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. The federal rule covering scheduled US airlines is explicit about this: 14 CFR § 121.589 requires a Part 121 certificate holder to have an approved carry-on baggage program and forbids boarding a bag that exceeds it, but the allowance itself is whatever that carrier has written into its own operations specifications. The regulator mandates that a limit exists and is enforced. It does not set the number. Part 121 covers most scheduled US-carrier passenger flights, mainline and regional alike — but air taxis and small commuter operators fly under different parts, and foreign carriers flying into the United States operate under different rules again, so this is not a universal requirement even for flights touching US airports.

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. Most large US carriers converged on roughly 22 x 14 x 9 inches (56 x 36 x 23 cm) including wheels and handles, and that figure is worth knowing as the common case — but it is a convention, not a rule, and low-cost carriers on both sides of the Atlantic permit less. A bag that has travelled comfortably for years on full-service carriers can be refused 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.

Source: 14 CFR § 121.589 — Carry-on baggage

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 two-place split is not purely commercial. For Part 121 carriers the same section lists where a bag may legally sit for takeoff and landing: in an approved stowage compartment, in an overhead rack fitted with restraining devices or doors, or under a passenger seat. A bag that fits none of those has nowhere to be. What the regulation does not do is put a size on any of them — the under-seat figure, like the overhead one, is the airline's own, and how strictly each gets checked is a matter of carrier policy and the individual gate rather than of federal law.

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.

Source: 14 CFR § 121.589(c)–(d) — permitted stowage

Liquids: the rule and the uneven exceptions

The familiar restriction limits each container to 3.4 ounces (100 ml), with all of them fitting inside one transparent resealable bag of about a quart (roughly a litre), and one such bag per passenger. In the United States this is known as 3-1-1. The two volumes are one limit rather than two, because 3.4 ounces is simply the conversion of 100 ml. The same 100 ml figure is what the EU applies, which is why packing to it usually travels — but each jurisdiction sets its own limit, and as the next paragraph shows they do change, so it is worth confirming rather than assuming.

It is worth knowing where the rule lives, because it is not written into the regulations the way people assume. TSA publishes the limits as screening procedure rather than codifying them, and 49 CFR § 1540.107 supplies the force: nobody may pass into the secure area or board without submitting to inspection 'in accordance with the procedures being applied' at that checkpoint. The consequence is practical rather than academic — the procedures can be revised without any change to the regulation, so the current published guidance is the only reliable version.

Newer computed-tomography scanners are often described as ending the liquid rule, and in the United States they have not. What they change is handling: in a CT lane you may be able to leave the bag inside your carry-on instead of taking it out, and often keep laptops in place too. The 3.4-ounce container limit is unaffected. Treating a scanner upgrade as permission to pack a full-size bottle is a costly mistake.

Limits themselves have moved in Europe, and recently. Airports that had installed cabin-baggage explosive detection systems were permitting containers above 100 ml until the European Commission reverted the maximum to 100 ml at those airports with effect from 1 September 2024, describing it as a precautionary measure addressing a technical issue rather than a response to any new threat. The Commission called the restriction temporary and said it was working on technical solutions, but set no date in the notice for lifting it, so treat the 100 ml limit as being in force unless a particular airport says otherwise. Note also what the reversion did not touch: airports that had never deployed the equipment, or had kept the 100 ml limit anyway, were unaffected. The limit therefore varied airport by airport rather than country by country, which is why what you can carry is decided by each airport where you pass security rather than by your destination — and on an itinerary that puts you through a second screening at a connection, that is more than one airport. So pack to the stricter standard: 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.

Two exemptions are worth carrying in your head because they are generous and widely missed. Medications in liquid form are permitted in reasonable quantities above the limit, as are infant and child nourishments including formula and breast milk. Declare them at the checkpoint and expect additional screening rather than refusal. Duty-free liquids bought abroad may also exceed the limit on a connection through a US airport, but every condition has to hold at once: the retailer's sealed tamper-evident bag, the original receipt, a purchase made within the previous 48 hours, and no sign the bag has been opened. Even then the contents still have to clear screening, and anything that alarms or cannot be resolved will not be allowed through.

Sources: TSA — Liquids, Aerosols, and Gels Rule; 49 CFR § 1540.107 — Submission to screening and inspection; European Commission — temporary restrictions on liquid screening, 1 September 2024

What counts as a liquid is broader than people expect

The rule covers liquids, aerosols, gels, creams and pastes, which is a wider category than most travellers picture and 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 are allowed rather than banned: TSA permits one that appears to hold under 3.4 oz of liquid — roughly tennis-ball size — provided the entire globe including its base fits inside your quart bag. Anything larger goes in the hold.
• Powders are not liquids and have no 3-1-1 limit, but a container over 12 oz (350 ml) has to come out into a separate bin for X-ray and may be opened. TSA states plainly that this changes nothing about what you may bring through a US checkpoint. The real restriction applies to flights into the US from an international last point of departure, where a powder over that size which cannot be resolved at the checkpoint is refused the cabin and disposed of — which is the argument for putting protein powder in the hold.

Sources: TSA — Liquids, Aerosols, and Gels Rule; TSA — Snow Globes; TSA — Protein or Energy Powders; TSA — policy on powders

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. On Part 121 flights the crew also has explicit regulatory backing here: § 121.589 requires passengers to comply with crew instructions about carry-on stowage, so a refusal at the door is not a negotiating position.

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.

Source: 14 CFR § 121.589(e) — compliance with crew instructions