Sweat Rate Calculator

Jump straight down to the sweat rate calculator

As a runner, it’s helpful to understand how much you sweat at any given effort level. This article shows how to calculate sweat rate and describes a simple test protocol, the results of which can be put into the sweat rate calculator below. The test will take an hour to perform. You can use this information in the longer runs so that you can rehydrate accordingly, and also not over overhydrate on your easy runs / ultras.

• Why should I do sweat rate tests?
• Sweat rate test protocol – how to calculate sweat rate
• Jump straight down to the sweat rate calculator
• Sweat rate formula

Why should I do sweat rate tests?

On shorter runs in normal conditions, most people don’t need to take water with them – ideally you should be in a well-hydrated state before you start the run, having had a normal amount of drink in the preceding 48 hours. However, once your running training reaches the point when you are running for ~80+ minutes, you might want to consider rehydrating mid-run or race. A sweat rate test will help you understand how much to drink so that you get the best performance in a race and can keep going longer in training.

Don’t get hung up on the exact minutes number there! Runners might need water earlier if they are newer to running. Or if it’s a hotter, more humid day. Or if you are wearing more clothes. Or at high altitude. Or faster / harder effort. There are many variables. The main point is that if you are just going out running for thirty minutes in normal conditions, most people won’t need to be clutching a water bottle. If they do need it, it’s probably because they have forgotten to stay hydrated outside of running, or have a medical condition that means they are probably way ahead of needing to read this article.

Once you do a sweat rate test, you can then run in similar conditions, replacing lost sweat as you go, keeping the body in a well-hydrated state for optimum training and performance. Additionally, you can avoid drinking too much, which can be both uncomfortable and potentially dangerous during very long events like Ultras (due to the risk of hyponatremia – this dilutes sodium in the bloodstream with some nasty side effects).

Sweat Rate Test Protocol – How to calculate sweat rate

To perform this test, you need to be able to run for an hour. This is to give enough time to sweat a measurable amount. We don’t go for much more than an hour because on much longer runs, weight loss from glycogen burned as fuel can skew the result.

Choose an effort level

Run the test at the effort level that’s important to your goal. You could do the test once for easy runs and another for marathon race pace for example.

Ideally, do not drink until all the steps below are complete – if you can’t run for an hour without a drink, you probably need to work on your hydration. If you live somewhere super hot and have to drink during hour-long runs, you’ll need to factor that in by holding the drink bottle while weighing yourself before and after.

Start and stop your run as close to the scales as possible, i.e. straight out of your front door.

1. Weigh yourself with some accurate scales, naked. Don’t scare people in the gym. Do this at home.
2. Get dressed.
3. Run for roughly an hour – use a stopwatch or sports watch to measure the minutes exactly.
4. Stop the watch!
5. Quickly, get undressed and dry yourself to remove as much sweat from the skin as possible.
6. Weigh yourself with the same scales, naked.
7. Enter the results into the sweat rate calculator below:

Sweat Rate Calculator:

Sweat rate formula

The formula used assumes that 1ml of water weighs roughly the same as 1ml of sweat. Given that 1ml of water weighs exactly 1 gram (0.001Kg), then for every gram of weight lost, during the test, we’ve lost 1ml of sweat.

Using the before and after weights to find the difference gives the total ml of sweat lost and then we can divide that by the number of minutes and adjust it to get ml/hour sweat rate.