The 3-1-1 rule explained
The TSA 3-1-1 rule allows liquids, gels, aerosols, creams, and pastes in containers of 3.4 oz, 100 ml, or less, all fitting inside one quart-size clear bag, one bag per passenger. Container size is what counts, not how full it is.
The name breaks down into the three numbers that make the rule easy to remember and easy to get wrong. Each individual container has to hold 3.4 oz or 100 ml or less, printed on the label. All of those containers have to fit inside one quart-size, clear, resealable bag. And each passenger gets exactly one of those bags, not one per checked item or one per family member sharing a bag.
Key takeaways
- 3.4 oz / 100 ml is the container’s printed capacity, not how much liquid remains inside it.
- One quart-size bag holds the entire allowance. Extra bags do not multiply the limit.
- The bag goes through screening separately in most standard lanes, though TSA PreCheck often changes this step.
The container-size trap is where most flagged bags come from. A nearly empty 6 oz shampoo bottle is still a 6 oz container, and it still gets pulled and either surrendered or sent to checked luggage. TSA screeners read the label, not the fill line. Buying genuinely travel-size containers, or decanting into ones you already own, is the only way around it.
What actually counts as a liquid (the surprising items)
The 3-1-1 rule covers more than water bottles and shampoo. TSA’s working definition includes anything that can spill, spray, spread, pump, or pour, and that catch-all sweeps in a long list of foods and personal-care items travelers do not think to plan around.
Counts as a liquid
- Peanut butter: spreadable, takes the shape of its container.
- Yogurt and hummus: same spreadable-substance logic applies.
- Jam, pate, and aloe vera gel: all classified as gels or pastes.
- Aerosol deodorant and spray sunscreen: aerosols fall under the rule.
- Contact lens solution: a liquid regardless of medical use.
Not restricted the same way
- Solid stick deodorant: not spreadable or pourable, no limit.
- Solid bar soap and solid perfume: exempt from the 3-1-1 bag.
- Dry snacks and solid food: no liquid, no restriction.
- Powders under 12 oz / 350 ml: allowed without extra screening.
A useful shortcut when a container is not obviously a liquid: run the spill, spray, spread, pump, or pour test. If it fails any part of that test, treat it as a liquid and size it accordingly. Powders get their own separate threshold. Anything over 12 oz / 350 ml is allowed but triggers additional screening at the checkpoint, so a large bag of coffee or protein powder is legal but will add time to the line.
TSA PreCheck and liquids
PreCheck is one of the most common sources of confusion at the checkpoint, because it changes the screening process without changing the rule. Enrolled travelers in most PreCheck lanes can leave the quart bag and laptop inside their carry-on rather than pulling them out for a separate bin. The 3.4 oz per container and one-quart-bag limit still apply in full. Nothing about membership raises the size cap.
The exception worth knowing: a growing number of checkpoints, PreCheck and standard alike, now run CT scanners that change what has to come out of the bag, covered next.
The new CT scanner exemptions
Computed tomography scanners, sometimes called CT or C3 lanes, build a 3D image of a bag’s contents instead of the flat 2D image older X-ray machines produce. That extra detail lets screeners see liquids more clearly without pulling the bag out, and at some equipped lanes it allows select larger containers through as well. TSA had installed these scanners at 255 US airports as of July 2025.
The catch: this is not a nationwide policy change. Coverage depends entirely on which specific lane at which specific airport has the equipment, and even at an equipped airport, older lanes without CT scanners still run the standard 3.4 oz rule. Treat any relaxed limit as a bonus at a specific checkpoint, never as something to plan a bag around before checking your departure airport’s setup.
International rules: the EU 100ml patchwork
Outside the US, the baseline is the European Commission’s 100ml rule: each container 100ml or less, all fitting in one transparent, resealable bag, at most airports across Europe. It runs on the same logic as the US 3-1-1 rule and travelers moving between the two systems rarely notice a practical difference, since 100ml and 3.4 oz land at nearly the same real-world size.
Where it gets inconsistent is the same CT-scanner rollout happening on the US side. A growing but uneven list of European airports, including select lanes at Frankfurt and Munich, now allow containers up to 2 liters through CT-equipped checkpoints, as of 2026. Airports like Dublin and Rome Fiumicino are part of the broader rollout conversation but coverage varies lane by lane, same as in the US.
Check both ends of the trip. A relaxed limit at your departure airport does not carry over to a connection or a return flight through a different airport. Confirm the rule at departure and at arrival before packing anything above the standard 100ml / 3.4 oz threshold.
Duty-free liquid exceptions
Duty-free liquids over the standard limit are allowed, but only under specific packaging conditions. The bottle has to be sealed in a transparent, tamper-evident bag, known as a STEB, with the receipt visible through the plastic, and the seal has to stay unbroken. The purchase has to fall within 48 hours of the flight to qualify.
Connecting flights break this exception. A duty-free bottle can fail re-screening at a connecting airport, especially on international itineraries through Europe, if the bag is opened, the seal is disturbed, or the connecting country’s screeners do not recognize the original STEB. TSA recommends packing duty-free liquids in checked luggage if the itinerary includes a connection, rather than risking confiscation partway through the trip.
Common mistakes people make
- Forgetting a water bottle in a bag pocket. Any liquid left in a bottle, even a few ounces, gets flagged. Empty it before the checkpoint and refill past security.
- Assuming a half-empty container is fine. Screeners size the container, not the contents, as covered above.
- Not realizing food spreads count. Peanut butter, hummus, and similar items are frequently confiscated because travelers do not think of them as liquids.
- Gate-checking a bag with the quart bag still inside. If a carry-on gets gate-checked, its liquids typically need to be removed and carried separately before the bag goes into the cargo hold, or the whole bag has to be reorganized on the spot.
How to pack your quart bag efficiently
Buy genuine travel-size containers rather than guessing
Containers printed at 3.4 oz / 100 ml or under, or a reusable set under that size, remove the ambiguity a “probably small enough” bottle creates at the checkpoint.
Decant instead of buying duplicate minis
Refillable silicone or plastic travel bottles cut cost on trips longer than a few days, since full-size product is usually cheaper per ounce than pre-packaged minis.
Swap to solids where the product exists
Solid stick deodorant, bar soap, solid perfume, and solid sunscreen sticks skip the liquid bag entirely and free up room for the items that have no solid equivalent.
Lay the bag flat in an outer pocket
Keeping the bag accessible, rather than buried under clothing, speeds up the standard screening lanes where the bag still needs to come out separately.
Planning what goes in a carry-on gets easier once flight costs and timing are locked in first, and the cheap-flights toolkit covers that earlier step.
Book the flight before you pack the bag
A practical toolkit for finding lower fares and timing a booking, so the packing decisions come after the cheapest ticket is locked in.
Liquid medication follows a different set of rules entirely, with its own exceptions for larger quantities and no requirement to fit inside the quart bag. See our separate guide on medication rules for that specific case rather than treating it the same as toiletries.
FAQ
01Does TSA PreCheck let you skip the liquid rules?
No. PreCheck only lets most travelers leave their quart bag and laptop in their carry-on during screening. The 3.4 oz per container and one quart-bag limit still apply.
02Is peanut butter considered a liquid by TSA?
Yes. TSA classifies it as a liquid because it takes the shape of its container. It must be 3.4 oz or less in your carry-on, or packed in checked luggage without limit.
03Can I bring duty-free liquids over 100ml on a connecting flight?
Only if it is sealed in a tamper-evident bag with the receipt visible and the seal is not broken. On international connections, especially through Europe, TSA recommends packing it in checked luggage instead.
04Do the new CT scanners mean I can bring bigger liquids now?
At some CT-scanner-equipped checkpoints, yes, for select items. But this is not nationwide yet in the US or standard across the EU, so check your specific departure and arrival airport before packing.
05Is the European liquid limit the same as the US 3-1-1 rule?
The core amount is similar, 100ml versus 3.4oz, but the EU’s default is still a flat 100ml per container with no CT-scanner exemption at most airports, while the US uses the 3-1-1 quart-bag system with a growing list of scanner-based exceptions.
06What happens if my liquid container is too big but not full?
It does not matter how much is inside. TSA and EU screeners go by the container’s printed size, so a half-empty 6 oz bottle is still not allowed in carry-on.
Quart bag: David Lisbona / Wikimedia Commons (CC BY 2.0). Security tray: Mattes / Wikimedia Commons (CC BY 2.0 DE). Duty-free bag: Munich Airport / Wikimedia Commons (Public domain).
