40 x 30 x 20 cm is the size of the personal item (small bag), not a cabin case. Airlines like Ryanair, Vueling and EasyJet let you carry one item of this size for free with any fare, in addition to your cabin case.
A small backpack, a travel bag or a large toiletry bag usually fit within this size. It's not the same as the cabin case, which measures 55 x 40 x 20 cm and comes with an extra cost on the most basic fares.
If you're travelling with hand luggage only, combining both pieces (40x30x20 bag + 55x40x20 cabin case) is the cheapest way to fly without checking a bag in.
Main formats of 40x30x20 underseat luggage
Within this size there are three distinct formats, each suited to a different type of traveller:
- Suitcase-opening backpacks (180°): the most efficient format. They let you pack clothes neatly with compression straps, similar to a hard-shell case, while still taking advantage of the flexibility of fabric corners.
- Pilot-style cases / soft trolleys: fitted with 2 or 4 wheels and a telescopic handle. Although they offer the comfort of rolling, the wheels and handles slightly reduce the usable internal capacity compared with a backpack of the same external size.
- Cabin-style travel bags / duffels: flexible bags with a hand grip and shoulder strap, very lightweight and adaptable if airport checks are strict on volume.
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