The 3-1-1 rule is the US TSA's shorthand for carry-on liquids: every liquid, gel or aerosol must be in a container of 3.4 oz (100 ml) or less, all of those containers must fit in one quart-size clear zip-top bag, and each traveler gets one bag. Other countries run very similar 100 ml rules, but the details differ — always check the airport you're flying from.
The rule itself is simple. What trips people up is the definition of "liquid," the exceptions for medicine and baby items, and the things that aren't liquids at all but still get pulled at the checkpoint. Here's how to breeze through security.
1. What counts as a liquid
Security rules treat far more than drinks as liquids. A good rule of thumb: if you can pour it, spread it, spray it or squeeze it, it counts. That sweeps in a lot of everyday items people don't think of.
- Gels and pastes — toothpaste, hair gel, some sunscreens, peanut butter, soft cheese.
- Aerosols — deodorant spray, dry shampoo, shaving cream.
- Creams and lotions — moisturizer, foundation, liquid makeup.
- Anything spreadable or squeezable — that jar of jam or tub of yogurt in your bag is a liquid to security, even if it's food.
Solid items are generally fine: a bar of soap, a stick deodorant, solid perfume and lipstick don't count as liquids. When you're unsure, ask whether it pours or spreads.
2. The rule at a glance
| The "3-1-1" | What it means |
|---|---|
| 3.4 oz / 100 ml | The maximum container size. It's about the container's capacity, not how full it is — a half-empty 6 oz bottle is not allowed. |
| 1 quart bag | All your liquid containers must fit inside a single quart-size (about 1 liter) clear, zip-top bag that closes. |
| 1 per traveler | One bag per person. Families can't pool everything into one giant bag, though each traveler carries their own. |
At the checkpoint, take the bag out and place it in the bin unless you're in a lane with newer scanners that don't require it — follow the signs and the officer's instructions.
3. Exceptions and special cases
- Medications. Liquid medications are generally allowed in larger, "reasonable" quantities and don't have to fit in the quart bag. Declare them to the officer; carrying a prescription label or doctor's note can help.
- Baby items. Formula, breast milk, and baby food are typically permitted in amounts above 3.4 oz when you're traveling with an infant. Declare them at the start of screening — they may be tested separately.
- Duty-free. Liquids bought after security or on a connecting flight are usually allowed if they're sealed in a tamper-evident bag with the receipt inside. On some itineraries a connection can still be a problem, so keep the sealed bag closed until you reach your final destination and check the rules for any country you transit.
- Powders. Powders aren't liquids, but large amounts (often above a certain threshold) may get extra screening and can be asked to be placed in a bin separately. Keep big powder containers accessible.
4. Simple ways to avoid a repack
- Decant into travel bottles. Small refillable bottles at 3.4 oz / 100 ml or under keep you compliant and save space; just label them.
- Carry an empty water bottle. Empty bottles pass security fine — fill up at a fountain past the checkpoint instead of buying water airside.
- Pack the bag on top. Keep your liquids bag near the top of your carry-on so you can pull it out fast.
- When in doubt, check it. Anything over the limit that you can't leave behind should go in your checked bag.