Common hiking wisdom says you need 0.5 to 1 liter of water for every hour of hiking. 0.5L for moderate hiking at moderate temps, and 1L for more extreme hiking and high, summer time temps. But how much water you need for hiking really depends on your baseline metabolic needs and your hiking sweat rate. And of course, those depend on your age, weight, height, and trail conditions, such as ambient air temperature, amount of shade on the trail, and exertional effort needed to conquer such.
And just to be clear, when referring to water, what I really mean is fluids, i.e. water + electrolytes. Your body will naturally lose sodium when sweating. And that needs to be replaced, just as much as the water it loses through metabolism and sweating.
Quick steps in calculating how much water to bring hiking
I see another article online giving you easy steps on how much water to bring on the trail. It goes something like:
- Figure out how many hours you’re going to hike
- Figure out the trail conditions: ‘easy and cool’ versus ‘hard and hot’
- If easy to moderate hiking, in cool to moderate conditions, bring 500mL per hour; if you’re going to experience hard and hot hiking, take 1L per hour.
- Multiply your hours hiking by your per hour water allotment to get an estimate on how much to take
- Modify this number if you are willing to refill, and filter at known water sources on the trail
- Modify this number if it seems contradictory to your personal water needs
I might add that you should bring extra for contingencies if you’re in the wilderness, away from water sources.
But this method, though simple and easy to remember, doesn’t really accurately factor in your personal metabolic need for water, which you should at least know because its easy to know, even if you don’t know the more critical component to the equation: your sweat rate when hiking.
Related: How to calculate your sweat rate, and Tutorial on how to measure your sweat rate when hiking, and Sweat rate calculator
Calculating your daily metabolic fluid needs
Your daily fluid needs are basically how much fluids you need to take in per day to run your body. This is outside of sweating, as previously indicated, which can add heavily to your daily needs if hiking. The reason this is important is because there is wide variability between hiker’s bodies, and thus wide variability of baseline fluid needs.
When I was getting my Nutrition Support Clinician certification, I noticed the nutritionists and CNSC’s would use multiple methods to calculate a person’s fluid need, then average them all together. So that’s what we’ll do below, using 3 methods.
Step 1 – the Holliday-Segar Method
The Holliday-Segar method actually is designed for sick people, in the hospital, that may have higher fluid and caloric needs than their well counterparts. However as hikers, we are active people, too with higher metabolic needs. Getting out on the trail burns more calories than sitting at home, watching TV. And you need water to burn calories. So I consider this a legit method for grabbing a hiker’s underlying fluid needs.
For an adult:
1,500mL + 20mL(W – 20kg) = daily fluid requirement in mL for people at or under 50 years old; where W = weight in kg
1,500mL + 15mL(W – 20kg) = daily fluid requirement in mL for people over 50 years old; where W = weight in kg
1kg = 2.2lbs
Example using the Holliday-Segar method
Say you’re a 35 year old, female hiker, that is 119 pounds. Convert your weight to kg. 119/2.2 = 54kg. Now plug that weight into the equation for under 50 years old. 1,500 + 20(54-20) = 2,180mL of fluid per day. That’s about 90mL per hour.
Step 2 – The Daily Calorie Method
The daily calorie method is based on the idea that for every calorie you burn, you use 1mL of water to do so. So basically all you need is your daily calorie needs. I’ve designed two hiking calculators that will calculate your daily calories, based on trail conditions, and what you are doing off trail.
I have a day hiker’s total daily energy needs calculator, for day hikers carrying less than 10 pounds in their packs. Just pick the calculator for your sex (there is a male and female calculator), then enter your age, weight, height, and how many minutes you spent on various trail conditions (for example moderate uphill hiking at a little under 2.5mph). You then add if you’re relaxing the rest of the day, or are up and active.
I also have a backpacker’s total daily energy needs, where you enter the same info, but also enter your pack weight and how much you slept at camp.
Example using the daily calorie method
Continuing with our example of a 35 year old, female hiker that is 119 pounds, and let’s further say, 5 foot 4 inches, let’s give her a 3 hour hike to do. This hike will be 1 hour up a moderate slope, 1 hour of flat hiking, and 1 hour down that moderate slope, for an out-and-back hike. Using my day hiker’s total daily energy needs calculator, you’ll find she will use 2,233 calories, if hiking then relaxing the rest of the day. This means her daily water requirements are 2,233mL, or 93mL per hour.
Step 3 – Age based fluids per kilogram nomogram
You can use the table below to find your daily fluid needs based on your age:
| Age | Fluid requirement per day |
|---|---|
| >65 | 25mL/kg |
| 55-65 | 30mL/kg |
| 25-55 | 35mL/kg |
| 15-30 | 40mL/kg |
Example using age based per kilogram nomogram
Again with our 35 year old female, who we’ve calculated to be 54kg, the nomogram says she would need 35mL/kg for her fluid daily needs. This works out to be 54 * 35 = 1,890mL per day or 79mL per hour. A little less than the other calculations.
Average of the 3 methods
You can then take the average of these three methods to find your baseline metabolic fluid needs. Using the numbers from the example, this would be (2,180 + 2,233 + 1,890)/3 = 2,101mL per day, or 88mL per hour.
Metabolic fluid needs run higher during the act of hiking
Keep in mind your metabolic fluid needs will be higher during the act of hiking than they are on average throughout the day. This is because your metabolism is higher during exercise.
So even though our example hiker only needs a baseline of 88mL per hour of fluids for the day, her hourly metabolic need could easily be double that, during her hike. She may end up needing half a liter of water just to run her metabolism for her 3 hour hike. That’s outside of the fluid she needs to replenish what she sweated.
Calculating your sweat rate
Though it’s important to know your total daily fluid requirements, your sweat rate will really define how much water you need, on the spot, while hiking.
Different people have totally different sweat rates, generally anywhere from 0.5L per hour up to 2L per hour. So your really have to measure your own rate under different hiking conditions to know how much you will sweat on the trail. Again check out my articles on measuring your sweat rate, measuring it while hiking, and my sweat rate calculator.
But it turns out athletes who’ve been measured have a median sweat rate of 16mL/kg/hour during exercise. Going by this, our example hiker, who was 54kg, would sweat on average 864mL per hour, when hiking. Now if most of that occurred on the uphill part of her hike, then her average hiking sweat rate could be lower, but just in case, she might want to bring 2.5 liters of water on the hike. After all, as we found before, she is using water just to make energy to push up the mountain, and then walk back down. Some of that water may actually be produced through glycolysis, but that’s a different story.
Preventing Dehydration
Knowing your sweat rate helps you prevent dehydration on the trail. It gives you a good idea on how much water to bring to replete what you’re losing to sweat.
But it’s important to note you may not be able to drink the same exact amount of fluid on the trail, that you’re losing to sweating, in that moment. And that’s okay. All you have to do is drink enough that you don’t lose so much water weight, that you put yourself at risk of dehydration, and/or loss of thermoregulation, i.e. your ability to keep your core temperature stable, and in normal range. Just ‘drink to thirst’ or drink until you’re no longer thirsty.
Then you can drink more after the hike for recovery. And indeed knowing both your metabolic water needs and your sweat rate will give you an idea on how much water, or fluids, to drink to take care of your full day’s needs. You’re baseline needs PLUS how much fluids you lost to sweat would be your total daily fluid needs.
Going back to our example, let’s say it turned out to be a hot hike, and she ended up sweating 2.5L during the hike. Her total daily needs would be that 2.5L plus her baseline of 2.1L per day. So she would need to get in 4.6L for the entire day.
Preventing Overhydration
Again knowing your sweat rate really helps you know the limit on how much you should be drinking per hour, when hiking. But the other factor in preventing overhydration is getting enough sodium when taking in water.
Though everyone’s sodium concentration in their sweat is different, on average it’s probably going to be a little less than a gram per liter of sweat produced, at least if they aren’t heat acclimated. Heat acclimatization requires you be in a high heat environment for 10-14 days, and be living, working and exercising in that heat. At that point you naturally sweat more, but lose less sodium.
So it’s important to bring a sodium source when hiking multiple hours. Electrolyte solutions like Gatorade, or salty snacks like chips, beef jerky, and salted peanuts can provide some sodium to your trail diet, to keep you from getting hypervolemic hyponatremia, i.e. you drank too much water and as a result your sodium concentration in your blood went down.
Further Reading
Thanks for reading my article on how much water to bring for hiking. I’ve built an entire article library dedicated to hiking/backpacking nutrition. Check that out for more hiking nutrition topics! See ya out there!