Above roughly 2,500 metres (8,200 ft), add 1–1.5 liters to your normal daily target. Most adults land between 3.5 and 4.5 liters per day at altitude because of faster breathing in dry air and altitude-induced diuresis.
At altitude the air holds very little moisture, and every breath you exhale is fully saturated with water vapour. Because oxygen pressure is lower, you also breathe faster and deeper. The combination can roughly double respiratory water loss compared with sea level — a loss you cannot see and rarely feel.
On top of that, the body responds to low oxygen with altitude diuresis: increased urine output during the first two to four days. That is a normal, healthy adaptation, but it means fluid leaves faster than usual exactly when you are least likely to notice.
A 75 kg person with a 3.0 L moderate-activity baseline therefore lands around 4.0–4.5 L on a trekking day at 3,500 m.
Headache, nausea, fatigue and poor sleep are symptoms of both dehydration and acute mountain sickness. This overlap causes real problems: people assume they are dehydrated, drink more, and keep ascending when they should be resting or descending. Staying properly hydrated from the start removes the ambiguity — if symptoms persist despite good hydration, treat them as altitude sickness.
Mountain environments are usually cold, and cold exposure reduces thirst sensation by around 40%. You will not feel like drinking, which is precisely why altitude hydration has to be scheduled rather than reactive. A practical rule: 500 ml every two hours while moving, plus a full bottle at each meal.
Long days of sweating combined with high fluid intake can dilute sodium. On multi-day treks, add an electrolyte tablet to one or two bottles per day, or make sure meals contain adequate salt. Persistent muscle cramps or headaches despite drinking well often point to electrolytes rather than water volume.
Urine colour still works at altitude, but note that the first days of diuresis produce large volumes of pale urine even if total body water is dropping. Combine urine colour with body weight: a loss of more than 1–2% overnight usually means fluid, not fat.
Three reasons: the air is very dry so you lose more water through breathing, you breathe faster and deeper at low oxygen pressure, and the body increases urine output during the first days of acclimatisation.
No. Hydration does not prevent acute mountain sickness — gradual ascent does. But dehydration causes headache, fatigue and nausea that look exactly like altitude sickness, so staying hydrated removes a confusing overlap.
Typical trekking guidance is 3–4 litres per day at 3,000–4,500 m, more on long walking days. Drink steadily rather than large volumes at camp, and include electrolytes if you are sweating heavily.