Hydration Calculator – How Much Water to Drink a Day

Hydration Calculator

How the Hydration Calculator Works

The Hydration Calculator estimates how much water you’ll need on your trek or hike. It factors in your weight, the number of hours you’ll be hiking, temperature, sun exposure, and how hard you plan to go. Here’s exactly how each value is calculated—scroll down for simple tables.


1. Base Water Needs (Weight)

Your base hydration is calculated from your body weight:

  • 0.4 oz per pound (13.2 ml per kilogram), then reduced by 20% for a safe, realistic baseline.
Weight (kg)Base Water (ml, after 20% reduction)Weight (lbs)Base Water (oz, after 20% reduction)
5052811035.2
6063413242.2
7073915449.3
8084417656.3
9095019863.3

2. Activity Level (per hour, after 15% reduction)

For every hour you trek, add the following based on your activity intensity:

Activity LevelAdd per Hour (oz)Add per Hour (ml)
Easy0.8525
Moderate1.750
Strenuous3.4100

3. Sun Exposure (per hour, after 10% reduction)

More sun = more water needed. Add this amount for every hour, based on the sun level:

Sun ExposureAdd per Hour (oz)Add per Hour (ml)
Low1.853
Medium2.780
High3.6106

4. Temperature Adjustment

Hotter temps mean higher water needs.
Multiply your subtotal (base + activity + sun) by the temperature factor:

Ambient Temp (°C)Hydration Increase (%)Multiplier
Below 20°C0%1
20–24.9°C0.051.05
25–29.9°C0.11.1
30–34.9°C0.151.15
35–39.9°C0.21.2
40°C or higher0.251.25

5. Final Safety Buffer

After all other calculations, add a final 10% to cover for any unexpected sweat loss or extreme conditions. The calculator also caps your total intake at 4.5 liters for safety.

Hydration Tips for Hikers

  • Start hydrated: Drink a full bottle before your hike—don’t start on empty.
  • Drink small amounts often: Sip regularly instead of chugging huge amounts at once.
  • Watch your pee: Clear or pale yellow urine means you’re hydrated. Dark = drink more.
  • Carry a backup: Always bring a bit more water than you think you’ll need, especially if you’re unsure about refill spots.
  • Use electrolyte tabs: For long hikes, hot weather, or if you sweat a lot, use electrolyte tablets or drink mixes to replace lost salts.
  • Know your route: Some trails have water sources (creeks, huts, taps). Bring a filter if you’ll refill from the wild.
  • Eat your water: Fruits (like oranges, apples, cucumber) provide hydration and taste great on the trail.
  • Adapt to altitude: You’ll need more water at higher elevations (above 2,500 meters/8,000 feet).
  • Monitor for dehydration: Headache, dizziness, muscle cramps, or confusion are red flags. Stop and hydrate!
  • Pack light, refill smart: If you know where clean water is available, you can carry less and refill as you go.

Hydration FAQ

How do I know if I’m drinking enough during my hike?

Pay attention to thirst, energy, and urine color. If you feel thirsty, low on energy, or your pee is dark, drink more.

Is it dangerous to drink too much water?

Yes, overhydration (hyponatremia) can be risky, especially if you’re not replacing lost salts. Don’t force water if you’re not sweating heavily or hiking long distances. For most hikes, sticking to the calculator’s cap (4.5L) is safe.

Should I drink sports drinks instead of water?

For short hikes, water is enough. For longer hikes, or in hot weather, adding electrolytes (like sports drinks or electrolyte tablets) helps replace salt lost in sweat.

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Catalin Geangos

Catalin is a writer and outdoor specialist who has been traveling in over 35 countries so far. He loves spending time in nature, enjoying mountains and nature adventures, and ultimately inspiring people to travel more. In his time off, he tests, analyzes, and reviews hiking, and other outdoor gear and accessories.
E-mail: [email protected]

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