A volume limit built around a screening problem
The 100-milliliter cap is not a point at which a liquid suddenly becomes dangerous. It is a practical boundary for screening: some liquid chemicals can be combined into explosives, while ordinary drinks, lotions, and other liquids may look similar inside a container. At busy checkpoints, officers need a rule that can be applied quickly and consistently.
The restriction followed the disruption of a 2006 plot to attack aircraft using liquid explosives. The concern was that dangerous ingredients could be carried in separate containers and combined later. Limiting each container’s size reduced how much liquid a passenger could bring through the checkpoint in this hard-to-screen category.
Why the bottle matters more than what is left inside
The limit applies to the container’s capacity, not just its contents. A nearly empty bottle labeled for a larger volume still exceeds the limit: checkpoint staff cannot establish a container’s original capacity simply by looking at how much remains. A bottle made to hold 100 milliliters, by contrast, stays within the rule even when full.
That distinction is useful when packing: check the size printed on the bottle, not the amount you plan to pour into it. The familiar 3.4-ounce figure is the rounded U.S. equivalent of 100 milliliters, so travel bottles often use the metric amount as their label.
