Liquids rule basics: 100 ml, LAGs and duty-free

The "100 ml rule" is the most-detested, most-misunderstood rule in air travel. It is set by national aviation authorities, not airlines — which is why it's nearly identical at Hong Kong, Singapore, London and New York, and why your airline can't waive it. Here is the rule as the official airport and security pages actually state it.

What counts as a liquid

Security agencies group everything that flows, sprays or smears as LAGs. The TSA's phrase is "liquids, aerosols, gels, creams and pastes"; HKIA and Changi say "liquids, aerosols and gels". In practice:

• Obviously: water, juice, perfume, shampoo, lotion, oil
• Often forgotten: toothpaste, deodorant (aerosol or roll-on), mascara, liquid foundation, sunscreen, hand sanitiser, contact-lens solution, honey, yoghurt, soup
• Borders on absurd but real: snow globes, jam, peanut butter spread

Solid cosmetics (lipstick in a stick, powder, solid deodorant, solid shampoo bars) are not LAGs — they fly without the bag.

The 100 ml detail people get wrong

The limit is on the container size, not the amount of liquid inside. A 200 ml bottle that is half-empty fails; Changi's page states this explicitly: containers exceeding 100 ml "will not be accepted, even if partially filled". Ten 100 ml bottles, by contrast, are each fine — as long as they all fit in the one bag.

The one-bag rule

HKIA: containers go in "a transparent re-sealable plastic bag with a capacity of 1 litre or less… Only one transparent plastic bag per passenger is permitted." Changi adds that the bag "must be completely closed" and "presented separately for examination". Roughly 20 × 20 cm is the practical size. Non-compliant LAGs are disposed of at screening — both airports state this plainly.

Exemptions that always apply

All three authorities carve out the same categories, with conditions:

• Medications — needed in quantities above 100 ml; Changi asks for a bona fide doctor's letter for some items; TSA requires declaring them to officers.
• Baby milk, food and breast milk — Changi requires the infant to be travelling with you; TSA calls this "infant and child nourishments".
• Special dietary requirements — HKIA lists these, subject to verification.

Keep exempt items accessible and declare them — they still get screened, usually with a quick extra test.

Duty-free and STEBs: the exception with a bag of its own

Liquids bought after security (duty-free) are allowed in larger quantities if they are sealed in a Security Tamper-Evident Bag (STEB) with the receipt visible or enclosed. The catch is connections: a STEB from Singapore is accepted at Hong Kong or London only if you don't open it, and rules at your final or connecting airport govern. TSA's version for flights to the US: duty-free liquids over 100 ml are allowed in carry-on for connecting itineraries if the STEB shows no signs of tampering and the original receipt is present. If you have a long transit or a red-eye where the transfer security re-screens you, the STEB — unopened, receipt attached — is what saves the bottle.

Power banks and lithium batteries: not LAGs, but the other bin-killer

The rule most travellers still miss in 2026: spare lithium batteries and power banks must travel in the cabin, never in checked baggage. HKIA limits passengers to two power banks in hand baggage and prohibits recharging them in flight; Changi has published additional power-bank restrictions for flights departing Singapore effective 15 April 2026. If your carry-on gets gate-checked (see our gate-guide), take the power bank out before the bag goes down the belt.

Practical packing list

1. One clear 1-litre zip bag, packed at home, in an outer pocket of your carry-on.
2. All toiletries in ≤ 100 ml containers — or solids, or checked baggage.
3. Medications and baby food outside the bag, ready to declare.
4. Duty-free purchases kept sealed in the STEB with the receipt until your last airport.
5. Power banks and spare batteries in the cabin bag, not checked, not in use.

Related guides

• What happens if your carry-on is too big at the gate
• Cabin baggage size & weight: 20 airlines compared