Carry-on liquids: the 3-1-1 rule, explained properly

The TSA 3-1-1 rule: liquids in your carry-on must be in containers of 3.4 ounces (100 ml) or less, all of which must fit inside a single quart-sized clear resealable bag, one such bag per passenger. The bag comes out of your luggage and goes in a tray at screening.

That is the whole rule, and the numbers are worth understanding precisely. The 3is the container size, not the amount of liquid inside it — a half-empty 6 oz bottle is not compliant, because screeners judge the container's stated capacity, not its contents. The 1 is the one-quart bag. The second 1 is one bag per passenger. Children each get their own allowance.

What counts as a liquid

Far more than most travelers expect. The screening category is "liquids, gels, aerosols, creams and pastes", which sweeps in a lot of things that do not pour:

Screening decisions are made by officers at the checkpoint and can vary. The TSA's searchable "What Can I Bring?" tool is the authority for a specific item; the rules also change, so confirm before you fly.

Peanut butter is the classic surprise, and so is anything "spreadable". If you could spread it or spray it, assume it is a liquid.

What is exempt from 3-1-1

Three categories are allowed in reasonable quantities above 3.4 oz and do not need to fit in the quart bag — but you must declare them to the officer at the start of screening, and they may be tested separately:

• Medications. Prescription and over-the-counter, liquid or gel. Keeping them in labeled original packaging makes the conversation shorter, though the TSA does not require it.
• Baby formula, breast milk and baby food, including the ice packs or gel packs needed to keep them cool. The child does not need to be present.
• Duty-free liquids purchased after security, in a sealed tamper-evident bag with the receipt inside — with important caveats on connecting flights, below.

Declaring these takes ten seconds and avoids the bag search that follows finding them undeclared. Do it at the front of the line, not after the X-ray.

The duty-free connecting-flight trap

This catches experienced travelers regularly. A liter of spirits bought airside is fine on the flight you bought it for. But if you clear security again at a connecting airport — which you almost always do when connecting through another country, and sometimes when re-entering the US — that bottle goes back through screening as an ordinary liquid over 100 ml, and it will be taken.

The sealed tamper-evident bag with the receipt inside is what usually saves it, and only when the receipt is recent and the seal is intact. If you are connecting and you care about the bottle, the safe move is to put it in a checked bag on the first leg, or buy it on the final leg instead.

International rules are similar but not identical

Most of the world runs a comparable 100 ml / one-liter-bag standard, so a US-compliant quart bag is generally accepted. The differences that matter:

• Some airports have installed newer CT scanners and relaxed the rule; others in the same country have not. Never assume based on a previous trip.
• Bag size is specified in liters rather than quarts in most of the world — a quart is slightly smaller than a liter, so a US quart bag is comfortably within a one-liter limit.
• Enforcement of the "must be transparent and resealable" requirement is stricter in some countries than in the US.

Our international carry-on size guide covers the size and weight side of flying outside the US, which trips people up more often than the liquids do.

The practical packing system

Keep a clear quart pouch permanently packed inside your toiletry bag holding only the flight-legal liquids. Pull the pouch at the checkpoint, put it back afterwards, and never repack from scratch. Everything larger travels in your checked bag or gets bought at the destination.

Two habits reduce leaks, which cabin pressure changes make more likely: leave a little headroom in each bottle rather than filling it, and pack bottles upright in elasticated slots rather than loose. Our toiletry bag roundup ranks bags on exactly that, and our carry-on packing guide puts it in the context of the whole bag.