How Much Water Should You Carry on a Hot Hike?
Water is the heaviest thing most day hikers carry, and on a hot day it is the thing they most often run short of. Carry too little and you risk dehydration and heat illness miles from the car. Carry far more than you need and the weight slows you down and makes you work harder in the heat. The Water Needs Calculator gives a planning figure from three inputs: how long you will be out, the expected temperature, and how hard you will be working. This guide shows how it works, runs it from a cool day up to serious heat, and covers what no calculator can do for you.
How the calculator works
It starts from a common field baseline of half a litre per hour for moderate hiking in mild weather, then multiplies by two factors.
Temperature factor:
- Below 10°C (50°F): 0.8
- 10 to under 20°C: 1.0
- 20 to under 28°C: 1.3
- 28 to under 33°C: 1.6
- 33°C (about 91°F) and above: 2.0
Exertion factor: easy (flat, relaxed) 0.8; moderate (rolling trail) 1.0; strenuous (steep, heavy pack) 1.3.
Litres per hour is 0.5 times the two factors; the total is that rate times the hours on the trail. It also flags when electrolytes are advised: when the hike is at least three hours long, the temperature is 28°C (about 82°F) or more, and the effort is strenuous. Long, hot, hard days are when sweat losses of salt add up.
From a cool day to serious heat
Here is the same kind of five-to-six-hour day hike at different temperatures and effort levels.
| Conditions | Litres per hour | Total | Electrolytes flagged |
|---|---|---|---|
| 5 h, 15°C (59°F), moderate | 0.5 | 2.5 L | No |
| 5 h, 24°C (75°F), moderate | 0.65 | 3.25 L | No |
| 5 h, 30°C (86°F), moderate | 0.8 | 4 L | No |
| 6 h, 34°C (93°F), moderate | 1.0 | 6 L | No |
| 6 h, 34°C (93°F), strenuous | 1.3 | 7.8 L | Yes |
The jump from a mild day to a hot, hard one is more than threefold: from 2.5 litres to 7.8. Each litre of water weighs about a kilogram, so the hottest row means close to 8 kg of water alone, before food or gear. Very few day hikers carry that comfortably, which leads to the most important practical point in this guide: on a really hot day, the answer is usually not "carry more water" but "change the plan". Start at first light, choose a shorter or shadier route, plan reliable refill points, or pick a cooler day.
The electrolyte threshold
The electrolyte flag is a hard line in the calculator, so two nearly identical hikes can land on either side of it. A strenuous hike at 28°C gives 1.04 litres per hour in both cases below:
- 3 hours: 3.12 litres, electrolytes advised.
- 2.5 hours: 2.6 litres, no electrolyte flag.
Your body does not know about the line, of course. Treat the flag as a reminder that on long, hot, hard days plain water alone is not enough, not as a precise cut-off. Eating salty snacks during the hike is the simplest way to replace some of what you sweat out; electrolyte drinks or tablets are another. If you have a medical condition that affects salt or fluid balance, or take medication that does, ask your doctor before relying on either.
Too much water is a risk too
It is possible to drink too much. Hyponatremia is a dangerously low level of sodium in the blood, and on long, hot hikes it usually happens when someone drinks large amounts of plain water while sweating out salt and eating little. Early signs, such as nausea, headache, confusion and tiredness, can look very like heat exhaustion or dehydration, which is what makes it dangerous: the instinct is to drink even more. It can progress to vomiting, seizures and collapse.
The safeguards are simple. Drink regularly but do not force down far more than you are losing; drinking to thirst is a reasonable guide for most healthy adults. Eat salty food along with your water on long, hot days. If someone is confused, vomiting or getting worse despite drinking, treat it as an emergency and call for help.
Heat exhaustion and heatstroke
Know the signs, for yourself and for everyone you hike with.
Heat exhaustion: heavy sweating, cool and clammy or flushed skin, weakness, dizziness, headache, nausea, muscle cramps, a fast pulse. Stop, get into shade, loosen clothing, cool the person with water on the skin and fanning, and give small sips of water, with something salty if they can eat. Rest until they recover fully, then head out by the easiest route. Do not carry on with the hike.
Heatstroke is a medical emergency. Warning signs include confusion, slurred speech, strange behaviour, fainting, seizures, and very hot skin that may be dry or still sweaty. Call the local emergency number at once, and cool the person as fast as you can while you wait: shade, water all over the skin, fanning, wet cloths at the neck, armpits and groin.
Dehydration on its own shows as thirst, a dry mouth, dark and scanty urine, headache and tiredness. Pale urine at regular intervals is a reasonable sign you are keeping up.
What the calculator cannot see
The figures are planning estimates for adults. Real needs can be much higher or lower depending on things the calculator does not ask about:
- Sun and shade. An exposed ridge or open desert in full sun is far harder than a shaded forest at the same air temperature.
- Humidity. In humid heat sweat does not evaporate well, and you heat up faster.
- Altitude. Breathing hard in dry mountain air increases water loss.
- Acclimatisation. People used to heat sweat more efficiently than visitors from cooler places.
- Individual differences. Body size, fitness and how much you naturally sweat vary widely between people.
- Your hike time. The total is only as good as your estimate of hours on the trail. Use the Hiking Time Calculator and add your breaks; in heat you will be slower than usual.
Refilling from natural sources
If the route passes reliable water, you can carry less and refill on the way. Always treat or filter water from streams, springs and lakes, however clean it looks: filters, chemical treatment, UV devices and boiling all work when used correctly. Do not count on a source you have not confirmed is flowing: seasonal streams dry up in exactly the conditions when you need them most. Check the land manager's current information and carry enough to reach the next sure source, plus a margin. Our guide to carrying water versus filtering it works through the weight trade-off with the Backpack Weight Calculator.
A hot-weather plan
- Estimate your hike time realistically, including breaks and a slower pace in heat.
- Run the water calculator for the hottest temperature you expect, at the effort level of the hardest part of the route.
- Carry that, plus a margin, and a way to treat more water if there is a confirmed source.
- Bring salty snacks, and electrolytes on long, hot, hard days.
- Start early, rest in shade, and wear a hat and light-coloured, loose clothing.
- Tell someone your route and return time, and know the local emergency number.
- Turn back early if anyone shows signs of heat illness, or if you have drunk half your water before the halfway point.
That last rule is worth remembering on its own. If half your water is gone and you are not yet halfway, the plan has failed, whatever the clock says. Turn around.