Backpack size calculator: liters to inches and cm
Turn a backpack's liters into a typical outside size, or a size into liters, and see on how many airlines it fits as a personal item or a carry-on.
Liters to size is an estimate from the makers' own specs of 46 bags. Brands count liters differently, so measure your bag if you're close to a limit.
Why liters don't tell you if a bag fits
Airlines limit the outside size of a bag, not its capacity. Liters are a maker's own measure of what fits inside, and there's no single rule for counting them. In our data, the middle half of travel bags hold between 84% and 100% of their outside box volume, and a few claim more liters than their outside box could hold.
So use liters to get close, then check the real size. Here are the bags we track, with their listed liters next to their outside size:
Want a backpack as your free personal item? Read is a backpack a personal item, or find bags that fit your airlines with the bag finder.
Questions people ask
What size is a 40L backpack?
Usually about 55 x 36 x 23 cm (21.7 x 14.2 x 9.1 in), but it varies by brand. The Osprey Farpoint 40, for example, is 55 x 35 x 23 cm.
Is a 40L backpack carry-on size?
On most airlines, yes. A typical 40 L travel backpack of about 55 x 36 x 23 cm fits the carry-on limit of 29 of the 39 airlines we track. It's too big to be a free personal item almost everywhere.
What size backpack counts as a personal item?
Around 20 L or less. A typical 20 L backpack of about 44 x 28 x 18 cm fits the personal item limit of 7 of the 30 airlines that publish one. Depth is what usually fails, so pack it flat.
How are backpack liters measured?
There's no single standard. Some brands count every pocket, some only the main compartment, and a few measure with the bag overfilled. That's why two 40 L bags can have different outside sizes. Airlines only care about the outside size, so measure the bag itself.