Whether a water bottle is allowed in your flight baggage depends entirely on the type of luggage and its contents. For checked baggage, you can pack almost any water bottle, whether full or empty, without restriction. For carry-on baggage, the rules are governed by the TSA's 3-1-1 liquids rule. You may bring an empty reusable water bottle of any size through security and fill it at a fountain or hydration station once you are in the departures area. However, if the bottle contains more than 3.4 ounces (100ml) of liquid, it will be confiscated at the checkpoint. An interesting exception exists for "frozen" water: if a bottle is 100% frozen solid at the time of screening, it is technically permitted, but if there is any visible melting or liquid at the bottom, it falls back under the 3.4 oz restriction. For 2026 travel, it remains a standard "travel pro-tip" to bring a high-quality empty insulated bottle to stay hydrated while avoiding the high costs of bottled water inside airport terminals.