Knowing your individual sweat rate:

A calculator for hot-weather races

Alan Couzens, M.Sc. (Sports Science)

Sep 19, 2017

When racing in hot environments (e.g. Kona :-) a key performance limiter is your ability to dissipate heat. No matter the size of the engine you have built, if you cannot keep said engine cool, it won't be of much use to you in a hot race!

One of the most important ways that our body 'keeps the engine cool' is via sweating. In a previous post, I broke down the relative contributions of the various mechanisms for ditching the heat that our bodies produce and showed how evaporation contributes significantly.

But... evaporation is, of course, dependent on having fluid to evaporate! When the well runs dry, we're down a heat loss mechanism! So, it becomes essential to athletes racing in the heat to not let the well run dry!

To accomplish this aim, it helps to know the 'emptying rate' of the well. I.e. how much fluid is going out each hour, so that you can make an intelligent hydration plan as to how much should go in.

While it is not essential (or in some cases even possible!) to replace all fluid lost, 'pacing' your dehydration and keeping body weight losses within a reasonable range is essential to keeping this heat dissipation mechanism functioning strong all the way through to the end of the race, especially on the run where it is needed most!

Importantly, athletes vary significantly in their sweat rates. Even more importantly, if you over-do it & take in more fluid than you're expending, the results can be deadly (see this post on hyponatremia)

So, we're left with one very important question - how do we assess just how much fluid is going out in sweat each hour for each individual athlete?

A simple way of approximating sweat rate is by measuring the change in bodyweight over the course of an exercise session. While other factors come into play to some extent, e.g. the depletion of CHO stores etc, by far, the largest component of bodyweight changes during exercise is fluid loss.

To this end, I built a calculator below that you can use to assess your own sweat rate via bodyweight change in a given workout (swim, bike or run)

Start Bodyweight (kg)
End Bodyweight (kg)
Duration (hrs)
Fluid Consumed (L)
Urine Produced (L)

Your sweat rate is L/hr

To use simply plug in your weight at the start of the session, your weight at the end of the session along with amount of fluid consumed and amount of urine produced (use a measuring cup) and you will have a good estimate of sweat rate for those conditions.

More practically, you can use the above calculator in a couple of ways....

1. Assessing max sweat rate

Knowing your maximal sweat rate is one useful measure in your ability to deal with the heat. Athletes who are well acclimated to performance in the heat tend to sweat more than those who aren't. If your key race is in the heat, this is an important number to know and develop.

2. Knowing your sweat rate under race specific conditions

Somewhat linked to the first, but not completely so, is knowing your sweat rate for conditions specific to your event. While, for some races, like Kona, we can safely assume that the event sweat rate and your max sweat rate will be one and the same :-) this isn't always the case. Some athletes have a tendency to apply the same nutrition/hydration strategy to events with very different environmental demands & consequently different sweat rates. This is not only suboptimal, but also can be dangerous. If your fluid plan is significantly greater than your sweat rate, the risk of hyponatremia (water intoxication) is significant. Therefore knowing your sweat rate under conditions specific to your event is important to your performance and to your health!

3. Knowing your sweat rate under conditions of heat acclimation

There is a disturbing trend in endurance sports towards tougher and tougher heat acclimation protocols. While acclimation is a key consideration for athletes preparing for races in hot environments, overdoing things can be very detrimental to the most important part - the training, and it can be flat out dangerous! By using sweat rate as a marker of what's 'hot enough' we can keep that balance and keep the protocol/conditions at a level that the athlete can deal with so that it doesn't adversely affect the training output. Specifically, by doing things like monitoring sweat rate under conditions of appropriate (race specific) power sessions, you may find that using simple protocols like short periods of no fan work on the trainer, are adequate to appropriately tax the heat management aspects of an athlete's physiology without detracting from the athlete's ability to do or adapt to the most important part of the whole equation - the training.

Hopefully, you find the above useful in your own preparation for your next warm location event & if that happens to be Kona - congratulations! Set yourself up to perform well and enjoy it!

Train smart,



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