TSA's 3-1-1 Liquids Rule Explained, Including the Real Exemptions

By Travelog Editorial Team ยท Updated August 9, 2026

TSA's 3-1-1 rule requires carry-on liquids in containers of 3.4oz (100ml) or less, all fitting in one quart-size clear bag, one bag per passenger. Specific exemptions apply: medications (no doctor's note required, reasonable quantities) and baby formula, breast milk, and toddler drinks (any reasonable quantity, declared at the checkpoint) aren't subject to the size limit. At airports with newer CT scanners, TSA has piloted exempting select items -- including baby formula in any quantity -- from the bag requirement entirely, though this varies by checkpoint.

The 3-1-1 rule has been in place long enough that most travelers can recite the basic numbers, and long enough that the exemptions -- the part that actually matters when packing medication or traveling with an infant -- get lost in secondhand summaries that don't cite TSA directly.

The Core Rule, Stated Precisely

Carry-on liquids, gels, and aerosols must be in individual containers of 3.4 ounces (100 milliliters) or less. Every container has to fit inside one quart-size, clear, zip-top bag, and each passenger is limited to one such bag. This applies to the container's labeled size, not how much liquid is actually inside it -- a mostly-empty 5oz bottle still counts as a 5oz container and isn't allowed, regardless of how much liquid remains.

Why the Rule Exists: A Specific 2006 Origin

The 3.4oz figure isn't an arbitrary round number -- it traces to a specific event worth knowing. In August 2006, UK authorities disrupted a plot to detonate liquid explosives aboard transatlantic flights from the UK to the US and Canada, disguised as ordinary soft drinks -- the plan reportedly involved 500ml bottles of commercial drink brands, with a sugary drink powder mixed with hydrogen peroxide (used as hair bleach) and other materials to create the explosive. Within days of the plot being disrupted, US authorities banned all liquids from carry-on bags entirely, a blanket restriction far stricter than today's rule. Roughly six weeks later, TSA introduced the 3-1-1 framework as a deliberate compromise between that total ban and unrestricted travel -- explosive chemists at the time reportedly calculated that a single container of 100 milliliters or less of a comparable liquid explosive couldn't cause catastrophic damage to a cabin on its own, which is the specific reasoning behind the 3.4oz (100ml) threshold that's remained essentially unchanged since.

The Medication Exemption

This is a genuine, documented exemption, not an informal courtesy some agents extend. Prescription and over-the-counter liquid medications are exempt from the 3.4oz limit, in reasonable quantities for the trip. No doctor's note or prior authorization is required to bring them. The practical guidance worth following, even though it's not strictly mandatory: keeping medications in their original, labeled packaging speeds up the checkpoint process, since an officer can visually confirm what a container is meant to hold rather than needing to ask.

The Baby Formula and Breast Milk Exemption

Baby formula, breast milk, and toddler drinks and juices are exempt from the 3.4oz limit in reasonable quantities, a specific accommodation distinct from the general medication exemption. The practical step that makes this exemption work smoothly: telling the TSA officer at the checkpoint that these items are being carried, rather than assuming the exemption applies automatically without being flagged. A genuinely useful, lesser-known detail: ice packs and gel packs used specifically to keep breast milk cold are also exempt from the standard size rule, even if they're still partially frozen or slushy at the time of screening.

A Newer, Airport-Specific Exemption: CT Scanners

Knowing this matters because it's a genuine change still in the process of rolling out unevenly, not a rule that applies everywhere yet. At airports equipped with newer CT (computed tomography) scanning technology, TSA has exempted a specific set of items from the standard 3-1-1 requirements at those particular checkpoints -- among them, baby formula and breast milk in any quantity, not just a "reasonable" one, and certain other medical-necessity liquids. The important caveat: this exemption applies only at checkpoints actually equipped with CT scanners, which is not yet every security checkpoint in the country, so travelers shouldn't assume it applies at every airport without checking.

Duty-Free Liquids: A Separate Exception

Liquids purchased after the security checkpoint -- duty-free alcohol, perfume, and similar items -- are handled differently from the standard 3-1-1 rule. These purchases are typically allowed through security in a sealed, tamper-evident bag provided by the retailer, along with the original purchase receipt, even in containers well over 3.4oz. The catch that trips up connecting travelers: this exception is designed for a single security screening, and a passenger with a connecting flight who has to clear security again at a layover airport can have that same duty-free liquid confiscated at the second checkpoint if the bag has been opened or the connecting airport doesn't recognize the same sealed-bag exception.

What Still Requires Checked Baggage

Anything genuinely over 3.4oz per container that doesn't qualify for one of the exemptions above needs to go in checked baggage rather than carry-on, full stop -- there's no size workaround for a standard toiletry that simply doesn't fit the container limit. This is the practical reason many travelers decant liquids into smaller, TSA-compliant travel-size containers before a trip, rather than relying on manufacturer packaging.

What Actually Happens to a Non-Compliant Liquid

Stating this plainly matters since it's rarely covered directly: a liquid that exceeds the 3.4oz limit and doesn't qualify for an exemption is simply not allowed past the checkpoint -- there's no fine or penalty involved, but travelers are typically given the choice to step out of the security line to check the item in luggage (if that's still logistically possible), mail it to themselves, or discard it, usually into a bin at the checkpoint with no compensation. This is the practical reason it's worth sorting toiletries into TSA-compliant containers before arriving at the airport rather than discovering a non-compliant bottle at the checkpoint itself, when the only remaining options are considerably less convenient.

What This Means for Planning a Trip

The practical takeaway is that the 3-1-1 rule itself is simple, but its exemptions are specific enough to be worth knowing precisely rather than assuming: medication and baby-formula exemptions are genuine and don't require special documentation, but they do require declaring the items at the checkpoint. Because TSA's own screening technology and specific checkpoint procedures continue to evolve -- the CT-scanner rollout being the clearest current example -- checking TSA.gov directly close to a trip's departure date is worth the extra few minutes, rather than relying on a rule remembered from a previous trip.