HYDRATION CALCULATOR
Water is one of the most important things to plan before heading outdoors. How much you need can vary considerably depending on your bodyweight, how long you’ll be active and the conditions you’re likely to encounter.
The Survival Guides UK Water & Hydration Calculator gives you a starting point for planning how much water to carry. Enter your bodyweight, activity duration and the type of climate or environment you’ll be in, and the calculator estimates a suitable hydration volume for your trip.
It is designed as a practical planning tool, not a substitute for listening to your body or making sensible decisions about water availability and conditions.
Calculate Your Water Requirement
Enter your details below to get an estimated hydration volume for your activity.
🧪 Hydration & Water Purification Calculator
Input your expedition conditions to calculate safe hydration volumes and identify the required filtration methods based on your field water source.
Base carrying capacity needed for transit.
The result gives you a useful starting point for deciding how much water you’ll need to carry. You should always consider the actual conditions, your route and whether safe drinking water will be available along the way.
What Affects How Much Water You Need?
There isn’t one amount of water that works for every outdoor activity. Your requirements can change significantly depending on the circumstances.
Bodyweight
Bodyweight is one factor that can be used when estimating hydration requirements. The calculator uses the weight you enter as part of its calculation rather than assuming everyone has the same needs.
Activity Duration
A short walk in cool conditions is very different from spending several hours walking, climbing or carrying a pack.
The longer you’re active, the more water you may need to plan for. If your activity could take longer than expected, it is sensible to allow some additional capacity rather than planning for the minimum possible duration.
Climate and Environment
Temperature, humidity, sun exposure, altitude and the type of activity you’re doing can all affect how quickly you lose fluid.
A route through cool woodland may have very different hydration requirements from a long walk in hot, exposed conditions. That’s why the calculator takes the environment into account when producing its estimate.
Planning the Water You Carry
Your calculated hydration requirement is only part of the planning process.
Think about how much water you can comfortably carry, where you could safely refill and whether those water sources can actually be relied upon. A stream, lake or other natural source should never automatically be assumed to be safe to drink.
If you are planning to use water from a natural source, you need to know how you will make it safe before drinking it.
Water Purification
Natural water can contain harmful microorganisms and other contaminants even when it looks clean.
Depending on the situation, water can be treated using methods such as filtration, chemical treatment or boiling. Different methods have different strengths and limitations, so make sure you understand the method you’re relying on before you need it.
For remote trips, don’t rely on finding water along the route unless you’ve checked that a suitable source is available and have a reliable way of treating it.
Use the Calculator as a Planning Tool
The figure produced by the calculator is an estimate to help with planning. Your actual needs can be higher or lower depending on the conditions and your individual circumstances.
Pay attention to how you’re feeling while you’re outdoors, plan for changes in weather and activity, and make sure you have enough water for unexpected delays.
For longer or more demanding trips, build some contingency into your water plan rather than carrying exactly the calculated minimum.
Keep Building Your Outdoor Skills
Knowing how to plan water is an important part of being self-reliant outdoors.
Explore our other water and outdoor skills guides to learn more about finding, carrying, treating and managing water when you’re away from home.