Summary: Airlines set limits in centimeters, not liters. A 20- to 25-liter backpack almost always fits as large carry-on, and 15 to 18 liters usually also fits under the seat as a small personal item. Always measure in centimeters.
Why liters are not included in the rules
A 30-liter backpack can be narrow and tall or wide and flat. The same capacity, but completely different dimensions. An overhead bin has a fixed opening, so the shape matters, not the volume.
That is why all airlines use three measurements in centimeters: length, width, and depth. Backpack manufacturers, on the other hand, use liters because that indicates how much the backpack can hold. Both make sense from their own perspective, and you are caught in between.
The rule of thumb
For a normally shaped backpack—not extremely wide or extremely narrow—it roughly comes down to this:
- 10 to 15 liters: a compact daypack that almost always fits under the seat
- 16 to 20 liters: the most common size, accepted by almost all airlines as either a small personal item or large carry-on
- 21 to 28 liters: usually fits as large carry-on in the overhead bin; check the dimensions
- 30 liters and up: too large for most small carry-on limits, but often suitable as large carry-on if the shape is right
- 40 liters and up: expect to have to check it
What airlines allow
Dimensions vary and change regularly, so always check your airline's website shortly before departure. Broadly speaking, you will see two categories.
Small carry-on luggage that must fit under the seat is around 40 by 20 by 25 centimeters with most budget airlines. That works out to approximately 15 to 18 liters.
Large carry-on luggage in the overhead bin is usually around 55 by 40 by 20 to 25 centimeters. That is roughly 35 to 45 liters of gross volume, although a backpack rarely reaches that capacity because it tapers toward the top.
Keep in mind that with many budget airlines, only the small bag is included and you pay extra for the large one.
Measure your backpack yourself
Do not blindly rely on the manufacturer's specifications, because they often measure without the protruding parts.
- Fill the backpack as you would when taking it with you, rather than measuring it empty.
- Stand it upright against a wall.
- Measure the height from the bottom to the highest point, including the carry handle.
- Measure the width at the widest point, including the side pockets.
- Measure the depth, including the back system and any bulging caused by the contents.
Compare those three figures with the limit. Stay below the limit on all three measurements, because staff measure with a sizer, not a calculator.
Where things go wrong
Depth. This is the measurement most often exceeded with backpacks. A fully packed backpack bulges, turning 20 centimeters into 28. It will not fit in the sizer then.
Protruding parts. Hip belts, compression straps, and a bottle holder count too. Tightening or tucking away the straps can save a few centimeters.
Weight. In addition to size, there is often a weight limit, usually between seven and ten kilos. A leather backpack already weighs more than a comparable nylon model, and that reduces the weight available for your belongings. See Leather bag as carry-on.
Advantages of a leather backpack when traveling
A leather backpack has one practical advantage as carry-on: it holds its shape. A soft nylon backpack expands when you overfill it and then gets wedged in the sizer. Sturdy full-grain leather gives less, keeping the dimensions more predictable.
The disadvantage is weight. Expect it to weigh half a kilo to a full kilo more than a comparable nylon backpack. That is noticeable with a seven-kilo limit.
Practical advice
If you fly regularly with different airlines, choose a backpack of around 18 to 20 liters with dimensions that stay below the strictest limit you encounter. Then you will never have to check again.
If you fly occasionally and know which airline you are using, you can choose a larger one. Just check that same airline's dimensions for the return flight, because they can differ with a codeshare.
Learn more about choosing a backpack in Buying a leather backpack and Which backpack suits you best?.
