"I'll drink when I'm thirsty" — if that's your mental model, it's the wrong model for the mountains. By the time you actually feel thirst, your body has already lost roughly 2 % of body weight in fluid. In town that's barely noticeable. On a hike, that level of dehydration drops your judgement, makes your legs cramp, and in the worst case puts you in the kind of situation that gets called "lost on the mountain." This guide walks through the what, when, and how much of hydration on a hike, in plain terms.
Hiking loses far more water than you'd think
In regular daily life, the rough water requirement is about 2 to 2.5 litres per day. Hiking changes the math. The work rate is higher, and heavier breathing increases what's called insensible loss — fluid you lose through your mouth and nose without noticing. As elevation rises, the air gets drier, and the body loses water through routes other than sweat as well.
The widely cited formula for estimating hiking water needs is body weight (kg) × hours moving × 5 ml. For a 60 kg hiker out for 6 hours: 60 × 6 × 5 = 1,800 ml. That's a starting point, not a hard answer — air temperature, humidity, wind, total elevation gain, and individual sweat rate all push the number around. In summer or on routes with sustained steep climbs, you can easily need more.
Here's where beginners get it backwards. "I want a lighter pack, so I'll bring less water." Water is heavy — 1 kg per litre. But cutting your water leaves you dehydrated, dehydration tanks your pace, your hike runs longer than planned, and now you're in a worse problem than carrying the extra kilo would have caused. Treat the weight of water as the price of staying safe.
Is water alone enough? — picking the right thing to drink
"Just bring water" is the obvious move, but it isn't actually right. With sweat, you also lose electrolytes — sodium, principally. Drink large amounts of plain water and you can dilute your blood sodium enough to trigger hyponatraemia: headache, dizziness, and in serious cases impaired consciousness. The symptoms can look like heatstroke, which makes it dangerous to misdiagnose in the field.
So what do you actually carry?
- Plain water: the base. Clean-tasting, easy to drink. On long days it tends to run short on electrolytes.
- Sports drinks: carry sugar and electrolytes; absorb fast. The sweeter ones can leave your mouth sticky and actually make you thirstier — pick a leaner formulation if that's a problem for you.
- Oral rehydration solutions: strongest electrolyte replacement. Worth having on hand in heavy summer sweat or if anyone in the party isn't well.
- Barley tea (mugicha): minerals, no caffeine, no real diuretic effect. A solid choice on day hikes on lower mountains.
One thing to watch: caffeinated drinks (coffee, green tea, energy drinks) are mildly diuretic and don't carry the main hydration job. A summit coffee is one of the best small pleasures in hiking — keep it as a pleasure, and run your real hydration off something caffeine-free.
A simple working pattern is to pair water with a sports drink. Lean on water for the bulk of intake, but cycle in a sports drink every one to two hours to keep the electrolyte side honest. Powdered electrolyte mix you stir into water at the trailhead works too, and saves weight.
The biggest variable is when you drink, not how much
The lever people most often miss is timing. As noted up top, "drink when you're thirsty" is too late. It's especially wrong on a mountain. In dry air at altitude, you can be losing fluid without any awareness of sweating at all. On a windy ridge, sweat evaporates so fast you tell yourself you're not even sweating, and meanwhile you're getting steadily drier.
Useful timing checkpoints:
- Before you start: once you're at the trailhead, drink a cup (150–200 ml) 15–30 minutes before stepping off.
- While moving: one or two sips (100–200 ml) every 15 to 20 minutes. Frequent small drinks absorb more efficiently than occasional big ones.
- On longer breaks: top up the deficit you've built up. Two cheap signals: how soaked your shirt is, and the colour of your urine (dark yellow is a clear dehydration flag).
- After you're off the mountain: you don't refill what you lost just by getting back to the car. Keep deliberately drinking that evening — it makes a real difference to how you feel the next day.
One practical detail. Make it easy to drink without taking your pack off — use a hydration system (bladder with a drinking tube), or keep a bottle in a side pocket you can reach behind your back without stopping. "I didn't drink because it was a pain to take the pack off" is one of the most common reasons people end up dehydrated on the trail. Build the habit into the gear and the rest takes care of itself.
Note: hydration needs vary a lot between people. If you have a medical condition or are unsure, follow your doctor's or qualified specialist's advice.
Summary
Three things to hold on hiking hydration. First, amount — body weight × hours moving × 5 ml as a baseline, with the awareness that conditions push it up. Second, what to drink — pair plain water with something that replaces electrolytes, and don't rely on caffeinated drinks for the main job. Third, when to drink — build a habit of small, frequent sips before you ever feel thirsty.
None of this requires technique. What it requires is knowing it. "Knowing" is the difference between a comfortable safe hike and a needlessly hard one. On your next outing, plan your hydration before you leave the house. The numbers above are general references — your specific needs will vary. When in doubt, ask a specialist or a mountain guide.