How Much Water Should You Carry?
Running dry with miles to go is the most common — and most preventable — emergency on the trail. But hauling too much is its own mistake: every extra liter is a kilogram on your back for the entire day.
This calculator gives you a defensible number to pack: liters per hour and a total for the hike, adjusted for temperature, altitude, pack weight, and sun exposure — so you carry enough, and not a bottle more.
Estimate your water needs
- Drink rate
- 25 fl oz per hour
Water to carry
3.6 L (123 fl oz)
≈ 4 × 1 L bottles
At this volume plain water isn't enough — add electrolyte tabs or salty snacks to replace sodium lost in sweat.
How this is calculated
The baseline is 0.5 liters per hour of moderate hiking — the midpoint of the 0.5–1 L/h range recommended by the US National Park Service and standard wilderness-medicine references. That covers a temperate day with a light pack.
Four multipliers adapt it to your actual conditions.Temperature is the dominant one: sweat loss climbs steeply past 25 °C, so the factor steps from 0.9 in cold weather up to 1.8 in extreme heat. Altitude adds 10% per 1,000 m above 2,500 m for respiratory moisture loss. Pack weight andsun exposure each add up to 20%, reflecting the higher work rate and radiant heat load.
The output is a planning volume for drinking water — cooking water and the extra safety liter for desert travel are on top. When the total passes 3 liters or the day is hot, the tool reminds you to add electrolytes, because replacing water without sodium is how endurance hikers get into trouble.
Frequently asked questions
How much water do I need per mile of hiking?
A useful rule of thumb is about half a liter (17 fl oz) per hour of moderate hiking in mild weather — roughly one liter every 2–3 miles depending on your pace. Heat, altitude, and a heavy pack can double that, which is exactly what this calculator adjusts for.
Why do I need more water at high altitude?
Above roughly 2,500 m (8,200 ft) you lose extra moisture with every breath: the air is drier and you breathe faster, and altitude also blunts your thirst signal. Dehydration and altitude sickness share symptoms, so drinking on schedule — not on thirst — matters more up high.
Can I drink too much water?
Yes. Drinking far beyond sweat losses without replacing salt can dilute blood sodium (hyponatremia), which is genuinely dangerous. On long or hot days, pair water with electrolyte tabs or salty food — the calculator flags when that matters.
Should I carry all my water or filter along the way?
Water is the heaviest thing in most packs — 1 L weighs 1 kg (2.2 lb). If your route crosses reliable streams or lakes, carrying a filter and 1–1.5 L between sources usually beats hauling the full day's supply. Check recent trip reports for whether sources are flowing.
Does the weather forecast temperature matter that much?
It is the single biggest factor. Sweat loss rises steeply once air temperature passes about 25 °C (77 °F) — our model adds 50% at that point and 80% in extreme heat, in line with wilderness-medicine guidance. Start early on hot days: the same trail costs less water at dawn.