Best Personal Item Backpack for Budget Airlines
The best personal item backpack is the one shaped to the allowance you actually fly. European budget carriers cluster at 40 by 30 by 20 centimetres; large US carriers mostly publish 18 by 14 by 8 inches. Those two boxes are different shapes, and one bag rarely clears both.
Two standards, not one
Almost every article on personal-item backpacks quotes 18 by 14 by 8 inches as though it were universal. It is a US convention. In Europe the common free under-seat figure is 40 by 30 by 20 centimetres, which is 15.7 by 11.8 by 7.9 inches — more than two inches shorter and two inches narrower. A bag designed to the American box is over the European limit before anything goes into it.
The exceptions run both ways. easyJet publishes 45 by 36 by 20 centimetres, the most generous in European budget aviation. United publishes 17 by 10 by 9 inches, four inches narrower than its US peers, which disqualifies most bags sold as personal-item backpacks. Both are set out per carrier in the airline allowances section.
What to look for, in order
Length first. It is the dimension that most often fails, and the one that cannot be packed around. Check the longest published side against the longest side of the allowance before anything else.
Then depth. A soft bag packed full goes round in the middle, and a sizer is rigid. Anything that removes volume — compression, or simply not filling it — comes out of the depth.
Then the laptop layer. A padded compartment against the back panel keeps the heaviest flat object near the spine and away from the compression, if the bag has any.
Weight last. Under-seat allowances are usually measured, not weighed. Weight matters on European carriers that publish a limit for the larger cabin bag.
Which model suits which allowance
The 18 by 14 by 8 model is built to the US convention and shaped to it: 47 centimetres on its longest side, which is 18.5 inches. Soft-sided bags have the advantage here that a shell does not — packed a little below capacity, the sides stay flat and it goes into the box. The 40-litre model measures 40 by 30 by 20 centimetres, exactly the common European under-seat figure it was designed around. United is the one carrier to check separately: its 17 by 10 by 9 is narrower than the standard these bags are built to, so there they travel as cabin bags.
Questions
What size is a personal item backpack?
There is no single size. European budget carriers commonly publish 40 by 30 by 20 centimetres; large US carriers commonly publish 18 by 14 by 8 inches. Check your own carrier before buying.
Does a backpack count as a personal item?
Yes on every carrier covered here, provided it measures within the published figure once packed and slides fully under the seat.
Which dimension fails most often?
Length on paper, depth in practice. A bag can measure correctly flat and still fail a rigid sizer once it is packed round.
Is a 40-litre backpack a personal item?
Usually not. Forty litres is cabin-bag volume. It only qualifies where the under-seat box happens to match the bag's own dimensions.