Liquid Rules on a Flight: What Decides at the Gate

Liquid rules on a flight are set by the airport’s security regulator, not by the airline selling the ticket. That single fact explains most of the confusion at the tray table: two passengers on the same carrier, departing the same day, can face different rules because their airports have different scanners. The airline’s contract of carriage governs the bag; the security regime governs what goes in it.

Where the rule is written

In the European Union, the 100 ml container limit comes from Commission Implementing Regulation (EU) 2015/1998, which lays down detailed measures for the common basic standards on civil aviation security. The United Kingdom applies equivalent requirements through the Department for Transport’s security directions. In the United States, the Transportation Security Administration sets the liquids rule under 49 CFR Part 1540 and publishes it as the 3-1-1 rule: containers of 3.4 ounces (100 ml) or less, in one quart-sized bag, one bag per passenger.

The number is a container limit, not a volume allowance. A 200 ml bottle filled halfway is still a 200 ml container and is not permitted under the rule. That is the part most passengers misread, and it is the part that decides whether the bottle is surrendered at the tray.

The 100 ml rule and the single transparent bag

Under the standard regime, each container must hold no more than 100 ml (3.4 oz). All containers must fit into one transparent, resealable plastic bag. The bag’s dimensions are not fixed by the regulation; airports and carriers publish their own guidance, and the figures differ. The bag is presented separately at the security checkpoint, not packed inside a carry-on.

Medically necessary liquids and baby food are treated as exceptions in most regimes, but the exception is not automatic. It usually requires declaration at the checkpoint and may require documentation. The conditions differ by regulator and by airport, so the figure that applies is the one published by the airport being used, checked on the date of travel.

Airports that no longer apply the 100 ml rule

Some airports have installed computed tomography (CT) scanners that produce a three-dimensional image of the bag’s contents. Where the regulator has approved the equipment, the 100 ml container limit and the single-bag requirement may be lifted. Passengers at those airports can leave liquids in the cabin bag, and the container size is no longer the deciding factor.

This is where the rule now differs by airport rather than by airline. A passenger departing an airport with CT scanners may carry a larger container; a passenger transferring at an airport without them will face the 100 ml limit at the transit security check. The regime that applies is the one at the checkpoint being passed, not the one at the point of origin.

Because scanner rollout is uneven and approvals change, no figure for a specific airport belongs on this page. The airport’s own security page carries the current position, and it should be checked before travel.

Batteries: what goes in the cabin and what does not

Lithium batteries are governed by the International Civil Aviation Organization’s Technical Instructions and, in the United States, by 49 CFR Part 175. They are treated as dangerous goods because a lithium cell can enter thermal runaway, and a fire in the cargo hold is harder to reach than one in the cabin.

The watt-hour figure is the one that decides. It is printed on the battery or the device, and it is calculated from the voltage and the amp-hour rating. Where the rating is not marked, the battery may be refused. The thresholds differ by regime and by carrier, and the figures belong on the per-airline pages where each is dated.

Power banks: the item most often confiscated

A power bank is a spare lithium-ion battery, and it is treated as one. It must travel in the cabin, it must be protected against short circuit, and it must not be placed in checked baggage. The reason is not bureaucratic: a power bank in the hold is out of reach if it fails, and the hold has no suppression system designed for a lithium fire.

The most common confiscation is not a banned item. It is a permitted item in the wrong place: a power bank packed in the checked bag because the passenger did not know it was a battery. The second most common is a power bank with no visible watt-hour marking, which security cannot verify.

Powders: the rule that is not the liquid rule

Powder rules on a flight are separate from the liquid rules, and they are applied by some regulators and not others. In the United States, TSA may require powders larger than 12 ounces (350 ml) to be removed from the bag for separate screening. In Australia, the limit is 350 ml or 350 g for inorganic powders. The European Union has applied restrictions on certain powders at various times.

The condition that changes the answer is the regulator and the date. A powder that is exempt at one airport may be screened separately at another. The figure is not universal, and the airport’s security page is the one that carries it.

What cannot fly at all

The list of what cannot fly is set by the same dangerous goods regime, and it includes items that are prohibited outright in both cabin and hold: explosives, flammables, oxidizers, toxic substances, and radioactive material. Some items are permitted only with carrier approval, and some are permitted only in the cabin or only in the hold.

The airline’s conditions of carriage may be stricter than the regulator’s minimum. A carrier can refuse an item the regulator permits, and it can refuse it without compensation because the refusal is a safety decision, not a breach of contract. The rule that applies is the stricter of the two.

Worked calculation: is this power bank permitted?

The watt-hour rating is not always printed. It can be calculated from the voltage and the amp-hour rating on the label.

Step 1: Read the label. Example: 3.7 V, 20,000 mAh.
Step 2: Convert mAh to Ah. 20,000 mAh / 1,000 = 20 Ah.
Step 3: Multiply voltage by amp-hours. 3.7 V x 20 Ah = 74 Wh.
Step 4: Compare with the threshold. 74 Wh is below 100 Wh, so it is permitted in the cabin under the standard regime.
Step 5: Check the carrier's rule. Some carriers require approval above 100 Wh; some permit up to 160 Wh with approval; above that, carriage is generally prohibited.
Step 6: Check the marking. If the watt-hour rating is not marked on the battery, security may refuse it regardless of the calculated figure.

The same calculation applies to a laptop battery, a camera battery, and a power bank. The figure that decides is the watt-hour rating, and the condition that changes the answer is the threshold published by the carrier and the regulator on the date of travel.

What people get wrong, and why

The first mistake is treating the 100 ml rule as a volume allowance. The rule is a container limit, and a half-empty 200 ml bottle is still a 200 ml container. The mistake is natural because the rule is usually described as a limit on how much liquid can be carried, and the container is the unit that is measured.

The second mistake is assuming the rule is the same at every airport. It is not, because the rule depends on the scanner at the checkpoint, and scanner rollout is uneven. A passenger who has carried a large bottle through one airport may have it refused at another, and the refusal is correct in both cases.

The third mistake is packing a power bank in the checked bag. The item is permitted, but the place is not. The mistake is natural because a power bank looks like any other accessory, and the battery rule is usually explained in terms of what is banned rather than where a permitted item may travel.

The fourth mistake is assuming the airline sets the rule. The airline enforces the regulator’s rule and may add its own, but it does not set the security regime. When a passenger asks the gate agent why the rule is different from the last flight, the honest answer is that the last flight departed a different airport.

Where the figures live

No specific airline’s fee, allowance or dimension belongs on this page, because each figure carries a date and a regime. The per-airline pages carry the checked figures: the cabin bag dimensions, the checked allowance, the battery thresholds the carrier applies, and the date each was verified. The airport’s security page carries the liquid and powder position for that checkpoint. The regulator’s page carries the instrument: Regulation (EU) 2015/1998, the TSA’s 3-1-1 rule, the ICAO Technical Instructions, 49 CFR Part 175.

The rule that matters is the one at the checkpoint being passed, and it is set by the airport’s regulator, not by the carrier.