3-1-1 rule

The 3-1-1 rule is the US Transportation Security Administration's limit on liquids, gels, and aerosols carried through security in a carry-on bag. Each container must hold 3.4 ounces or 100 milliliters or less, all of those containers must fit inside a single quart-sized clear bag, and each passenger is allowed one such bag at the standard security lane.

The rule applies to the container's stated capacity, not how much liquid is actually inside it, so a half-empty 8 ounce bottle still counts as an 8 ounce container and will usually be pulled aside. It covers everyday items people forget about, including toothpaste, contact lens solution, and some solid deodorants that soften enough to count as a gel.

Most airports outside the US enforce a similar but not identical rule, often 100 ml per container, so it is worth checking local rules when connecting through another country. A common mistake is packing several small bottles that individually meet the limit but do not all fit in one quart bag, which forces a rushed repack at the checkpoint. TSA PreCheck and some newer scanners are gradually relaxing this rule at a small number of airports, but it should not be assumed without checking first.