Optimising Hydration - Why and How to Measure Your Sweat Rate

Sweating is your body’s natural cooling mechanism and is a vital physiological function that prevents overheating, particularly when you are engaging in strenuous activity. Understanding your sweat rate (the amount of sweat you lose per hour) is relatively simple yet very important if you want to tease out those extra performance and recovery margins. 

Each person’s sweat rate is different and yours can be influenced by a few factors such as: the intensity of activity, environmental conditions (temperature and humidity), your level of heat acclimation and the clothes you’re wearing. Other characteristics like muscle mass, body size and the number of sweat glands you have also play a part and go some way in explaining why such large differences are seen between seemingly similar people. 

Understanding your sweat rate forms an integral part of the hydration equation. Pairing this information with your unique sweat sodium concentration can help gauge net sodium losses; enabling you to develop a hydration blueprint that is tailored to your personal needs.

A good understanding of your sweat rate can also help determine how much fluid you should be consuming during and after exercise, which will subsequently boost energy, reduce risk of dehydration or over-hydration, and optimise recovery.

How to measure your sweat rate 

Unlike sweat sodium losses, which remain relatively stable over a lifetime, your sweat rate is highly variable as a result of the above-mentioned factors. As such, measuring your sweat rate should be an ongoing process with regular testing in different conditions as necessary.

To determine your sweat losses, you will need to record the following measures: 

• Your pre-workout weight in kilograms (A)
• Your post-workout weight in kilograms (B)
• Workout duration in hours (C)
• The temperature (and humidity if this is relevant) 
• The intensity or type of the workout (i.e. a recovery session versus a tempo session)

For the most accurate results, try to empty your bladder before taking your pre-workout weight and step on the scales wearing as minimal kit as possible; keeping what you wear the same pre- and post exercise. Also, towel-dry before the post-workout measure to remove as much sweat as you can.

If you consume any food or drink during the workout, this should also be accounted for in your calculations since it impacts your weight. To factor this in, weigh your food and drinks using kitchen scales before and after exercise and calculate the difference (D). 

The calculations are then pretty simple. Every kilogram of bodyweight you have lost* is equivalent to approximately one litre of sweat lost. As such, you can calculate your sweat rate by dividing the number of litres lost by the number of hours of exercise performed.

*Note: If you have gained weight, you are most likely over-drinking during your training session!

Calculation without fluids/food consumed:  

Sweat rate = (A-B)/C

Example: Athlete A weighs 66 kg before (A) going out for a 1-hour (C) run. During the run, they do not eat or drink anything. Afterwards, the scales read 64.5 kg (B). Athlete A’s sweat rate is 1.5 litres per hour (66 kg - 64.5 kg / 1 hour = 1.5 litres per hour) 

Calculation with fluids/food consumed: 

Sweat rate = ((A-B)+D)/C

Example: Athlete B weighs 72 kg before (A) going out for a 2-hour (C) cycle. During the ride, they drink 1 litre of fluid (D). Afterwards, the scales read 70 kg (B). Accounting for the fluid the athlete has drunk, they have lost a total of 3 kg during the workout (equivalent to ~3 litres). Athlete B’s sweat rate is 1.5 litres per hour ((72 kg - 70 kg) + 1 kg) / 2 hours = 1.5 litres per hour)

To make your testing relevant, we recommend testing your sweat rate under similar conditions to those of an upcoming competition or your usual training setting where possible. 

To account for variations in sweat rate, you should also consider conducting tests under a range of conditions (hot versus cold) and intensities (hard versus easy). Of course, the hotter and harder the exercise, the more you sweat. This will provide a more comprehensive understanding of how your sweat rate fluctuates and will assist you in developing an effective hydration strategy regardless of changes in context and/or conditions.

Putting your sweat rate into perspective

Once you’ve calculated your sweat rate, you might be eager to know which category you fall into: ‘high’, ‘moderate’ or ‘low'. 

Sweat rates tend to range from less than 0.5 litres per hour up to ~2.5 litres per hour, though more extreme sweat rates have been observed (up to 5 litres per hour in some cases!). Those who demonstrate higher sweat rates are generally larger individuals engaging in rigorous training or competition under very hot, and usually humid, conditions. Whilst such high rates do exist, it is best to consider them the outliers in the grand scheme of things.

As it stands, no official guidelines outline what constitutes a ‘low’, ‘moderate’ or ‘high’ sweat rate. However, from the available data and research, it’s sensible to assume that a sweat rate of less than 1 litre per hour can be considered as ‘low, 1 to 1.5 litres per hour as ‘moderate’, 1.5 to 2 litres per hour as ‘high’ and certainly, anything above 2.5 litres per hour as ‘very high’!

Putting your sweat rate data into practice 

Once you know your sweat rate, try not to fall into the thinking that you must now drink 100% of these fluid losses during your workout (i.e. a 'like-for-like’ replacement). Often, this is not recommended or practical; especially for anyone with a higher sweat rate (more than 1.5 litres per hour). In this instance, consuming an equal volume of fluid to that lost is rarely feasible and is likely to exceed your natural thirst instincts. 

In fact, drinking too much during exercise can have negative effects such as bloating, discomfort and/or a sloshing sensation in the gut. In extreme cases, it may lead to ‘hyponatremia’; a condition characterised by low sodium levels in the blood due to excessive water intake that can lead to nausea, vomiting, loss of energy and confusion.

As such, sweat rate data should be used to inform a sensible fluid replacement strategy. Depending on your sweat rate and the environmental conditions, most individuals will find  success by consuming between 0.3-1 litre during every hour of exercise. Typically, the more a person sweats, the higher in this range they should drink, but it is very individual and context is key.

Rehydrating after exercise 

Your sweat rate data can also be used to optimise rehydration following exercise. Restoring your hydration status is key to your recovery. This is because rehydration increases your blood volume, which in turn increases the delivery of oxygen and nutrients to your damaged muscles and tissue. 

To effectively replenish fluid losses, it is recommended that you drink 1.5 times the volume of fluid you have lost. 

Putting this into practice, if you end a workout in a fluid deficit of 1 litre (calculated by the difference between your pre- and post-workout bodyweight), to replenish your fluid levels you should drink 1.5 litres across the following 2-4 hours. 

Why 1.5 times the amount? After exercise you will keep sweating for around 30 minutes and you may also lose fluid through urine. Most people forget this, but it’s good practice to drink a bit more than you have lost to ensure you’re rehydrating effectively and are ready to take on the rest of your day.

Most importantly, you may want to be fully rehydrated before commencing your next training session. Keep in mind that adding some electrolytes to your fluids will also increase the rate of rehydration by helping your body better retain the fluid you take on.

Interested in expert guidance? Reach out to our team at The Edge HPL for a chat. We’ve helped hundreds of people optimise their hydration for improved energy, health and performance.