The rule for liquids on a plane is based on the container's capacity, not the amount of liquid inside it. According to TSA and international security standards, you can bring a half-empty bottle in your carry-on only if the bottle's total capacity is 3.4 ounces (100ml) or less. If you have a 500ml water bottle that is only 10% full, it will still be confiscated because the container itself exceeds the limit. However, you are completely free to bring a totally empty bottle of any size through security and refill it at a water fountain or bottle-fill station once you have passed the checkpoint. This is a common travel hack for staying hydrated without paying high airport prices for bottled water.