Measuring your sweat rate when hiking is a little different than your standard sweat rate measurement you can perform at the field or gym. I’ll walk you through some tips and tricks to calculate your hiking sweat rate, which could yield different numbers than when you are cycling, running, or participating in a sustained, max exertion endurance exercise. But first I’ll answer the more obvious questions when it comes to this topic.
How do you measure your hiking sweat rate?
Basically you weigh yourself right before and right after your hike, and also measure how much fluid you drank, and how much urine you peed during your hike. You will also time how long the hike was. These five numbers will let you calculate your sweat rate. I’ve published the equation to use below.
If you need a step by step guide on measuring your data and calculating your sweat rate, read my article about doing so.
If you just want to collect the data and put it in a calculator, I also built a calculator to do so.
How is your hiking sweat rate measurement different than that of other exercises?
When hiking, you’re not participating in a homogeneous task, where you’re sustaining work within a relatively narrow exertional range, like you would if you were running on a flat track or field, rowing for a given period, going through a pre-prescribed amount of resistance reps, or similar.
Instead, you may be working at low exertion on a flat bit of terrain for 30 minutes, then working at high exertion for 20 minutes on steep terrain, then working at low to moderate exertion after clearing a pass and heading downhill for 30 minutes, then stopping to take pictures, etc. Every new hike is going to have its own sweat rate profile for your body, based on the ratio of hard parts to easy parts of the trail.
Your situation, as a hiker/backpacker, is further compounded in that you have to decide how much water you need to bring with you, before the next time you can fill up your bottles or bladders again. Common hiking wisdom says to just bring 1L of fluid for every hour of hiking. But what if you’re acclimated to a high heat environment and actually sweat close to 2L of fluid every hour?
It’s therefore imperative for you to capture good sweat rate data for multi hour, multi terrain hikes to come up with a rage to work with. You will be better served in hydration planning when doing so.
Hiking sweat rate equation
I’ve said this before on my other sweat rate articles, but I’ve spent the time with the CDC’s, and National Athletic Trainers’ Association’s sweat rate equation, and have been able to modify it a little, by emphasizing the order of operations (order of what gets added and subtracted first) with parentheses. Otherwise, it’s easy for you to get confused on when to add or subtract, and come up with the wrong sweat rate.
(pre-hike weight in kg + fluid intake on the trail in L) – (post-hike weight in kg + urine output on the trail in L) = sweat volume in L
Sweat volume in L/time spent hiking in hours = hiking sweat rate in L/h
If you didn’t drink anything after your pre-hike weighing or before your post-hike weighing, use a zero for your ‘fluid intake on the trail’ in the equation.
If you didn’t urinate after your pre-hike weighing or before your post-hike weighing, use a zero for your ‘urine output on the trail’ in the equation, regardless of how full your bladder was at the post-hike weighing.
Tips for collecting your sweat rate data when hiking to find your sweat rate
Multiple conditions affect sweat rate
Keep in mind your sweat rate for a given hike will be affected by multiple conditions. How sunny or overcast is was that day. What the ambient temperature was. So you may have to perform multiple runs on multiple days to get a good range to work with. But you probably do want to know the upper extreme of your range, so pick a hot and sunny day to grab that data.
You will sweat more when acclimated to the heat
If it’s just now starting to get to summer conditions, you should know that it takes about 10 to 14 days for your body to acclimate to a higher temperature environment. And at that point you will sweat more. You’ll also retain more sodium in your body and sweat less of it out.
This is also important to know because if you only calculated your sweat rate during the colder seasons, you may be underestimating your summer sweat rate.
Start you hike hydrated
You will sweat more when perfectly hydrated, so make sure you start that way, so you don’t get an underestimate of your sweat rate.
National Athletic Trainers’ Association pre-excercise hydration protocol
According to NATA, you should consume 500-600mL of water, or sports drink, 2 to 3 hours before your hike, and another 200-300mL of water, or sports drink, 20 to 30 minutes before the hike.
Source: National Athletic Trainers’ Association Position Statement: Fluid Replacement for Athletes
Medscape’s pre-exercise hydration protocol
Medscape recommends that you slowly drinking 5-7mL/kg 4 hours before, and another 3-5mL/kg 2 hours before, if you are not producing enough urine, or your urine is darker than what is consistent with good hydration. Adjust this fluid intake to where you are producing lightly colored urine color. (See a urine color chart.)
Make sure you eat something salty and get some electrolytes if just hydrating with water. The median sodium loss through sweat for an exerciser is a little under a gram per liter of sweat; maybe less if you are acclimated to the heat.
Find a parking spot near the trailhead
You’re going to want a parking spot near your trailhead so that you can bring a scale in your car to weigh yourself right before and right after the hike to get the most accurate measurement.
Weigh yourself consistently
Whatever clothes or gear you had on during your pre-hike weighing needs to be on you for your post-hike weighing. If you come back after the hike, and your base layer is saturated with sweat, then momentarily take it off, and wring it out, before putting it back on to weight yourself.
And it’s best not to have a pack on, or water bottles, etc. when doing your weighings, to get the best results.
Have a clear plastic urinal with graduated markings in your pack
Because you’re doing a full hike, and not just a small trial, which may not provide as good results, you’re probably going to have to urinate at some point. And you need to know exactly how much you urinated, so have a urinal with graduated markings with you in your pack.
Pee into your urinal. Take the measurement. Then dump it out. Take the measurement down to the nearest milliliter marking on the urinal.
Follow the trail rules. Bathrooms are still the best place to do the measurement, as if they have running water, you can rinse your urinal out immediately after.
Urinate right before your first, pre-hike weighing
Urinating right before your hike means you may be able to get away with not urinating on the trail, and having to deal with the measurement. Or at least it could allow you to get away with just one measurement, as opposed to multiple measurements. Make it easy on yourself and pee right before your pre-hike weighing.
Know your fluid intake volume
Have a method of measuring down to the milliliter how much fluid you drank during the hike.
One easy method is to just use commercial water bottles. Look on the side of your water bottle for the published volume, then finish the full bottle before getting back on the scale at the end of the hike. That way you’ll know exactly how much fluid you took in.
Another method is just weighing your water bladder before and after, separately from when you are weighing your body. 1L of water is equal to 1kg of water. So if you started with say, 2 liters, and it weighed in at 2.1kg (because of the weight of the bladder plus water), and at the end of the hike it weighed 0.6kg, then you know you drank 1.5 liters of water during the hike. 2.1 – 0.6 = 1.5L
Measure you time in minutes but convert to partial hours
Note the time between your pre-hike weighing and post-hike weighing in minutes. But then convert that number to hours and partial hours, out two digits after the decimal mark. To do the conversion, just divide your minutes by 60.
Make sure to add and subtract your measurements in the right order
- Add your pre-hike weight to your fluid intake on the trail
- Subtract your post-hike weight
- Subtract your urine output volume
- Divide that resulting number by your hours spent on the trail between weightings
- The resulting number is your sweat rate in liters per hour
Further Reading
Thanks for reading my article on measuring your sweat rate when hiking. Check out my other hiking nutrition topics for ideas on hiking nutrition timing, calorie calculators based on trail conditions, etc.