Calculator · Water

Water Needs Calculator

How much water to drink, cook with and carry — by weight, heat, effort and altitude.

How much water do you really need on the trail? Enter your weight, the temperature, your effort and how far apart the water sources are – and get your daily need, the amount to carry and the total for the trip.

Body weight

Daytime temperature

°C

Active hours per day

Effort

Light Moderate Hard

Food

Dehydrated Mixed Ready-to-eat

Altitude

Trip days

days

Hours between water sources

Water per day 5.3 L

Carry at once 2.0 L incl. 20 % reserve

Whole trip 10.6 L

Drinking only 4.1 L/day

Where the water goes (per day)

Basic needs2.0 L
Sweat (activity)2.7 L
Cooking & drinks1.2 L

Treatment for the whole trip

≈ 11 purification tablets (1 per litre — check your brand)
Filter capacity: at least 11 L
Containers: 2.0 L total, at least one boil-safe metal bottle

How is this calculated?

Basic need 0.033 L per kg body weight minus ~20 % that comes with food + active hours × sweat rate (light 0.25 / moderate 0.45 / hard 0.75 L/h, max 1.2 L/h) × temperature factor (0.6 below 5 °C … 2.1 above 34 °C) + altitude (+0.5 L from 1,500 m, +1 L from 2,500 m) + cooking water (0.4–1.2 L); half of the cooking water counts towards drinking (tea, soup, rehydrated meals). Guideline values — individual sweat rates vary a lot.

How to use the water calculator

Enter your body weight and the expected daytime temperature.
Set your active hours per day and the effort (light, moderate, hard).
Choose your food: dehydrated meals need the most water, ready-to-eat food the least.
Add the altitude, the number of trip days and how many hours you walk between water sources.

You get water per day, how much to carry at once, the total for the trip and a checklist for tablets, filter capacity and containers.

How it is calculated

The basic need is 0.033 litres per kg of body weight, minus about 20 % that you take in with food – roughly 2 L for a 75 kg person. On top comes sweat loss: active hours × a sweat rate (0.25 L/h light, 0.45 L/h moderate, 0.75 L/h hard) × a temperature factor from 0.6 below 5 °C to 2.1 above 34 °C, capped at 1.2 L per hour – about as much as the gut can absorb. Above 1,500 m the calculator adds 0.5 L per day for faster breathing, above 2,500 m a full litre.

Cooking water is 1.2 L for dehydrated food, 0.8 L for mixed and 0.4 L for ready-to-eat. Half of it counts towards drinking, because tea, soup and rehydrated meals are fluid too – but drinking never drops below the basic need.

The carry amount is what you drink between two sources plus a 20 % reserve, rounded up to half litres; from 8 hours apart, dinner water is included.

75 kg, 6 h moderate effort, dehydrated food	Drinking	Total per day
10 °C	≈ 3.5 L	≈ 4.7 L
18 °C	≈ 4.1 L	≈ 5.3 L
25 °C	≈ 5.0 L	≈ 6.2 L
30 °C	≈ 6.0 L	≈ 7.2 L

Practical tips

These are planning values – sweat rates differ a lot between people. Check your urine colour: pale straw is fine, dark means drink more.
Drink small amounts regularly instead of large amounts at once, and add electrolytes in the heat.
Treat all wild water: boil, filter or use tablets. A filter does not remove viruses unless it is rated for them.
In winter thirst is weak – drink on a schedule and keep bottles from freezing.

FAQ

How much water should I drink per day when hiking?

In mild weather plan roughly 0.25–0.75 L per hour of activity, depending on effort, on top of a basic need of about 2 L. For 6 hours of moderate hiking at 18 °C that is about 4 L of drinking water, or 5.3 L a day including cooking. Heat and altitude add more – up to about 1.2 L per hour in hot weather.

How much water should I carry between sources?

Carry what you will drink until the next reliable source plus a reserve of about 20 %. If a source might be dry, plan with the next one after it. For long dry stretches also carry the water for cooking dinner.

Do I need more water at altitude?

Yes. Faster, deeper breathing in dry mountain air increases water loss. The calculator adds 0.5 L per day from 1,500 m and 1 L per day from 2,500 m.