Anyone who flies a lot for work — consultants, sales, frequent flyers — knows the feeling: after the flight, dry eyes, a scratchy throat, headache, fatigue. A large part of that is simply dehydration. The air on board is extremely dry, and over hours the body loses fluid unnoticed.
The tricky part, as at altitude: the loss is insensible — through breathing, not visible sweat. You drink too little because you do not feel very thirsty — and arrive with a deficit that costs concentration and performance, exactly when a meeting is waiting.
Why flying dehydrates you
1. Extremely dry cabin air
Relative humidity in an aircraft cabin is usually only 10–20% — comfortable would be 40–70%. That is drier than many deserts. The air at cruising altitude holds almost no moisture, and the air-conditioning dries it further. Through skin and especially breathing, the body loses water continuously.
2. Cabin pressure like 6,000–8,000 ft
Cabins are pressurized to roughly a 6,000–8,000 ft (1,800–2,400 m) altitude equivalent. The lower pressure makes you breathe a little faster — similar to being on a mountain — which raises water loss through the airways. Modern jets like the Boeing 787 and Airbus A350 keep a lower cabin altitude and higher humidity, easing symptoms.
3. Coffee and alcohol on board
The welcome drink and the in-flight coffee are mildly dehydrating and add to the deficit in the dry cabin. Alcohol also worsens sleep — and poor sleep lengthens jet lag.
How much should you drink when flying?
A common rule of thumb: about one cup of water (roughly 250 ml) per hour of flight. On a long-haul flight the need adds up accordingly:
| Flight duration | Guideline water on board | Plus |
|---|---|---|
| Short-haul (1–2 h) | approx. 250–500 ml | 1 glass before boarding |
| Medium-haul (3–5 h) | approx. 0.75–1.25 litres | limit alcohol |
| Long-haul (6–10 h) | approx. 1.5–2.5 litres | consider electrolytes |
| Ultra-long-haul (>12 h) | approx. 2.5–3 litres | move regularly |
Dehydration and jet lag
Dehydration amplifies exactly the symptoms people mistake for jet lag: headache, fatigue, poor concentration, irritability. Drinking enough eases these noticeably.
7 hydration tips for frequent flyers & business travellers
- Reach the gate hydrated — drink enough before the flight, don't start on board.
- Bring your own bottle — refill after security; on board, actively ask instead of waiting for the trolley.
- ~250 ml per hour of flight as a rough guideline.
- Cut back on alcohol & coffee — especially on red-eyes before important meetings.
- Electrolytes on long-haul — over many hours a pinch helps more than plain water.
- Keep drinking at the destination — the deficit is not gone the moment you land.
- Track the travel day — seeing your higher need makes you drink more consistently. Brim's Flight Mode raises the goal on travel days automatically.
💧 Brim knows your flight day
Brim's Smart Goal has a Flight Mode: on flight and travel days Brim raises your goal and also factors in weather, altitude, steps and workouts. With the Apple Watch app you log on the go — at the airport, on board, at the hotel — without reaching for your iPhone.
Free on the App StoreHow Brim helps frequent flyers
Most water apps show the same goal every day — whether it's a desk day or an 11-hour long-haul. For people who fly a lot, that is too crude.
- ✈️ Flight Mode — the daily goal is raised on flight/travel days
- 🌡️ Weather at the destination is factored in (Apple WeatherKit)
- 👟 Steps & workouts — hotel-gym sessions and walking count too (Apple Health)
- ⌚ Apple Watch + complications — log on the go without opening the app
So frequent flyers see their higher need on a travel day automatically — and arrive in better shape.
Frequently asked questions (FAQ)
How much water should you drink when flying?
About one cup (roughly 250 ml) per hour of flight. Over 10 hours the body loses 1.5–2 litres through breathing alone. Drink well before the flight and limit alcohol and coffee.
Why does flying dehydrate you?
Cabin air is only 10–20% humidity (comfortable: 40–70%) and the cabin is pressurized to a 6,000–8,000 ft altitude equivalent. Both raise insensible water loss through breathing.
Does drinking help with jet lag?
Dehydration amplifies jet-lag symptoms like headache and fatigue — drinking eases them. But jet lag itself comes from the body clock; light, sleep timing and time adaptation are what help. Hydration is the foundation, not a full fix.
Avoid alcohol and coffee in flight?
Both are mildly dehydrating and add to the deficit in the dry cabin; alcohol also worsens sleep and lengthens jet lag. Before important meetings, prefer water and electrolytes.
Which app adjusts your goal for travel days?
Brim has a Flight Mode: the goal is raised on flight/travel days, plus weather, altitude, steps and workouts. Loggable on the go with Apple Watch.