What influences sweat rate? (And how to calculate it)

From the desk of
Robb Wolf
ScienceWhat influences sweat rate? (And how to calculate it)

We all sweat to stay cool. But some folks seem to breeze through their gym sessions with barely a glisten on their foreheads. Others, like me, start sweating just thinking about jiu jitsu class. So what gives?

Sweat rate depends on a myriad of factors. Ambient temperature influences sweat rate, of course, but so does humidity, exercise intensity, exercise duration, genetics, heat acclimation, hydration status, body weight, and even time of day.

I may sweat several liters in a 2-hour jiu-jitsu session. That’s a lot! In fact, humans sweat more on a size-matched basis than any other organism besides horses. Interestingly, the oft-uttered phrase “sweating like a pig” makes little sense because pigs can’t sweat at all.

Understanding your sweat rate helps with hydration planning. If you know that you typically lose X liters of sweat per hour during a given activity, you can bring along an equivalent amount of electrolyte water.

Near the end of this article, I’ll share a method for calculating your sweat rate during exercise. If that’s what you’re here to learn, click that link to skip to the appropriate section. But if you want to dig into the science with me, I’ll first cover why we sweat, a typical range of sweat rates, and factors that most influence our sweat rate. Let’s get started.

Sweat Rate and Thermoregulation

Sweat’s primary function is to keep your body temperature from rising too high (thermoregulation). In order to thermoregulate, the hypothalamus in your brain monitors both internal and skin-based thermoreceptors. Sweat glands are then activated as skin or core body temperatures rise. Simply put, the primary determinant of sweat rate is how much you need to be cooled.

Two main things heat your body: Your metabolism and the environment. When you exercise, metabolic heat increases your core temperature, so you sweat. And if it’s a hot day, environmental heat raises the temperature of your skin, triggering perspiration. High ambient temperatures don’t raise core temperature directly, but they do make it harder to dissipate heat, which means you need to sweat more to stay cool. The key takeaway: the more you need to cool down — from internal or external factors — the more you will sweat.

Typical Sweat Rates

Before we talk sweat rate numbers, understand that the term “typical sweat rate” is a bit misleading. You can give a range or average, but individuals’ sweat rates vary wildly, and are determined by many factors I’ll cover shortly.

With that in mind, here’s what some literature says about sweat rate:

  • A 2017 analysis found that sweat rates during exercise tend to fall between 0.5 and 2.0 liters per hour (L/h).
  • A chapter in Nutritional Needs in Hot Environments: Applications for Military Personnel in Field Operations found that “sweating rates of 1 liter per hour are very common.”
  • A study on 506 athletes under varying conditions found that the average sweat rate was 1.21 L/h.

There are outliers, like Alberto Salazar’s voluminous sweat rate of 3.7 L/h during the 1984 Olympic marathon. And another study on athletes from tropical climates found that a 200-pound man would exceed 2 L/h in sweat, while a 160-pound woman would be closer to 1.2 L/h.

The bottom line? Most exercise-driven sweat rates fall between 0.5–2.0 L/h.

Environmental Factors Influencing Sweat Rate

Let’s cover how the environment affects sweat rate, then move to lifestyle and individual factors.

#1: Ambient temperature

Ambient temperature is the most obvious factor influencing sweat rate. As temperatures rise, skin temperatures rise, triggering sweating. That sweat then evaporates, dissipating heat.

It’s hard to quantify how temperature affects sweat rate because so many factors are in play. One 1967 study assessed the average sweat rates of 7 men after 2 hours of exercise at various temperatures:

  • 87°F: 0.4 L/h
  • 90°F: 0.9 L/h
  • 93°F: 1.6 L/h
  • 96°F: 1.8 L/h
  • 98°F: 2.2 L/h

You’ll notice that going from 87°F to 93°F increased subjects’ sweat rate by 4x. Thereafter, the effect decelerates. We can’t generalize these results for everyone, but it does seem like sweat rates jump considerably in this range.

Another study on elite male soccer players examined sweat rates between 60°F and 84°F. The transition increased sweat rate by about 47% during low-intensity exercise and about 46% during high-intensity efforts. So it seems below 87°F, fluctuations in temperature have a much less pronounced effect on sweat rate. Still, the trend is clear if not obvious: hotter weather means greater sweat rates.

#2: Humidity

Sweat rate increases in humid vs. dry conditions. Why?

First, it’s important to note that your body doesn’t cool off immediately when you sweat — it cools off when that sweat evaporates. Since more humid air has less “room” to hold the water vapor from your sweat’s evaporation, your sweat evaporates more slowly. Inevitably, you lose some “cooling power” when sweat drips off your body instead of evaporating. Therefore, your body must sweat more in humid conditions to achieve the same degree of cooling that you experience in a dry environment.

Beyond this paper on sweaty horses — yes, horses — I couldn’t find much science isolating the effect of humidity on sweat rate. But one study — on humans, yes! — found that fully wet skin decreased sweat efficiency (the ratio of sweat needed to provide evaporative cooling) to 67%. In other words, when the surrounding air is humid, evaporation is less efficient and you must sweat more to compensate. It’s reasonable to infer that humid conditions could increase sweat rate by 20–30% vs. dry conditions.

#3: Airflow

Airflow cools your skin directly and assists in the evaporation of sweat. This reduces your overall cooling requirements and, along with it, sweat rate. One study on 12 heat-acclimated men found that increasing the breeze velocity from 0.2 to 1.1 meters per second decreased sweat rate by about 20% during exercise.

Lifestyle Factors Influencing Sweat Rate

Now we come to lifestyle factors, including movement and hydration. 

#4: Exercise intensity

Athletes (horses included) sweat most during high-intensity efforts in the heat. The harder you go, the more your core body temperature rises, the more you sweat. Strenuous efforts also raise skin temperature, which logically explains why people sweat more while exercising vs. recovering before their core temperature has decreased.

Recalling the study on soccer players. It found that shifting from low-intensity to high-intensity exercise nearly doubled sweat rates, regardless of ambient temperature.

The takeaway is that high-intensity exercise leads to greater increases in core temperature and a significant increase in sweat rate.

#5: Exercise duration

Exercise duration is trickier. Yes, you sweat more in total during longer workouts. But as skin becomes saturated with sweat, sweat glands can become blocked, decreasing sweat rate.

This phenomenon (hidromeiosis) typically begins after 1–2 hours of sufficiently vigorous physical activity, especially in humid conditions. Why care about hidromeiosis? Because it makes it harder to thermoregulate during extended workout sessions. (You’re sweating less due to blocked sweat glands AND less sweat is evaporating.) This bumps up the risk of heat illness, so don’t go for a personal record in a jungle marathon, folks.

#6: Hydration status

The less hydrated you are, the more your body conserves fluids, and the less you sweat at a given core body temperature. Dehydration also raises your threshold body temp to begin sweating.

A 1985 Journal of Applied Physiology study analyzed 9 men exercising under different hydration conditions. A glance at the figures reveals that slightly dehydrated men (1–3% net water loss) had a ~5–15% lower sweat rate, while severely dehydrated men (5–7% water loss) had a ~50% lower sweat rate. When you’re severely dehydrated, the body really wants to retain fluids.

Individual Factors Influencing Sweat Rate

I won’t quantify most of these factors but I’ll give color when possible.

#7: Heat acclimation

Heat acclimation — regimented training in hot conditions — has been studied extensively. Exercising in the heat trains your sweat glands to produce more sweat, lowering your risk of heat illness. Interestingly, heat acclimation also blunts the effect of hidromeiosis (again, blocked sweat glands).

The literature suggests that heat-acclimated athletes sweat between 0–20% more than non-acclimated controls. For a deeper dive into this topic, check out our article on heat acclimation.

#8: Aerobic training

Like heat acclimation, aerobic training unlocks adaptations that increase sweat rate. In one study, endurance-trained men and women had 123% and 144% higher sweat rates than sedentary controls, respectively. Another paper found that older folks had similar sweat rates to younger folks when matched for aerobic capacity. Fitness keeps your sweat glands functioning.

#9: Other Individual Factors 

I’ve lumped several other minor factors that influence sweat rate here:

  • Age. Older adults tend to experience impaired sweat rates, though this is likely often due to declining physical fitness and lack of heat acclimation than age.
  • Body weight. Larger people tend to sweat more to compensate for greater metabolic demands.
  • Skin surface area. More surface area = more sweat glands = higher sweat rates.
  • Biological sex. Women have a significantly greater sweat gland density, whereas men have a greater sweat production per gland. But from an absolute perspective, men tend to sweat more than women because men tend to be larger than women.
  • Genetics. If mom or dad sweat like a Thoroughbred, you probably will too.
  • Clothing. The less your clothing breathes, the more you must sweat to cool off.
  • Medications. Excessive sweating (hyperhidrosis) is one side effect of many pharmaceuticals.
  • Medical conditions. Cystic fibrosis, Addison’s disease, renal failure, heart failure, infections, and many other conditions increase sweating.
  • Menstrual cycle phase. Women usually sweat more during the luteal phase. 
  • Circadian rhythm. Sweat rates tend to be higher in the morning.
  • Sweat gland size. The size of these glands can differ by a factor of 5 between individuals.

Understanding these factors is neat, but you needn’t worry about them to learn more about your individual sweat rate.

How to Calculate Your Sweat Rate

Here’s a step-by-step procedure to calculate your sweat rate:

  1. Weigh yourself naked after peeing. For simplicity and if possible, record your weight in kilograms.
  2. Exercise for 45 minutes to 2 hours. (No need to be naked!) Track how many liters of water you consume. Note that 1 liter of water (or sweat) weighs exactly 1 kilogram.
  3. Towel your entire body dry and weigh yourself naked again in kilograms.
  4. Subtract your post-exercise weight and the weight of liquids consumed from your pre-exercise weight.
  5. Divide your result by the number of hours you exercised, and voila! That’s your sweat rate in liters per hour (L/h).
  6. Note that your sweat rate will vary according to all of the factors we talked about today! If any of these factors change significantly, it’s worth repeating this test.

Now you have an idea of your sweat rate. What’s next?

Replacing Sweat Losses

Don’t worry about replacing every drop of sweat the moment it leaves your glands. Moderate dehydration is normal in athletics, and it doesn’t impair performance. Besides, aggressive rehydration strategies are risky. Athletes who drink beyond thirst (a common practice during marathons) can develop a dangerous low-sodium condition called hyponatremia.

My standard hydration advice is to drink electrolyte water to thirst before, during, and after exercise. Drinking to thirst prevents severe dehydration, and adding electrolytes (especially sodium) helps prevent low sodium levels. After the session, you’ll rehydrate until your body achieves equilibrium.

Knowing your sweat rate is still useful. It informs how much you should pack for a workout session or activity. But don’t worry about 1:1 replacement. Doing the math is no substitute for listening to your body.

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