Hand luggage rules trip up experienced travellers as often as first-timers, mostly because the two big constraints — the 100ml liquids limit and the cabin bag size allowance — are set differently by different authorities and airlines, and a scanner upgrade under way at some airports is starting to change the liquids rule in ways that aren't yet consistent everywhere.
The 100ml liquids rule, and why it exists
Most of the world's airports still apply the same basic rule: liquids, gels, pastes and aerosols in hand luggage must be in containers of 100ml or less, all packed together in a single clear resealable bag of about a litre in size (commonly a 20x20cm bag). This applies whether the container is full or not — a half-used 150ml bottle is not allowed even though it currently holds under 100ml, because the rule is based on the container's printed capacity, not its contents. The rule dates back to a 2006 transatlantic liquid-explosive plot and has remained largely unchanged at most airports since, despite periodic predictions that it would be scrapped.
The new CT scanners: what's actually changing
A number of airports — more in some countries than others — have begun rolling out newer CT (computed tomography) scanners at security, the same 3D-imaging technology used in medical scanning, which can see the contents of a bag in far more detail than older 2D X-ray machines. At airports where these are fully installed and enabled for passenger use, some travellers can leave liquids (and in many cases laptops) inside their bag rather than removing them, and larger liquid containers are sometimes permitted.
The catch is that this rollout is uneven and still changing: some airports have the scanners installed but not yet running in the new mode, some have upgraded only some lanes, and the rule can vary literally by which specific security lane you're sent to on the day. Airport and airline guidance both tend to say "check specific signage on arrival at the airport" rather than promising a blanket change, and that remains the honest, current answer — treat the classic 100ml/one-bag rule as your safe default and treat any relaxation as a pleasant surprise rather than something to plan around.
Cabin bag size limits: there is no single global standard
Unlike the liquids rule, which is broadly consistent worldwide, cabin bag size and weight limits are set individually by each airline, and vary more than most travellers expect — a bag that fits comfortably within one carrier's allowance can be too large for a budget airline's stricter limit on the same route. Full-service and legacy carriers tend to allow a somewhat larger main cabin bag, often with a separate small personal item as well, while low-cost carriers frequently enforce tighter dimensions and may charge a fee at the gate for a bag that doesn't fit their sizer, even if it would have been fine on a different airline.
Always check the specific limit for the airline and even the fare class you've booked before you fly, rather than relying on a bag that has "always worked before" — a size that cleared easily on one airline can be turned back at the gate on another, sometimes at a cost of paying to check it there and then at an inflated last-minute rate.
Electronics and other restricted items
Laptops and larger electronics have traditionally needed to come out of the bag and go through security separately in their own tray, though this too is starting to change at airports with the newer CT scanners installed and enabled. Spare lithium batteries and power banks generally must travel in hand luggage rather than checked baggage, since a rare battery fire is far easier to detect and manage in the cabin than in the hold — check your airline's specific watt-hour limit if you're carrying a large power bank or camera battery, as very high-capacity ones can require prior approval or be refused entirely.
Duty-free liquids bought after security
Liquids bought airside after the security check, including duty-free alcohol and perfume, are generally exempt from the 100ml rule regardless of container size, provided they're sealed in a tamper-evident bag with the receipt visible and you don't open it before a connecting security check. This last point matters for anyone connecting through a second airport: if you have to clear security again at a connection, an oversized duty-free liquid bought on the first leg may be confiscated at the second checkpoint unless the connecting airport specifically recognises the tamper-evident bag scheme, which not all do. See our guide to connecting flights and checked baggage for more on what does and doesn't carry through smoothly at a connection, and if you booked two separate tickets rather than one through itinerary, our guide to self-transfer flights explained covers the wider risks of that approach.
Medicines, baby food and other exceptions
Medically necessary liquids, baby food and formula for accompanying infants are generally allowed above the 100ml limit, but usually need to be declared to security staff separately from your main liquids bag, and you may be asked to demonstrate or explain the item. Carrying a prescription or a doctor's letter for medical liquids or medication is a sensible precaution, particularly for injectable medicines or anything unusual-looking on a scan.
What to check before you fly
- Your specific airline's cabin bag size and weight allowance for the fare you've booked — not a generic figure from memory.
- Whether your departure and any connecting airport have upgraded security scanners, and what that means for liquids and laptops on the day.
- Any watt-hour limits on spare batteries or power banks you're carrying in hand luggage.
- Whether medically necessary liquids or baby food need declaring separately at your specific airport.
Packing smart matters even more on a long-haul sector — see our guide to how to survive a long-haul flight for what's actually worth carrying in your cabin bag beyond the security basics.
If you're facing a tight connection between two airports with different security setups, our guide to minimum connection times covers how much buffer is sensible, and if a long layover means an overnight wait, see sleeping in airports for the practicalities.
Frequently asked questions
How big can liquids be in hand luggage?
Generally 100ml or less per container, all packed together in a single clear resealable bag of about a litre, at most airports worldwide — this is based on the container's printed capacity, not how much is left inside it.
Do the new CT scanners mean I can bring bigger liquids now?
At some airports with the newer scanners fully installed and enabled, yes — but the rollout is uneven and can vary by security lane, so check specific signage on the day rather than assuming, and pack under 100ml as a safe default.
Is the cabin bag size limit the same for every airline?
No — cabin bag size and weight limits are set individually by each airline and vary noticeably, particularly between full-service and low-cost carriers. Always check your specific airline's current limit.
Can I bring duty-free liquids through a connecting flight?
Usually yes if they're sealed in a tamper-evident bag with the receipt visible, but if you have to clear security again at your connecting airport, check that airport recognises the scheme — otherwise the liquid may be confiscated.
Sources and further reading:
- Security screening rules: TSA (United States) and UK Government guidance on hand luggage restrictions.
- IATA for general background on cabin baggage standards and airline practice.
- Individual airline websites for current cabin bag size and weight allowances.