How To Calculate Your Sweat Rate - Camp Hike Live California

Knowing how to calculate your sweat rate helps you in planning your hydration strategy for whatever activity you’ll be participating in, that involves significant sweating. For example, on this site, we’re concerned about hiking, camping, and backpacking, so knowing our sweat rate helps us plan how much water we have to carry for a certain hike or backpacking trip.

But for other activities, like cycling, running, sports, or resistance training, knowing that rate helps you determine maintenance fluid/carbohydrate repletion, and could also help you determine when that 2-4% loss of body weight will occur, which may correlate with a reduction of your performance, and/or dehydration, and loss of thermoregulation.

Equation for calculating sweat rate

My equation for calculating sweat rate is a bit of an improvement over the Center for Disease Control’s, or the National Athletic Trainers Association’s, in that I add the parentheses to the equation to help you determine the proper order of operations when solving. That way you’re not accidentally adding when you should be subtracting or visa versa.

As a reminder, you must solve the problems inside the parentheses first, then solve for the remaining steps.

(pre-exercise weight in kg + fluid intake during exercise in L) – (post-exercise weight in kg + urine micturated during activity in L) = sweat volume in L

Sweat rate = sweat volume in L / time in hours between your two scale weighings

Los Padres National Forest flat hiking for my sweat rate calculation — Hiking at Los Padres in the 100°F weather. I had a 58L backpack on. Sweaty conditions indeed!

Step by step instructions to calculate your sweat rate

1. Plan a relatively short activity

Plan for a relatively short bout of your activity, say around 1 hour, so you don’t have to worry about collecting, and measuring your urine volume during that activity.

2. Start fully hydrated

Your sweat rate is a function of how hydrated you are. You will sweat more when fully hydrated. This is how you want to start any exercise activity, so be consistent and start this one hydrated as well.

3. Urinate right before you weigh yourself

If you urinate right before you weigh yourself for your pre-excersie weight, chances are you won’t have to urinate again till after your post activity weighing, which again means you don’t have to keep track of your urine volume during the activity.

4. Your scale usage should be consistent

Whatever you’re wearing, when you first weigh yourself before your exercise, should be what you’re wearing when you reweigh yourself after your exercise.

If you have a drenched base layer after your exercise, you should temporarily take it off, wring as much sweat out of it as possible, then put it back on before your final weighing. Even better is just weighing yourself without the base layer before and after exercise, to keep it more consistent.

5. Weigh yourself right before your exercise and take note of the time

Weight yourself directly before your exercise. Don’t weigh yourself at home then go to the gym, hiking trail, or track. Bring your scale with you to wherever you are, then weigh yourself. Make note of your weight.

Also take note of the time you weighed yourself, and directly begin to exercise.

6. Know your fluid intake amount during exercise

If you take any fluids in during your exercise, you have to precisely know how much you took in. That means if you drink part of a Gatorade, or sports drink bottle during your exercise, you should finish it before getting back on the scale, post-exercise. That way, you just have to record the volume printed on the bottle to know how much you drank.

For example, if you drank half of a 20oz. (591mL) sports drink bottle, during your exercise, just gulp down the rest of it before weighing yourself again, during your post-exercise weighing. That way you know you took in 591mL of fluid.

Also it will be important to know this volume in milliliters, or liters, as opposed to ounces. But you can do the conversion from ounces to liters, to calculate your sweat rate, either way.

If you don’t want to mess with adding fluid back in on the equation, then go hardcore and don’t take in any fluid until you’ve completed your post-exercise weighing.

7. Weigh yourself directly after exercise and take note of the time

Towel off any excess sweat you have on your body and weigh yourself. Don’t wait and go home, where you could accidentally drink more, or kick off your shoes, or whatever. Again it’s best to have your scale at your site of exercise and use it directly afterward. Note your weight for your calculation.

Also note how long in hours you exercised. Basically you’re noting the time between your first weighing and your second weighing.

8. Convert you pre and post exercise weight to kg

If you didn’t record your weight in kg, this is the time to do the conversion. Divide your weight in pounds by the number 2.2. That will give you your weight in kg. Convert both your pre and post exercise weighings to kg.

9. Convert your fluid intake to liters

If your fluid intake was measured in mL then divide it by 1000 to put it into L format.

If your fluid intake was measured in US fluid ounces, then divide it by 33.814 to put it into L format.

Use Google to convert your fluid intake to liters if all else fails. For example if you drank 25oz. of fluid, go to the Google search bar and type in ’25oz. = L’ and Google will do the calculation for you.

10. Add your pre-exercise weight in kg to your fluid intake in L

For example if your pre-exercise weight was 70kg and your fluid intake was 0.591L then add those two numbers together for a total of 70.591kg. The reason this works is because 1 liter of water or sports drink weights 1 kg.

If you took in no fluid, then this number will simply be your weight.

11. Subtract your post exercise weight in kg from the number you just calculated to get your sweat volume

Take your step 10 number you just calculated, and subtract your post exercise weight from it. This number is your sweat volume in liters.

Continuing with the example, if your post exercise weight was 69.34kg then subtract that from the 70.591kg number from step 10. The final result will be 70.591 – 69.34 = 1.251 = 1.25kg = 1.25L of sweat.

12. Convert your time of exercise to hours

If you haven’t already done so, convert the amount of time you exercised to hours. If you recorded your time in minutes then divide that by 60 to convert it to hours.

For example if you exercised 68 minutes then the number of hours you exercised is 68/60= 1.133 hours.

13. Divide your sweat volume by your hours to calculate your sweat rate

In the example, you sweated 1.25L of sweat over 1.133 hours. So divide 1.25L by 1.133h to get your sweat rate. 1.25L/1.133h = 1.1L/h

Your calculated sweat rate would therefore be 1.1L per hour.

Things to know about your sweat rate calculation

Your sweat rate is variable based on exercise conditions

Multiple conditions you are exercising in determine your sweat rate, at the time of gathering your data:

the ambient air temperature
your exertion level
were you in the sun or in the shade?
were you hydrated before exercise, or were you dehydrated?
if you were in a high ambient temperature environment, were you acclimated to that environment? It takes 10 to 14 days to get used to a high heat environment, and your body will acclimate to it by increasing its sweat rate

This means you will likely have to do multiple runs of your sweat rate calculate for different conditions to know what your range is.

For example, I’m most concerned about my sweat rate in the summer time at my local day hiking areas. I might hike an hour in the shade, on easy terrain, and record my data, on a nice summer’s morning. Then a few days later, repeat the calculation, but this time hiking 1000 feet up a mountain, at full exertion, in direct noontime sunlight on a hot day. This will give me a range to play with.

What is a normal sweat rate?

A normal sweat rate for an athlete is between 0.5L/h to 2L/h, with the median being 1.2L/h. But this accounts for athletes of all body sizes. When factoring in weight of the athlete, then the normal sweat rate would be somewhere between 10 and 25mL/kg/h, with a median around 16mL/kg/h, or 1.12L/h for a 70kg person.

The source of these numbers is the study, Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability, which seeks to answer the question of what a ‘normal’ sweat rate is.

The study also says a typical sodium loss through sweating for athletes is in the 10 to 70mmol/L (0.23 to 1.61g/L), with the median being 36mmol/L, or 0.83g/L of whole body sweat. Gatorade typically has 0.46g/L of sodium, as a reference point.

Typical sweat rates for athletes, both in absolute terms for athletes of any body size, and relative terms, corrected for weight of the athlete. Source: Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability

Your sweat rate helps determine your needed hydration, but how do you interpret your sweat rate calculation data?

Once you’ve gone through your sweat rate calculation several times, and have a typical range for your body, how do you use that range to properly hydrate, during your next excursion?

First, the higher extreme of your sweat rate can help you find your time to dehydration. For example, let’s say that you’ve found through experience that once you’ve lost about 2% of your body weight through sweating, your performance level drops, and once you’ve lost 4%, you experience the typical signs and symptoms of dehydration to the point you have to quit your activity.

Now if you know that you have the capacity to lose 1L/h of sweat at high exertion, which is 1kg/h since 1L of sweat weighs 1kg, and you know you are normally 70kg, then you know your performance is diminished at a 2% loss of weight, or 1.4kg. That means if you sweat at that more extreme end of your sweat rate range, you will experience performance loss at 1.4 hours into your activity, if you do not hydrate during such.

2% of 70kg = 1.4kg; 1.4kg / 1kg/h = 1.4 hours

Maintenance carbohydrate and fluid intake based on sweat rate

You don’t necessarily have to replenish your entire fluid deficit during your activity though. All you have to do is take in enough to avoid that 1.4kg loss before your activity has concluded. And also take in enough to keep your sweat rate from diminishing, reducing your ability to thermoregulate. Indeed you may not even be thirsty enough to perfectly match whatever sweat you’ve lost during your activity.

But if you’re in full exertion mode for an hour or more, your glycogen stores are going to become taxed. And according to the International Society of Sports Nutrition, you’ll need to be taking in fluids and carbohydrates in solution, via a sports drink, to alleviate that glycogen deficit.

International Society of Sports Nutrition Recommendation on maintenance carbs

The ISSN recommends consuming carbs at a rate of 30-60g per hour, by way of a 6-8% carbohydrate solution (Gatorade is a little over 6%) using 6 oz. every 15 minutes. ISSN’s specific stance is 6-12 oz. every 10-15 minutes, but applying the maximal possible dosage under this formula may result in a overloading of carbs, at least by their stated dosing. It’s also important to have one with some sodium in it.

Going back to our example, say you were trying to take in 36g of carbohydrate per hour via a Gatorade sports drink, which has 6.1g of carbs for every 100mL of fluid. In order to get 36g of carbs, you have to drink 591mL of Gatorade (1 x 20oz. bottle) over 1 hour’s time. And let’s also say your activity typically lasts 2 hours.

Over two hours, to satisfy your maintenance carbohydrate needs, you’ve taken in 2 x 591mL of Gatorade, or 1.2 liters of fluid. You’ve lost 2 liters from sweating. This means you’ve only accrued a 0.8L deficit over that 2 hour period, but have avoided the 1.4L loss that would hurt your performance, much less the 2.8L loss that would take you out of the activity, based on your personal experience.

Hiking Joshua Tree, CA in the summer. Another high sweat zone in CA. Bring your water and your electrolytes!

Drinking to thirst and repleting your sodium lost through sweating

Maybe you’re not working near full exertion at your activity, like me when hiking. Because I’m not in a sustained bout of exertion, when not climbing at a high grade, I’m not putting my glycogen stores at risk and don’t need as much maintenance carbs to get me through my hike.

But it may be hot, and so I still need lots of fluids and sodium repletion. I don’t therefore need to bring Gatorade, but instead enough water to keep me inside my performance weight, and a salty snack to keep my sodium levels in my blood at a consistent level, to avoid hypervolemic hyponatremia, which can have neurological consequences, like seizures, or worse.

Knowing my sweat rate prevents hyponatremia

In this case, I’m just going to ‘drink to thirst’, or drink when thirsty to replenish some of my fluids. And thus knowing my sweat rate helps me stop myself from possibly drinking too much fluid, and putting myself at risk of hyponatremia.

I know if I typically sweat 1 liter per hour climbing uphill in hot weather, but once I get to the top of the mountain, chill for bit, make my way back down, and only sweat 1/3rd a liter per hour at that point, that I shouldn’t drink a liter on the way up, then drink another liter when I’m cooling down up there, then drink another 1/2 liter on the way down.

Or at least I know not to do this without taking in a few grams of sodium to avoid a sudden reduction in my sodium levels. Often it’s not your absolute sodium levels that are important, but how fast they change over a day’s time, that can get you in trouble with regard to the neurological symptoms of hyponatremia.

In my example, my sweat rate calculation allowed me to avoid too much hydration when drinking to thirst, as hikers sometimes do, but also let me be aware of the need for sodium repletion so that I could be ready with a salty snack.