Sweat rate calculation
Estimating your sweat rate can be a useful exercise when you’re trying to figure out how much and what you need to drink (in terms of fluids and electrolytes) during training and events.
Sweat rate varies considerably from person to person and it can also vary quite a lot for any given individual because things like how hard you’re working, the ambient temperature and humidity, your clothing choices, genetics and heat acclimation status all play a role in determining how fast and how much your body perspires.
So, sweat rate measurement is something that should ideally be done on a number of occasions and in a range of conditions if you want to extrapolate the results to help you in specific contexts, like planning your likely hydration needs for an upcoming race.
Here’s a guide for collecting the data you need to get a reasonably accurate idea of your sweat rate. And then some ideas for what to do with the data once you have it.
Equipment you'll need to calculate your sweat rate
1. An accurate set of weighing scales
2. A dry towel
3. Possibly a small, accurate kitchen scale to weigh your water bottles (if you’re planning to drink during the sessions where you’re measuring your sweat rate)
How to calculate your sweat rate
1. Go for a pee and then record your body weight, ideally with no clothes on. That’s A.
2. Perform your session (or event) and record exactly how much you drank.
This is easy if you drink from a single bottle or two.
Weigh your bottles before (That’s X) and after (Y) and record the difference. 1 gram = 1 millilitre.* (Z)
* If you use different measurement units, e.g. fl oz, you’ll need to convert all values to litres (via Google!). Make sure all units are in kg or litres
3. After exercise, towel yourself dry and then record your weight. Again no clothes on is best, as your clothes will hold some sweat. That’s B.
4. Now subtract your post-exercise weight (B) from your pre-exercise weight (A) to get the weight you lost during the session.
Weight lost (C) = A-B
5. Also subtract the weight of the bottle(s) before (X) and after (Y) to obtain the amount you consumed (Z).
Volume consumed (Z) = X-Y
6. You can now calculate your sweat rate…
(C+Z) / time.
Note: It’s best to aim not to pee during these sessions as this can skew the results. However if you do have to go, it’s not a bad estimate to assume a fluid loss of ~0.3l (300ml) per bathroom stop.
You then just need to subtract 300ml (0.3kg) from your estimated sweat rate at the end.
I’d generally recommend trying to limit data collection to sessions lasting ~1 hour to 2 hours. This is because anything shorter than that can be prone to errors of multiplication in the equations and anything longer can start to be skewed by things like fuel utilisation (you inevitably burn glycogen during exercise and this can affect your body weight results too).
To make the above calculations really easy, you can collect all of the data into this spreadsheet (link to Sweet Loss Calculator) along with some relevant notes about your session (mode of exercise, duration in minutes rough intensity and temperature, whether it was outside or inside etc).
The sheet will then spit out a % bodyweight loss figure for that workout and also an estimate of your sweat rate expressed in litres per hour. Like this:
You can record numerous sessions in the sheet and this will help you to get a handle on what kind of range of sweat losses you see for different sports, in different weather conditions and at different intensities.
If you do this enough, you’ll become very good at ‘guesstimating’ your sweat rate in the future; a dinner party trick of dubious value if nothing else!
Plus, if you’ve also had an Advance Sweat Test with us, you can add your sweat sodium concentration data in and it will estimate your hourly and total sodium loss numbers too, helping you get a good idea of what you’re losing so you can more adequately replace it.
What to do (and not do) with the data
Once you’ve collected a reasonable amount of sweat rate data, the obvious question is what can you do with the numbers? And unfortunately, the answer is not as straight forward as many athletes would like it to be.
Often you’ll see athletes performing these kind of sweat rate tests and then proceeding to work out that based on, for example, a rate of 1l/hr when running hard that they obviously need to drink 1l/hr when running (i.e. to replace 100% of their losses).
There’s a nice simplicity to the concept of ‘1 out = 1 in’ and for a long time it was assumed that 100% sweat loss replacement during exercise was likely to deliver an optimal performance.
But time and research has shown that this logic is fundamentally flawed (the maintenance of performance in an exercising body is way to complex) and few, if any, credible sports scientists or nutritionists would advocate ‘like for like’ replacement of sweat losses during activity these days.
In fact, as 100% replacement often requires drinking beyond the body’s natural thirst instincts, it can be very dangerous. It carries the risk of hyponatremia (low blood sodium levels resulting in some nasty symptoms) if taken too far and this alone is enough to strongly discourage 100% fluid replacement as something to shoot for.
So, what % of your losses should you aim to replace?
You can actually tolerate quite a bit of dehydration (as defined by body weight loss) during training and competition - assuming you start well hydrated. The exact amount you need to replace at any given time is unclear though; it’s probably highly individual and most likely varies a bit day to day too.
It’s not productive to try to use sweat rate data to try to create a pre-determined, inflexible strategy for fluid and electrolyte replacement.
Measuring your sweat rate should be about getting to a pretty decent ‘ballpark’ figure for how much sweat (and sodium if you know your sweat composition) you’ll likely lose over a period of time, at a certain intensity and in a particular set of environmental conditions.
If you do enough of this testing in and around the type of scenarios you encounter in training and competing, it can be very helpful in guiding you in setting the approximate levels of fluid and sodium intake that you’ll then go on to experiment with in order to optimise your performance.
A few examples in practice
Let’s say you’re losing ~0.5l of sweat per hour. It’s unlikely that you’re going to benefit from doing much more than drinking to thirst as, even over several hours, your total fluid and sodium losses are unlikely to get especially high.
But, if you’re losing more than 1.5l per hour then, during prolonged exercise, you’re likely to benefit from getting in front of the dehydration/sodium depletion curve with a pretty aggressive approach to hydration.
Taking proactive action during the early stages of big sessions and long events will help mitigate your inevitably high losses over the long haul. That’s especially the case if you also have a high sweat sodium concentration.
By measuring your sweat rate (and sweat concentration) and experimenting with differing levels of intake, you’re much more likely to iterate your way to the kind of hydration plan that will serve you well when it matters most.