H2–Oh, You Got This: Trail Running Hydration Basics

The basics of trail running hydration, written for beginner to intermediate trail runners.

By on April 18, 2017 | Comments

Once trail running feels a bit more comfortable, or maybe you have race on the schedule, you may find yourself wanting to hit the trails for a longer duration of time. More time means carrying your own resources and that means figuring out how to do that. You may be used to stashing water along your road-running route. Or you may sometimes carry a big backpack full snacks, maps, and water bottles when you go hiking. When you trail run, though, you want to feel comfortable and svelte while still having to access to everything you need. This month, we will talk about what you need or at least need to consider in order to stay hydrated for a run up to two hours in duration. This article, like all the articles in this column, is geared toward newer trail runners, whether you come from road running, other outdoor activities, or are new to running altogether.

Basic Principles of Trail Running Hydration

Two hours on the trails is a considerable amount of time—especially if it’s hot and/or humid. When it comes to trail running, it’s long enough that you’ll need water to stay happy and healthy. If you don’t drink water for that length of time, most people will begin to see a decline in their performance and, in some cases, experience thirst and symptoms of dehydration. As a result, for any run that is longer than 60 to 90 minutes, you will want to consider carrying water with you to maintain a healthy hydration level and optimize your performance. Much shorter than this, and you can likely rehydrate in a short period of time after your run is over. How much water do you need to carry and drink? The answer is simple: it depends.

Requirements for hydration vary widely based on the individual. Very generally, people lose about 32 fluid ounces/0.95 liters of sweat/water in volume—which weighs 2.1 pounds/0.95 kilograms—for every hour they exercise. This can and will be affected by a lot of factors, including the temperature, humidity level, how dehydrated you are when you start, your level of fitness, and more. In order to determine your own sweat loss while running, weigh yourself before you run and then again after. Calculate the difference between your pre and post-run weight, add in the weight of any fluid you ingested during your run, and you will know your sweat rate:

Your weight loss + Weight of any fluid intake = Your sweat rate (in water weight)

Of course, you’ll need to convert that weight back to its representative volume of water, an easy conversion—and there are online calculators that can do it for you, too. Simply, 1 fluid ounce of water equals 0.065 pounds of water and 1 liter of water equals 1 kilogram of water.

To determine how much you would need to drink each hour to counteract your fluid loss, divide your sweat rate (now converted to water volume in your favorite unit of measurement) by the number of hours you will be running. The difference gives you a rough idea of how much water it takes each hour to keep you completely hydrated while running:

Your sweat rate in water volume ÷ Number of hours running = Volume of fluid needed each hour

If you track this change every time you run, in a training log perhaps, you will be able to recognize patterns and start to understand what it takes to keep you hydrated in various types of conditions and on various types of runs. Studies show that performance starts to dip when a person has lost 2% or more of their body weight in sweat. For a 120-pound/54.4-kilogram female, losing 2.4 pounds/1.1 kilograms) will hinder performance. Knowing that the average person loses about 2 pounds of water for every hour he or she exercises, it makes sense that an individual can only run about 60 to 90 minutes without hydrating before performance begins to suffer.

Stephanie Violett - 2016 TNF50 Mile - handheld water bottle

Stephanie Howe Violett using a handheld water bottle to carry water during the 2016 The North Face Endurance Challenge 50 Mile Championships. Photo: iRunFar/Meghan Hicks

Although it may seem counterintuitive, it may not be prudent to replace 100% of your fluid loss while running. Drinking too much can cause serious health emergencies and reduce your ability to perform. Replacing all fluid lost may also cause gut discomfort and bloating, especially if you are running at a challenging-for-you effort. The goal is to stay ahead of that 2% loss so that your performance does not suffer and you do not become dehydrated. Once you know your sweat rate, you can determine how much water you need to carry on a run. To demonstrate this, let’s assume a sweat rate of 32 fluid ounces/0.95 liters per hour. If you are planning to go for a two-hour run, the math is pretty easy–multiply your sweat rate by the number of hours you plan to run to calculate the amount of water it would take to replace 100% of your fluid loss. In this case, we would need to ingest 64 ounces/1.9 liters in order to completely replace the fluid lost while running. Here’s the equation:

Sweat rate (in water volume) x Number of run hours = Total fluid replacement needed (in water volume)

Remember that you don’t necessarily have to drink and carry 100% of the fluids you expect your body to lose during your run. Let’s shoot for a 1% fluid loss during our two-hour run, in our example. For a 120-pound/54.4-kilogram female, that is 1.2 pounds/.54 kilograms–or about 18.5 ounces/0.55 liters that we can afford to lose. Subtract that number from your total fluid replacement needed to calculate the amount of fluid you need to carry with you to ensure your performance doesn’t suffer. In this case 64 ounces/1.9 liters – 18.5 ounces/0.55 liters = 45.5 ounces/1.35 liters. In equation form:

(Sweat rate x Number of run hours) – Affordable fluid loss = Fluid volume needed for your planned run

There is a case to be made that you should carry 100% of your total fluid-replacement needs. You may be starting your run a bit dehydrated, or you may become lost, sidetracked, or experience an unplanned change in weather or temperature. Each of these variables would require a potential need for more fluid intake. If you aren’t certain of your route or are concerned about changing variables, you may want to consider carrying the extra fluid. If you are fairly certain of the route and your needs on that route, the planned 1% variation in need should be an adequate buffer.

All of this said, our current scientific understanding of hydration can be boiled down to the basic principle of drinking when you are thirsty and not drinking if you are not–that the thirst mechanisms of the human body do a good job of helping us self-regulate our hydration. The guidelines we describe here are meant to help you understand basic ideas about sweating during endurance running and they should also help you estimate how much fluid you may want to carry with you when you are venturing into longer runs up to about two hours in length. Once you are out there, and pretty much all the time, follow your thirst.

How to Carry Your Fluids

Now that you know how much you will want to consider drinking while you are running, let’s figure out how to carry that with you. There are a variety of ways to carry fluid. Below you will find a very basic analysis of the types of packs on the market. As with any gear, there are advantages and disadvantages to every piece of gear. I will try to explain these based on my experiences on the trail as well as my experience in running retail. We will start with the most minimal solution and progress to the most maximal.

Water Stash

This option is typical of road runners and a great option if you will be crossing the same point multiple times on your run. Prior to beginning your run, stash some water in an inconspicuous place. Each time you pass that place, take a drink of water. You do run the risk of needing water away from your stash and not having it. If the time or distance between passes is short enough, this is a great option as you have an unlimited capacity for both quantity and variety. Pair this with a handheld water bottle for a fool-proof option.

Handheld Water Bottle

You can typically carry about 12 to 20 fluid ounces/0.35 to 0.6 liters in a handheld water bottle. If you have one on both hands, you double your carrying capacity. There is very little storage capacity for other gear so if you are planning to carry much more than a phone and a few gels, this option likely won’t work. These can often feel like running with a hand weight and over the course of several hours can cause fatigue in the arms. If you like to run with minimal gear or you have the option of refilling throughout your run, this is a great option.

Ian Sharman - 2016 Western States 100 - handheld water bottles

Ian Sharman running the 2016 Western States 100 with two handheld water bottles. Photo: iRunFar/Meghan Hicks

Portable Filter/Straw

A somewhat recent introduction into trail running is the filtered straw. This is a wide-mouth straw with a filter on one end that allows you to dip the straw right into a source of water and drink directly from it without running the risk of waterborne illness. If you want to carry the absolute least amount of gear possible, a straw filter is the best option. It is lightweight and the only capacity limitation is the number of times you come across a water source. If you are running in a desert where water is scarce, however, this may not be the best option. This will also add time to your run as you stop to drink versus sipping from a bottle on the go.

Waist Pack

You can typically carry about 20 to 30 fluid ounces/0.6 to 0.9 liters around your waist. This can be carried in one big bottle or several small bottles. If you prefer multiple fluid choices (sports drink, coconut water, and regular water as examples) this may be a great option for you. Some women may dislike this option as the running belt must be worn at the waist, not the hips, in order to prevent bouncing. This doesn’t seem to affect men as much. One big advantage of this option is that it doesn’t hold in heat as much as a vest or backpack will, as it only covers a small part of your torso. It also doesn’t hold as much gear as a backpack or a vest. If you are planning to carry a jacket, gloves, or other gear, this may not have enough storage capacity.


This seems to be the most popular solution out there and, as a trail runner, you will find an abundance of choices in terms of style and carrying capacities in this category. You can carry anywhere from 20 to 100 fluid ounces/0.6 to 3 liters in a vest in just a hydration bladder with options for adding additional bottles to carry a variety of beverages in other pockets. In addition, you will also have more carrying capacity for clothing, trekking poles, and other gear you may want.  A vest is different from a backpack in its construction and fit on the body. A vest is designed to sit close to the body while utilizing the straps and pockets for easy access to and distribution of gear both on the front and the back of the body. It should bounce very little.

If you are starting with shorter runs in hopes of increasing to longer runs, this is a consideration. If you plan to run recreationally with two to three hours as your ceiling, you will want to look for a small vest or look to a less-complex option. If you run hot or have a high sweat rate, bear in mind that this option traps the most heat as it covers much of your front and back torso.

Moab trail running - running vest

An example of a running vest, used to carry water and extra supplies for trail running. Photo: iRunFar/Meghan Hicks


A backpack is similar to a vest in its carrying capacity but is different in the construction. Instead of using a vest-like fit that is snug to the body to prevent bounce, a backpack will use straps to support the weight using your torso and/or hips, and most of the gear is carried on your back. It will also be more versatile in its use. If you like to mountain bike or day hike and you only want one piece of gear, this may be the best option for you.

Kinds of Fluids to Hydrate With

Once you determine which type(s) of contraptions you would like to use to carry your hydration, the final step is to determine how best you would like to hydrate. Of course, water is the go-to option. However, it is no secret that there are a variety of poisons out there from which you can pick. Here are a few things to consider adding to your water to make it more potent.


Electrolytes are forms of salt that are required for your nerves to work properly. Nerve function is vital to muscle function so without these salts, your performance can suffer. There are seven main electrolytes and some of them are more vital than others to runners. Electrolytes leave your body in your sweat, and you may (or may not) need to ingest more of them when you run to maintain the proper balance of water and electrolytes inside your body. This article won’t go into the nitty gritty of electrolytes but it is here to make you aware of the concept and to inform you that adding them to water is a common way to replace the electrolytes that come out in your sweat. It is common to use a powder, tablet, or an additional liquid to achieve the water-electrolyte balance that you need. I recommend finding a nutritional source that you trust, such as a local running store’s experts, an online source you use regularly (these two articles here on iRunFar provide an in-depth look at hydration and electrolyte requirements for long-distance running), or a book (like Where the Road Ends: A Guide to Trail Running by iRunFar’s editors) for advice in this area. Then experiment on your shorter runs, 45 to 90 minutes, to find something that feels right before taking it out on longer runs.


I know it may come as a shock to you but you burn calories while you run! Your body only stores a short supply of easy-access calories before it starts to turn to fuel sources that can impact your performance. Some people choose to add calories to their water to replenish their quick-access supply of calories as they are running so they can keep up the good work. There are many options for replacing calories and every option will have its reasons for why the formula is perfect. If this sounds interesting to you, you may want to read this article [broken link removed] here on iRunFar to learn more. Also, check back here next month because we will talk about the basics of nutrition during long-distance running.


If something tastes good, you are more likely to drink it, and that’s why there’s a case for running with flavored water—especially if you are someone who has a hard time drinking while running. If you choose to add just straight flavor, be conscious of what else is going in there with you as some artificial sweeteners may cause gut discomfort. If you are truly going for flavor only, consider adding some fruit to a pitcher of water the night before your run. Before you pour the water into your pack, take out the fruit. This will give you the flavor without any calories or bacteria build-up in your bottle or mouthpiece.

Now that you know how much fluid you will need to drink while running and you are familiar with how to carry it and what you can put in it to help you drink and perform, it’s time to put your newfound knowledge to the test. If you are worried about creating a hydration plan, just remember, H2–Oh, you got this!

Call for Comments (from Meghan)

  • Have you ever tracked your sweat rate for running or in other sports? How has it varied based upon your condition or the conditions through which you are running?
  • Do you already use some sort of gear to carry your water? What do you personally like and what hasn’t worked so well for you?
  • Do you have any other helpful hydration tips that you can share?
Luis Alberto Hernando - 2016 IAU Trail World Championships

Luis Alberto Hernando using a running vest during the 2016 IAU Trail World Championships. Photo: iRunFar/Meghan Hicks

Rhielle Widders
Rhielle Widders is passionate about introducing her favorite sport to newcomers. She created and directed the Park City Trail Series, a four-race series designed to get people running on dirt, from 2010 to 2014. When she isn’t in Park City, Utah, where she lives, you will find her traveling to try out new dirt. Follow her on Instagram.