According to Bacheler, the human body loses about 8 ounces of water per hour on an aircraft at altitude, mostly from breathing. As such, the Aerospace Medical Association (AsMA) recommends drinking about 0.25 liters (or 8 ounces) of water per hour while on an airplane.

For a 13-hour long-haul flight, medical experts and the Aerospace Medical Association generally recommend drinking approximately 8 ounces (240ml) of water for every hour spent in the air. This means you should aim to consume roughly 104 ounces (just over 3 liters) of water throughout the duration of your flight. Airplane cabins have extremely low humidity levels, often below 20%, which can lead to significant dehydration, dry skin, and respiratory irritation. Staying hydrated is also a key factor in reducing the risk of Deep Vein Thrombosis (DVT) and mitigating the effects of jet lag. It is highly recommended to bring your own large reusable water bottle to fill after passing through security, as the small cups provided by cabin crew may not be sufficient to meet this goal. Additionally, you should limit the consumption of alcohol and caffeinated beverages like coffee or tea, as these act as diuretics and can exacerbate fluid loss during your journey.