How Much Water to Drink Hiking: Hydration, Dehydration Signs & First Aid

Before you read on

Why do you lose so much water outdoors?

Your body loses water all day, even sitting still: through urine, through breathing and through the skin. Outdoors three things add to it: effort, weather and height. The loss is gradual, which is why it is easy to fall behind without noticing.

• Sweat. Sweating is how you get rid of heat. The harder you work, the warmer the air and the heavier your pack, the more you sweat. Sweat rate varies a lot between people.
• Breathing. Every breath leaves the body saturated with moisture. Hard breathing on a climb loses more than strolling.
• Cold, dry air. Cold air holds little moisture, so breathing loses water even when you hardly sweat. Cold also makes the body pass more urine, and thirst is weak, so people drink less exactly when they need to.
• Altitude. Higher up you breathe faster and the air is drier. The calculator adds 0.5 L per day from 1,500 m and 1 L from 2,500 m.
• Wind and sun. Wind dries skin and sweat evaporates unnoticed, so you may not feel how wet you are.

How much water should you drink when hiking?

There is no honest fixed answer. Health bodies give everyday guidance for adults at moderate temperatures – the DGE, for example, speaks of about 1.5 litres of drinks per day on top of the water in food – and active outdoor days need more. Our water calculator turns your situation into a planning figure. It starts from a basic need of 0.033 L per kg of body weight (minus about 20 % that comes with food, so roughly 2 L for 75 kg), adds sweat for your active hours, effort and temperature, adds altitude, and counts half of your cooking water (tea, soup, rehydrated meals) as drinking.

These are guideline values. Treat the result as your starting point and let thirst, urine colour and how you feel adjust it.

Worked examples from the calculator

For a 75 kg hiker with six active hours at moderate effort, dehydrated food and 500 m altitude, the calculator gives:

Cold and altitude change the picture less than people expect. For the same hiker at 0 °C with five hours of light effort and mixed food, the calculator gives about 3.1 L per day in total (2.3 L drinking). At 2,600 m with the same inputs it gives 4.1 L (3.3 L drinking) – the extra litre for altitude is built in. The body can absorb only so much per hour: the calculator caps sweat replacement at 1.2 L/h and warns when the daily drinking figure becomes extreme. If it does, the answer is less activity in the heat and more rest in the shade, not more bottles.

What do the numbers mean for your bottle?

At 18 °C, 0.3 L per hour on the move is about 100 mL – a few good swallows – every 20 minutes. That is easy to do if you sip regularly and nearly impossible to catch up in one go at the end of the day. How much to carry at once depends on the gap between sources: in the calculator's example, 2 hours between sources means about 1 L, 4 hours about 2 L. See water containers for bottles and bladders and finding water for reading the map for sources.

Drinking strategy: little and often, before you are thirsty

Thirst is a useful signal on easy days, but it lags behind in heat, during hard effort, in the cold and with age. Drinking only when thirsty means you are always slightly behind. The simple strategy is the one most guidance converges on:

• Start hydrated. Drink normally the day before and have a glass or two with breakfast. Do not force large amounts – pale urine before you set off is enough.
• Sip little and often. A few swallows every 15 to 20 minutes while moving, plus a proper drink at every break. The body can absorb a small amount continuously; a big gulp mostly passes straight through as urine.
• Tie drinking to routine. Drink at each rest stop, each junction, each time you eat. Add salty food to your snacks.
• Drink with meals. Hot drinks at breakfast and dinner count as fluid, and dehydrated food needs water anyway.
• Keep the bottle within reach. If you must stop and unpack to drink, you will drink less.

Carrying versus refilling

Water is the heaviest thing in your pack – one litre weighs one kilogram – so plan around sources instead of carrying everything. Refill at every safe source, treat the water (boiling for 1 minute is enough, 3 minutes above 2,000 m) and carry the amount the calculator gives for the next dry stretch plus its 20 % reserve. If the map shows a long dry stretch, plan the route differently, start earlier and use the cool hours (hiking time calculator, trip planning). In an emergency a rain catch or a stream you can reach beats an empty bottle you are "saving".

Signs of dehydration: what to look for

Dehydration builds in steps. The early signs are easy to ignore on the trail, which is why it is worth knowing them. The table is a general guide – people do not follow tables neatly. If in doubt, treat for the more serious stage.

Urine colour as a field check

Urine colour is the simplest check you can do outdoors, and the calculator uses it too: pale straw means fine, darker means drink more. Check it when you pass urine, ideally at every long break. Do not chase a colour chart with numbers; the general direction is enough. The first urine of the morning is naturally darker.

Colour can be misleading. Some vitamins turn urine bright yellow, some foods and medicines change the colour, and drinking a lot in a short time makes it clear without hydration being "better". Use colour together with how much you pass and how you feel. Passing little, dark urine is a reason to drink and slow down. Passing none for many hours, together with dizziness or confusion, is a reason to call for help.

Heat exhaustion and heat stroke: the short version

Dehydration and heat belong together. Heat illness is a spectrum from cramps and heat exhaustion to heat stroke. This is only an overview; the details belong in a first aid course.

Prevention is mostly planning: start early, rest in the shade at midday, wear a hat and light clothing, wet a bandana, reduce effort when it is hot and drink to plan. See reading the weather and clothing and layering.

Dehydration in cold weather

Cold-weather dehydration is sneaky. You sweat less visibly, thirst is weak, cold air is dry, heavy layers trap sweat, and drinking cold water is unappealing. People also avoid drinking because they dislike undressing to pass urine. The result is the same: dark urine, headaches and fatigue that make you colder and clumsier.

• Drink on a schedule, not by thirst: warm drinks at every break.
• Keep bottles from freezing: carry them upside down in the pack, in an insulated sleeve or inside your jacket, and use a vacuum-insulated bottle for hot drinks (containers). Never heat a double-wall bottle in a fire.
• Do not eat snow when you are cold; melt it first (snow water).
• Dehydration and cold feed each other. A dehydrated, tired person chills faster, so watch the signs of hypothermia.

Salt, electrolytes and drinking too much

Sweat contains salt as well as water. On most day hikes, water and normal food replace both. On long, hot, sweaty days, or when you have been sweating heavily for many hours, plain water alone may not be enough, and salty food helps: nuts, cheese, crackers, broth, instant soup. Commercial electrolyte tablets and oral rehydration powders exist; follow the packet instructions and do not dose beyond them. People with high blood pressure, kidney or heart conditions, or on diuretics should ask a doctor before adding salt or electrolytes.

Drinking too much is a real risk too

Hyponatraemia means the blood salt level falls too low. It is rarer than dehydration but real. It can happen when someone drinks far more than they lose, over many hours, especially plain water and with heavy sweating. Signs are nausea, bloating, headache and confusion – which look like dehydration. The sensible rules: drink to plan and to thirst, do not force fluids "to be safe", eat salty food on long days, and do not treat confusion by pouring more water into the person. If a person is confused, call 112.

Children, older people and illness: extra attention

Children lose water relatively fast, often do not say they are thirsty and forget to drink when playing. Offer a drink at every stop, make it a game, and watch for dark urine, a dry mouth and tiredness. See bushcraft with kids. For anything beyond mild signs in a small child, get medical advice early.

Older people feel thirst less and may take medicines that affect fluid balance, such as diuretics. Remind them to drink on a schedule, and avoid long hot stretches.

Vomiting and diarrhoea cause fast fluid loss, for example after drinking untreated water. Mainstream advice is small sips often and an oral rehydration solution from a pharmacy (packed in sachets, made up as directed). Seek medical help if someone cannot keep fluids down, is a child or elderly, or shows signs of moderate or severe dehydration. Prevent it by treating water properly (purification).

What to drink: water, tea, broth – and what not

Practical systems: bottle, bladder and schedule

The best system is the one that makes drinking effortless. A bladder with a hose encourages regular sips but hides how much is left, is harder to refill and its hose freezes first in winter. Bottles in side pockets are easy to fill and monitor, but you must reach for them. Many hikers carry one of each. An insulated bottle for tea is worth its weight in the cold. See containers for details and backpacks for pocket placement.

Dehydrated: what now?

Checklists

Common hydration myths