The TSA Liquids Rule (3-1-1), Explained

The liquids rule is the single most common reason a carry-on gets opened at security, and it is genuinely simple once the three numbers are clear. TSA calls it 3-1-1, and each digit is one constraint.

The rule, in three numbers

Anything in a container larger than 3.4 oz should travel in checked baggage. Every figure here comes from TSA’s own liquids rule page, cited in the sources at the foot of this guide.

The detail that catches people: capacity, not contents

This is the most common and most avoidable mistake. The limit applies to the container’s stated capacity, not how much is in it. A 12 oz bottle of shampoo with an inch left in the bottom is still a 12 oz container, and it will be removed at the checkpoint. Arguing that it’s nearly empty does not work, because the screener is reading the label, not estimating the volume.

The fix is decanting into travel bottles of 3.4 oz or less — and buying a set of those is the cheapest travel purchase there is.

“Liquid” means more than you think

TSA’s rule covers liquids, aerosols, gels, creams and pastes. That sweeps in a lot of things people don’t think of as liquid:

• Toothpaste — a paste, and the most commonly forgotten item.
• Sunscreen — a cream, and often sold in bottles well over 3.4 oz.
• Gel or roll-on deodorant — solid stick deodorant is not a liquid.
• Mascara, lip gloss, foundation, moisturizer — all creams or gels.
• Peanut butter, yoghurt, soft cheese, hummus — spreadable means liquid.

The working test: if it can be poured, spread or sprayed, plan for it to count.

The real trick: carry fewer liquids

The quart bag is a fixed budget, so the best strategy isn’t packing it cleverly — it’s removing items from it entirely. Several everyday toiletries have solid equivalents that don’t count against the bag at all:

• Shampoo and conditioner bars instead of bottles.
• Solid stick deodorant instead of gel or spray.
• Toothpaste tablets instead of a tube.
• Bar soap instead of shower gel.
• Solid sunscreen sticks instead of lotion.

Swapping two or three of these is usually the whole difference between a quart bag that closes and one that doesn’t — and it removes leak risk, which is the other reason liquids are a nuisance in a carry-on.

The second lever: most trips don’t need full-size anything. Hotels supply the basics, and shampoo is buyable in every country you can fly to. Pack for the first two days, not the whole trip.

Exceptions worth knowing

TSA allows formula, breast milk and juicein quantities greater than 3.4 oz in carry-on baggage, and these do not have to fit inside the quart bag. Medications are also handled separately. In both cases, declare the items to an officer at the start of screening — they will be screened separately, which is quicker than having them found in a bag.

Flying internationally

TSA’s rule governs US checkpoints. Most other countries run a closely comparable 100 ml limit with a one-liter clear bag, but details vary — the exact bag size, whether it must be zip-top, and how strictly it’s enforced. Some airports have newer scanners with relaxed rules and others explicitly do not, and this has been changing in both directions.

The safe approach on any international trip: pack to the strictest rule you’ll encounter, and check the rules for your departureairport on the way out and the way back. A kit that clears TSA will almost always clear a 100 ml regime, which is why 3-1-1 is a sensible default even when you’re not flying from the US.

Where the liquids bag goes in your pack

At the top of your carry-on or in an outer pocket — somewhere you can pull it out in one movement without unpacking anything else. That’s the difference between five seconds and a repacked bag at the end of the belt.

A dedicated toiletry bag makes this straightforward, and the good ones are designed around it: a removable clear pouch for the quart-bag items, with the solids and non-liquids kept in the main compartment. That way the compliant part lifts out at the checkpoint and the rest of your kit stays packed. Our packing guide covers where it sits relative to everything else.

The short version

3.4 ounces per container, one quart bag, one bag per person. Judge by the container size printed on the label, not by how full it is. Treat anything pourable, spreadable or sprayable as a liquid. Then cut the list down with solid alternatives, keep the bag where you can reach it, and the checkpoint stops being something you have to think about.