question is not whether 40 litres fits, but whether 18 to 32 litres does.

Travel pack or hiking pack: two shapes of 40 litres#

The difference is geometry, and it shows in the published specifications.

The travel packs are built as the airline's box with straps on. The backpacking packs are tall and narrow, because a load that sits close to the spine carries better, as packing a backpack shows with the moment arithmetic. That same shape is what puts them 18 to 24 cm over the height limit when full.

Note also the Farpoint's 9.1 inch depth, against the 9 inch figure Delta, United and American print. On paper it is 0.1 inch over. In practice a soft pack compresses, which is why travel packs are sold this close to the line, but a stuffed front pocket can push any of them past 23 cm.

How much a 40 litre roll-top holds at 56 cm#

Many ultralight backpacking packs, including both above, close with a roll-top, so they can be shortened. The useful number is how much kit is left in the pack once it is short enough to fit. Neither maker publishes that, so we model it from the specifications above.

The result is the same for both packs, which is not a coincidence. Two packs with the same perimeter, held to the same depth and height, hold the same volume. So the rule carries over to any roll-top body with a perimeter near 90 cm, which is where both of these 40 litre packs sit: pack about 25 to 28 litres of kit and roll it down, and it can meet a 56 x 35 x 23 cm limit. Pack the full 40 and it cannot.

The pockets complicate this in both directions. Zpacks' 40 includes 13 litres of side and centre pockets, and Hyperlite lists 11.5 litres of external volume on top of its 40. Pockets add depth: a full front pocket on a pack already 23 cm deep takes it over. Empty pockets cost nothing. For the flight, move what is in them into the main body, or into a personal item.

When the frame decides for you#

A roll-top can shrink. A frame cannot. Zpacks lists three frame heights for the Arc Haul Ultra 40: 43, 51 and 58 cm. The 58 cm tall frame is longer than every overhead limit in the table above before a single item goes in. The 51 cm medium leaves 5 cm under a 56 cm limit, which the hip belt, the rolled top and the bottom seam have to share.

The practical test is simple. Find the frame height or the torso range in your pack's specification, and add a few centimetres for the bottom of the pack and the closure. If that is more than 55 cm, the pack is a checked bag, or a gate-check gamble. Torso size sets frame height, so tall hikers are the ones caught by this, and backpack fit covers why the right frame for your back is not negotiable for the trail.

Weight, the second limit#

Size gets a bag into the sizer. Weight gets it past the gate agent on airlines that weigh. Of the pages in the table, Lufthansa caps a carry-on at 8 kg, Qantas international Economy at 7 kg, easyJet at 15 kg, and Air Canada says no weight limit applies. The three US airlines state no weight on the pages read here.

Seven or eight kilograms is a demanding figure for a backpacker. The pack itself is 0.84 kg for the Hyperlite Southwest 40 and 2 kg for the Tortuga Pro 40L, by the makers' figures. A traditional kit is over 9 kg base weight by the convention in base weight, explained properly, and only an ultralight kit under 4.5 kg leaves room for clothes. If the itinerary includes a 7 or 8 kg airline, the cap on the cabin bag can force the split of kit between bags before the dimensions do.

Some of the kit has its own rules regardless of size or weight. Fuel cannot fly in either bag, as whether a camping stove can fly sets out, and tent stakes are checked only under TSA and CATSA, covered in trekking poles on a plane, which also has the collapsed pole lengths that fit a 56 cm bag. Power banks are the reverse: cabin only, and whether you can use one on board now depends on the airline.

Measure your own pack before the airport#

Ten minutes at home replaces the gate agent's verdict with your own. You need a tape measure and either a cardboard box or a doorway and two books.

1. Pack it for the flight, not for the trail. Everything that will travel in it, pockets as they will be, poles and stakes out.
2. Roll the top down until the main body is firm but closed, and clip the side compression straps.
3. Measure the height from the floor to the top of the rolled closure or the top of the frame, whichever is higher, with the pack standing upright. This number must be 55 or 56 cm or less, depending on the airline.
4. Measure the depth at its fattest point, usually across the front pocket, by pressing two books against the back panel and front and measuring between them. It must be 23 cm or less (25 cm at easyJet).
5. Measure the width across the back panel and side pockets. It is the dimension most packs pass easily: 35 cm at the US airlines, 40 at Air Canada and Lufthansa.
6. For a Delta flight, add the three. The total must be 114 cm or less.
7. If any figure fails, take kit out of the main body, roll again and repeat. The volume you removed tells you what your pack holds at carry-on size.

A cardboard box cut to 56 x 35 x 23 cm inside is a sizer of your own. If the pack slides in without forcing, it will slide into the airline's.

Common mistakes#

Buying a pack for its litres. A 40 litre label says nothing about the 56 cm height that decides the question. Read the dimensions in the specification, and if there are none, assume a hiking pack is too tall.

Measuring the empty pack. A roll-top pack is shortest when empty and longest when full. Measure it packed, as step 1 above says.

Forgetting the fare. The overhead bag on United Basic Economy or Lufthansa Economy Basic short haul usually does not exist. Check the ticket before the pack.

Leaving the hip belt and straps loose. Most airlines include handles and wheels in the measurements. A hip belt hanging below the base adds height. Clip it round the front of the pack or tuck it away.

Stuffing the front pocket. It is the fastest way past 23 cm of depth. Put the soft items in the main body and leave the outside pockets flat.

Assuming a sizer is the only check. On the pages read here, Lufthansa and Qantas also set weight limits. An 8 kg limit fails a pack that fits the box easily.

Frequently asked questions#

Is a 40L backpack carry-on size?#

A 40 litre travel backpack usually is: those packs are cut to about 55 x 35 x 20 cm, inside the 56 x 35 x 23 cm limit used by Delta, United and American. A 40 litre backpacking pack usually is not when full, because it is built 74 to 80 cm tall. Rolled down with about 28 litres of kit or less, most roll-top hiking packs can meet a 56 cm height limit.

What size backpack is carry-on in litres?#

Airlines do not set a limit in litres. The common US limit of 56 x 35 x 23 cm is a 45.1 litre box, and the 55 x 40 x 23 cm limit at Air Canada and Lufthansa is 50.6 litres. A soft pack's label can read more or less than the box: Tortuga's Pro 40L is 55 x 35 x 20 cm, a 38.5 litre box. Check the outside dimensions, not the litre label.

Can I bring a hiking backpack as a carry-on?#

Yes, if it fits the airline's dimensions when packed, including straps. Most hiking packs fail on height, not width or depth, because they are built tall and narrow. A roll-top pack can be shortened to about 56 cm by packing less and rolling it down. A pack with a rigid frame taller than about 53 cm cannot be shortened and has to be checked.

Is a 45 litre backpack too big for carry-on?#

Not necessarily. A 56 x 35 x 23 cm box holds 45.1 litres, so a 45 litre travel pack shaped as that box can fit. A 45 litre backpacking pack built for the trail will be much taller than 56 cm when full. Measure the packed height and depth: 55 or 56 cm tall and 23 cm deep are the limits on most airlines.

Does a backpack count as a personal item?#

A small one does, if it fits the personal item size. United allows 43 x 25 x 22 cm, American 45 x 35 x 20 cm, Air Canada 43 x 33 x 16 cm and Lufthansa 40 x 30 x 15 cm. Those boxes hold 18 to 32 litres, so the pack's packed shape has to fit them, whatever its label says. easyJet's free under-seat bag is 45 x 36 x 20 cm.

Do airlines measure backpack straps?#

Most airline pages say measurements include handles and wheels, and an airport sizer measures whatever goes into it, straps included. A loose hip belt hanging below the base adds height, and full side pockets add width. Tighten every strap, clip the hip belt around the front of the pack and empty the outside pockets before you queue to board.