I don’t understand why you can go through with an empty bottle (no matter the size) but you can’t go through with a large bottle that has less than 100ml of liquid in it. All these regulations are supposedly there for safety reasons but it doesn’t seem like really.

  • 2
    How would a security person go about evaluating whether there is more or less than 100 ml in a large container? Oct 12, 2019 at 20:27
  • 1
    Estimating the liquid volume in containers of almost-infinite sizes and shapes would not be feasible given the press of travelers lined up to pass security. Oct 12, 2019 at 20:37
  • Two words: security theatre.
    – jcaron
    Oct 12, 2019 at 21:53

2 Answers 2


Let's say I have a 600ml water bottle. It's opaque.

I tell the security I have under 100ml in it.

How are they supposed to judge this? Do you expect them to pour it into a measuring container? Can't weigh it - different liquids weigh different amounts. Estimate? Yes 1ml of liquid is obvious to most of us that it's less than 100ml, but where do you stop? What if it's 99ml or 101ml? I'm certainly not capable of doing this estimate, even if I could see it.

The easiest answer from a policy - only allow 100ml bottles, and clearly empty bottles. That's the only way to guarantee there's no more than 100ml of liquid contained within.

Whether or not this is an effective form of security is a different conversation and debate, but that's the reason for 100ml bottles only.


The prime factor I consider is convenience. With a 100 ml or less bottle you know exactly there are less and 100 ml of liquide in there.

In long security queues, and bottles being all sizes it can be difficult to measure the number of liquids in each bottle and can cause long queues.

Not the answer you're looking for? Browse other questions tagged .