What Size Carry-On Backpack Can I Take?

A backpack of up to about 40 litres that measures no more than 55 × 35 × 20 cm will go in the overhead bin on almost every major UK, European and US airline. If you're booking the cheapest fare and want to fly with just one free bag, you'll need something smaller: roughly 18–24 litres, small enough to fit under the seat in front of you.

The catch is that litres are how backpacks are sold, but they're not how airlines check them. At the gate, nobody asks how many litres your bag holds. They ask whether it fits the metal sizer. That's why two bags both labelled "30L" can get very different treatment, and why a bag that sailed through on one airline can cost you a fee on the next.

So in this guide we'll go airline by airline in centimetres, look at what a 20L, 30L or 40L bag really holds, and explain why expandable bags are the ones most likely to get pulled aside at boarding. At the end, we'll show you where each Mous backpack fits.

Cabin bag limits for major UK, EU and US airlines (cm)

Most airlines give you two allowances: a small bag that goes under the seat in front of you, and a larger one for the overhead locker. On full-service airlines you'll usually get both. On low-cost airlines, and increasingly on the cheapest fares of the big airlines too, only the under-seat bag is free. The table below shows where each airline draws the line. All measurements include wheels, handles and outside pockets.

If you look across the overhead column, the numbers seem close, but a couple of centimetres is all a sizer needs to reject you. The two limits that catch most people out are Air France and KLM's 35 cm width and Ryanair's 20 cm depth. A bag that fits both will fit almost anywhere. Under the seat, United is the one to watch: its 25 cm width is narrower than any other US airline's, so a daypack that's fine on American can be too wide on United.

What 20L, 30L and 40L actually hold

A 20-litre bag is a day-and-a-night bag. It holds a laptop of up to 16 inches, a tablet, chargers, a change of clothes, a toiletry bag and a water bottle. It's the size most people use for commuting, and on many airlines it's the largest bag you can take free on a basic fare. Just check the height: the under-seat limit is usually 40 cm, and plenty of 20L backpacks are taller.

At 25–30 litres you've got a long weekend. Add two or three outfits, a light jacket and something to sleep in, and you're set for two or three nights. This size goes in the overhead bin on every airline we've listed, but it's too big for most free under-seat allowances, so on budget fares you'll be paying for a cabin bag.

At 35–40 litres, you can manage a full week with packing cubes, a second pair of shoes and all your tech. Travel brands such as Tortuga treat 40L as the practical ceiling if you want one bag that also works on European airlines. Above that, at 45 litres or more, you're fine on US domestic flights but likely to be stopped in Europe once the bag is full.

One caveat: capacity figures aren't measured consistently between brands. Some include outside pockets, and some quote the expanded volume as the headline number. A tall, slim 25L bag can fail a 40 cm sizer that a short, wide 30L bag gets through. So use litres to judge what you can pack, and the height × width × depth to judge where it can go.

Expandable bags and gate-check risk

Depth is usually the problem here. Most expansion zips add 2–5 cm, and Ryanair's paid cabin bag allows 20 cm of depth. A pack that measures 20 cm closed and 23 cm expanded is fine on Wizz Air, which allows 23 cm, but will be stopped on Ryanair. Roll-tops have the same problem with height: unrolled, they can grow by 20–30 cm, which takes them well past any airline's limit.

Airlines are also checking more closely than they used to. United started rolling out automated bag sizers in late 2025, and American and Delta have been testing similar systems. Getting caught costs more than paying in advance. Gate fees on US budget airlines run from roughly $60 to $100, and Ryanair and easyJet both charge more at the gate than online.

Which Mous backpack fits which airline

Every Mous backpack fits as an overhead cabin bag on all the airlines above. None of them is designed for the strict 40 × 30 cm under-seat sizers, so on a Ryanair, Wizz Air or Basic Economy fare, add a cabin bag when you book. Beyond that, it comes down to how long you're away for.

For a work trip or a night away, the Flap Backpack is the lightest option. It's a 20-litre bag weighing 1 kg, measuring 46.5 × 29.5 × 13.5 cm, with a sleeve for a 16-inch laptop. It's slim enough to go under the seat on many full-service flights, especially when it isn't packed full. At 46.5 cm tall, though, it's taller than the strictest 40 cm sizers, so don't rely on that.

If you carry more tech, the 25-litre Zip Backpack (formerly the Extreme Commuter) adds AiroFoam® drop protection around your laptop. At 48 × 33 × 17 cm, it fits every overhead limit we've listed, including Air France and KLM's narrow 35 cm width.

For two or three nights, the Protective Roll Top is the most flexible. Rolled down, it's 47 × 31 × 19 cm and holds 25 litres. Unrolled, it holds 35 litres but stands 77 cm tall, which is far too tall for any overhead limit. So keep it rolled down for the flight, and unroll it at your destination for gym kit or shopping.

For a week away with one bag, the Travel Backpack holds 40 litres and expands to 45. At 55 × 35 × 20 cm, it matches the tightest European overhead width, Air France and KLM's 35 cm. At 40L it also fits Ryanair's 20 cm depth limit. Expanded, it's 23 cm deep, which is still fine on Wizz Air, Lufthansa, BA, easyJet and the US airlines, but not on Ryanair. It weighs 2.2 kg empty, which leaves about 5.8 kg for packing under Lufthansa's 8 kg limit.

We've checked these sizes against airline guidelines, but rules vary by route and fare, so it's always worth a quick look at your airline's page before you fly.

FAQ

What is the biggest backpack I can take as a carry-on? About 40–45 litres. US airlines allow 56 × 35 × 23 cm, which works out at roughly 45 litres. In Europe, 40 litres at 55 × 35 × 20 cm is the safer choice.

Is a 40L backpack carry-on size? Yes, on every major airline in this guide, as long as it measures no more than 55 × 35 × 20 cm. On basic fares, you'll need to pay for the overhead bag.

What size backpack fits under the seat for free? In Europe, 40 × 30 × 15 cm is guaranteed, and Ryanair and Wizz Air allow 20 cm of depth. easyJet allows 45 × 36 × 20 cm, and in the US, American allows 45 × 35 × 20 cm. That's roughly 18–25 litres.

Do airlines weigh backpacks? Some do. Lufthansa has an 8 kg cabin bag limit, and Air France and KLM allow 12 kg across both your bags. Most US airlines don't set a weight limit for domestic flights.