A carry-on backpack is usually one that fits within 55×40×20 cm, which works out at about 44 litres and is a common cabin limit with many airlines, while US airlines often allow 22×14×9 inches. For travel, 30–45 litres suits a weekend and 35–45 litres is enough for a week with carry-on only, but always check your own airline’s size and weight limits before you fly.
Key takeaways
- Day trips: 15–25 L. Overnight: 25–35 L. Weekend: 30–45 L. One week, carry-on only: 35–45 L.
- A common cabin limit is 55×40×20 cm, which is about 44 L.
- IATA’s guideline size is 56×45×25 cm; many US airlines allow 22×14×9 in.
- Airline rules differ and change, so check yours before every trip.
- Packing light matters as much as the bag’s size for a one-bag week.
What does backpack size in litres mean?
Backpack size is normally given as volume in litres, which tells you how much the main bag can hold. A 20-litre bag holds roughly what 20 one-litre bottles would fill. Makers measure volume in slightly different ways, so two bags with the same number can feel different in use.
Litres tell you capacity, but airlines check external dimensions. A soft rucksack (backpack) can bulge past its listed size when it is stuffed, so a 40-litre bag packed tightly may measure larger than it did empty. When you compare bags, look at both the litres and the stated height, width and depth.
What size backpack is carry-on size?
There is no single worldwide carry-on size. Many airlines use a cabin limit of 55×40×20 cm, which is about 44 litres of space. The International Air Transport Association (IATA) has a guideline size of 56×45×25 cm, but individual airlines set their own rules.
In the US, many airlines commonly allow carry-on bags of 22×14×9 inches. Budget airlines often have smaller free allowances and charge for a full-size cabin bag, and some also set a weight limit. Always check your airline’s current rules on its own website before you pack, and check each airline separately if your trip has more than one.
Carry-on bag vs personal item
Many airlines allow one larger cabin bag for the overhead locker and one smaller personal item that must fit under the seat in front. The personal-item size is smaller and varies a lot between airlines. If you only pay for a personal item, a small day backpack is usually a safer choice than a large travel pack.
How many litres do you need for each trip length?
The figures below are a practical starting point for travelling light. They assume you pack for mild weather and can wash a few items along the way.
| Trip length | Suggested volume | What it usually holds |
|---|---|---|
| Day trip | 15–25 L | Water bottle, jacket, snacks, laptop or tablet, small essentials |
| Overnight | 25–35 L | One change of clothes, wash kit, charger, daily items |
| Weekend (2–3 nights) | 30–45 L | Two or three outfits, spare shoes if light, toiletries |
| One week, carry-on only | 35–45 L | Capsule wardrobe you re-wear and wash, plus basics |
For a full breakdown of what each size looks like in practice, see our backpack size guide.
What size backpack do you need for a weekend?
For two or three nights, a bag of 30–45 litres is usually enough. A smaller bag in that range works if you wear your bulkiest shoes and jacket while travelling. A bag near 40 litres gives space for an extra pair of shoes or a warmer jumper (sweater).
If you are flying, a weekend bag near the top of the range is often close to the common 55×40×20 cm cabin size. Measure it packed, since an overfilled front pocket can push the depth over the limit.
How many litres do you need for a week?
For a week with carry-on only, 35–45 litres is enough for most people who pack with care. The trick is to plan outfits that mix and match, bring about four or five days of clothes and wash smaller items during the trip.
Going bigger than about 45 litres usually pushes you out of many airlines’ cabin limits. If you need more space, it may be simpler to check a larger bag and carry a small daypack on board.
How to pick the right travel backpack size
- Check your airline first. Find the cabin bag and personal item dimensions and weight limit for every airline you will fly with.
- Match volume to trip length. Use the table above as a starting point and adjust for the climate and your activities.
- Compare external dimensions. Make sure the bag’s height, width and depth fit your airline’s limits, including wheels, handles or straps that stick out.
- Test it packed. Load the bag with your real kit, then measure it and see how it sits on your shoulders.
- Weigh it. Some airlines weigh cabin bags. A light empty bag gives you more allowance for what goes inside.
- Think about access. A bag that opens like a suitcase (clamshell) makes packing and unpacking easier than a top-loading bag.
Is a bigger backpack always better for travel?
No. A bigger bag tempts you to pack more, gets heavier to carry and may not fit in the cabin. Bags that are too large can also be hard to lift into an overhead locker, and on busy flights larger cabin bags are sometimes moved to the hold at the gate.
It often works better to choose the smallest bag that holds what you need, then pack to that size. Packing cubes help keep clothes compressed and sorted, and rolling soft items tends to save a little space.
Features that matter more than size
Once you know the volume you need, a few features make a travel backpack easier to live with. None of them change the size, but they change how the bag works on the road.
- Padded laptop sleeve: useful for work trips and quick security checks.
- Compression straps: pull the bag in when it is not full so it stays within limits.
- Luggage pass-through: a sleeve that slides over a suitcase handle.
- Lockable zips: handy in hostels and on trains.
- Stowable straps: hide the shoulder straps if the bag has to go in the hold.
Travel backpack sizing tips before you fly
Pack a day or two before you go and measure the bag while it is full. Keep liquids in an easy-to-reach pocket if your departure airport asks you to take them out at security. If your trip includes a small regional airline, check its limits separately, as they can be tighter than the main carrier’s.
If you are choosing a new bag, you can compare the backpacks in our shop against the sizes above, and find more packing and bag care advice in the Luxeurs Journal.
FAQs
Is a 40 L backpack carry-on size?
Often, yes. Many airlines’ 55×40×20 cm cabin limit is about 44 litres, so a 40-litre pack can fit when it is not overstuffed. Measure it packed and confirm with your airline.
Is a 30 L backpack big enough for a week?
It can be for a very light packer in warm weather who does laundry mid-trip. Most people find 35–45 litres more comfortable for seven days.
What size backpack fits under an airline seat?
That depends on the airline, since personal-item sizes vary widely. Look up the exact under-seat dimensions for your flight and compare them with the bag’s packed measurements.
How many litres is 22×14×9 inches?
Multiplying those dimensions gives a box of roughly 45 litres. A soft backpack rarely fills a perfect box shape, so its usable volume will usually be a little lower.
Does a backpack’s hip belt count towards the size limit?
Airlines usually measure everything that sticks out, including straps, pockets and handles. Tuck belts and straps away before your bag is checked at the gate.