Backpacking Daily Water Needs Calculator

Dehydration is the most common preventable problem on backcountry trips — and the most common excuse for cutting a trip short.
The standard advice to “drink before you are thirsty” is sound, but it does not tell you how much to carry.

The baseline human requirement for water at rest is about 35 ml per kilogram of body weight per day.
A 70 kg (154 lb) person needs about 2.45 liters just to maintain basic function at comfortable temperatures while doing nothing.

Exercise multiplies that dramatically.
An easy day hike adds about 50% to baseline.
A full day of loaded backpacking with significant elevation gain can double or triple it.
Scrambling or technical terrain above baseline adds another significant margin.

Two environmental factors compound this: heat and altitude.

Heat: your body sweats to cool itself, and sweat is water you need to replace.
Above 25°C (77°F), plan for an extra 0.5-1 liter per day just for temperature compensation.
Direct sun exposure makes this worse, especially in desert or alpine environments with no shade.

Altitude: the air at high elevation is drier, and you exhale more moisture with each breath due to the effort of breathing at lower oxygen pressure.
Above 2400 m (8000 ft), add 0.3 liters per day.
Above 4000 m (13000 ft), add 0.6 liters.

Cooking adds about 0.5 liters (rehydrating freeze-dried meals, boiling pasta, making coffee).
Basic hygiene — washing hands, brushing teeth — adds another 0.3 liters.

Carry a minimum water filter or treatment tablets on all trips longer than one day.
Carrying enough water for even two days at full hydration is impractical in terms of pack weight — filtering as you go is the only realistic strategy.