Carry-On Size and Liquid Rules, Explained

Why there is no single carry-on size

Cabin bag dimensions are set by each airline individually, not by any regulator. Industry bodies have proposed voluntary standards more than once and none has been widely adopted, because the allowance is a commercial decision rather than a safety one.

That produces a spread wide enough to matter. A bag that has travelled comfortably for years on full-service carriers can be refused by a low-cost airline on a short European hop, and the difference is often only a couple of centimetres in one dimension.

The practical consequence is that the phrase 'carry-on sized' means nothing on its own. A bag is carry-on sized for a specific airline, and only that airline's published dimensions settle it.

The personal item is the more important allowance

Most airlines distinguish between a full-size cabin bag for the overhead locker and a smaller personal item that fits under the seat in front. On ultra low-cost carriers, the personal item is frequently the only thing included in the fare, with the overhead bag charged separately.

The personal item also matters on flights where you have a bag but board in a late group. If the overhead space runs out, your cabin bag goes into the hold at the gate — and the personal item is what stays with you. Anything you cannot be separated from belongs there, not in the roller bag.

That means medication, travel documents, keys, chargers, a change of clothes on a long trip and anything valuable or fragile. This is worth doing as a habit rather than a reaction, because the gate is a bad place to be repacking.

Liquids: the rule and the uneven exceptions

The familiar restriction limits containers to 100ml, carried in a single transparent bag. It has been in place across most of the world for many years and is the assumption to travel on.

Newer scanning technology at some airports removes the need for it, and where installed, the liquid limit and the requirement to remove electronics may be relaxed. The complication is that rollout is uneven — not just between countries but between terminals at the same airport, and sometimes between checkpoints in the same terminal.

Rules have also been reinstated at airports that had previously relaxed them. The only safe approach is to pack to the stricter standard, because arriving prepared for the old rule costs you nothing, while arriving prepared only for the new one can cost you the contents of your bag.

What counts as a liquid is broader than people expect

Security treats a range of things as liquids that most travellers do not, which is where confiscations usually come from:

• Creams, gels, pastes and waxes, including toothpaste, sunscreen and many cosmetics.
• Aerosols of any kind, including dry shampoo and deodorant sprays.
• Soft cheeses, yoghurt, hummus, jam and peanut butter, all of which have caused arguments at checkpoints.
• Snow globes, which are the classic example, and which cannot be carried through at all in most cases.

Practical ways to avoid the problem

Most cabin baggage trouble is avoidable with a few decisions made before you leave home:

• Measure your bag with its wheels and handles included, because airlines do. The published dimensions are external, not internal.
• Check the dimensions for the specific airline on the specific booking, and check again if a leg is operated by a partner with different rules.
• Buy the bag allowance when you book rather than at the airport. On carriers that charge for it, the price rises at every stage and is highest at the gate.
• Switch to solid alternatives where they exist — bar shampoo, solid sunscreen, toothpaste tablets — which sidestep the liquid rule entirely.

If your bag is challenged at the gate

Gate agents measure against a sizer, and the sizer is the arbiter regardless of what has been accepted previously. Arguing that the bag has flown before is accurate and irrelevant.

If it does not fit, the bag goes into the hold and a fee usually applies. At that point the useful thing is not to negotiate but to move anything essential into your personal item before handing it over, which takes thirty seconds and prevents the genuinely bad outcome of being separated from your medication or passport.