The 3-1-1 Rule, Decoded
Every confused traveler at the security line is confused about the same three numbers. 3: each container may hold no more than 3.4 ounces (100 milliliters). 1: all containers must fit inside a single 1-quart (roughly 1-liter) clear, resealable bag. 1: one such bag per passenger. That is the entire TSA liquids rule, and it applies to liquids, aerosols, gels, creams and pastes in carry-on baggage — the same framework, with small local variations, is used by airport security worldwide.
The rule exists for a specific historical reason: the 2006 transatlantic liquid-explosives plot, after which regulators concluded that small quantities of liquids could not practically be screened with the technology of the time. The limits are therefore about container capability, not about your actual shampoo being dangerous. Understanding that distinction explains the rule's most misunderstood feature: the limit applies to the container's labeled capacity, not how much is left inside it.
The Container-Size Trap
The single most confiscated item at checkpoints is the half-empty large bottle. A 200 ml shampoo bottle containing only 50 ml of shampoo will still be taken, because officers check the container's rated capacity, not its contents. “But it's almost empty” is the most uttered — and most useless — sentence at airport security. If the label says more than 100 ml / 3.4 oz, it goes in checked baggage or stays home.
The fix is decanting: refillable travel bottles (silicone squeeze tubes are the frequent flyers' favorite) let you carry exactly what you need in compliant containers. Buy the 100 ml empties once; refill from your full-size products forever. It is cheaper than buying travel sizes repeatedly, and you never discover mid-trip that the hotel-shop “travel size” was 150 ml.
How Many Bottles Fit in a Quart Bag?
The TSA sets no fixed bottle count — the constraint is physical: everything must fit “comfortably” in one clear quart-size bag (about 7 × 8 inches), and the bag must close. In practice, travelers fit roughly 7–10 standard travel containers, depending on shape. Flat pouches pack tighter than round bottles; tall pump bottles waste the bag's corners.
The visual bag on this page models that reality: each container you add takes a slot, and the fill meter warns you as the bag approaches capacity. Nine containers is the practical planning limit — beyond that, you are betting the bag still zips, and an overstuffed bag that will not close is treated the same as an oversized one. The quart bag itself must be transparent and resealable — a zip-top freezer bag is the canonical choice; dedicated TSA-approved toiletry pouches work too.
What Counts as a Liquid? (More Than You Think)
The rule's reach surprises people because “liquid” at the checkpoint means anything that can be sprayed, squeezed, pumped, spread, smeared or poured. Toothpaste counts (it is a paste). Mascara counts (gel-based). Lip gloss counts; solid lipstick does not. Gel and spray deodorants count; solid stick deodorant does not. Peanut butter, hummus, jam, yogurt and sauces count — the “is it a liquid?” test famously fails on foods, and full-size jars get surrendered daily.
The lookup deck on this page covers the 24 most-asked items with yes/no verdicts, because the pattern to memorize is simple: solids are free, everything else goes in the bag. Switching to solid alternatives — bar shampoo, solid deodorant, toothpaste tablets, sunscreen sticks — is the highest-leverage packing move there is: every solid swap frees a quart-bag slot and removes a confiscation risk entirely.
The Exceptions: Medicine, Babies, Duty-Free
Three categories escape the 3-1-1 limits. Medically necessary liquids (prescription medicine, insulin, saline, contact-lens solution in reasonable quantities) may exceed 3.4 oz — declare them separately at the checkpoint for additional screening. Baby formula, breast milk and baby food are exempt in reasonable quantities, also declared separately. And duty-free liquids bought after security may exceed limits if sealed in the tamper-evident bag with the receipt visible — though on connecting international itineraries, the connecting country's rules apply and may be stricter, so confirm before you buy the large perfume bottle on a layover.
International Variations
The 100 ml / one-liter-bag framework is effectively global — the EU, UK, Canada, Australia and most of Asia use the same numbers (the EU expresses it as 100 ml containers in a 1-liter transparent bag). Enforcement details vary: some airports require the bag out of your carry-on in a separate tray; airports with newer CT scanners increasingly let you leave everything inside. A few European airports have begun trialing relaxed rules with advanced scanners, but as of 2026 the 3-1-1-style limit remains the rule almost everywhere — pack for it regardless of rumor.
CT Scanners: Is the Rule Ending?
You may have heard the 3-1-1 rule is on its way out. The reality is slower and messier. New computed-tomography (CT) scanners create 3D images of bag contents and can already let travelers leave laptops and liquids inside their bags at equipped checkpoints — the TSA is rolling them out across US airports, and several European airports have trialed relaxed liquid limits with the new machines. But rollout takes years, rules differ by terminal even within one airport, and a relaxed rule at your departure airport does not help if your connection or return airport still enforces the old one.
The sane posture for 2026 and beyond: pack as if 3-1-1 still applies everywhere. If you get a CT lane, the experience is simply easier; if you do not, you are compliant. Travelers who pack “for the new rules” and hit an old-rules checkpoint are the ones surrendering bottles. Watch the announcements, enjoy the faster lanes where they exist, and keep the quart bag packed.
Checked vs. Carry-On: The Full-Size Strategy
Sometimes the right answer is to stop fighting the quart bag. If you are checking luggage anyway, full-size toiletries ride in the hold with no limits at all — the 3-1-1 rule applies only to carry-ons. The winning hybrid: decant a minimal quart bag for the cabin (what you need on travel day plus one backup day), and put full sizes in checked baggage. If your bag is lost or delayed, the quart bag covers you; if everything arrives, you have your real products.
One caution for checked liquids: pressure and temperature changes in the hold make bottles leak. Squeeze air out, seal caps with a strip of tape or cling film under the lid, and bag everything in a zip-top inside your suitcase. A shampoo explosion inside a checked bag ruins more trips than any security line ever has.
The Solid-Swap Strategy
The travelers who never stress about the quart bag are not better packers — they carry fewer liquids. A realistic solid-swap kit: bar shampoo and conditioner, solid stick deodorant, toothpaste tablets, a sunscreen stick for touch-ups (keep one compliant spray for full coverage), solid perfume, and a safety razor with the blades in checked baggage (or buy blades at the destination). Each swap is one fewer container fighting for bag space, and solids are immune to the “container capacity” trap entirely.
What remains liquid goes in the bag in priority order: the items you would rebuy at great expense or cannot find at the destination go in first. Everything else is replaceable — and most destinations sell perfectly good toothpaste.
Packing the Bag So It Closes
Order matters. Put flat, flexible items (pouches, sachets) at the back, rigid bottles in front where you can see the layout, and small vials in the gaps. Squeeze air out before sealing — trapped air is the difference between “closes” and “almost closes.” Keep the bag accessible: it comes out at most checkpoints (unless directed otherwise), so pack it at the top of your carry-on, not buried under clothes. And pack a spare empty quart bag: they split, and a backup weighs nothing.