How to Test Your Sweat Rate
- Claire Shorenstein, MS RD CSSD CDN
- Dec 7, 2023
- 4 min read
Updated: May 8
Hydration mishaps are all too common among endurance athletes. Sometimes it's a fluid issue, other times an electrolyte issue, or it may be both.
Fluid and salt needs vary greatly among athletes, which is why these two things must be individualized.
In this post, I'll teach you how to test your sweat rate (the amount of fluid you lose per hour during exercise) using a home scale so you can personalize your hydration plan for training and competition.
But First, Some Background on Sweat
Water is essential both in everyday functioning as well as during exercise. In fact, water makes up about 60-70% of our body!
You need adequate water to transport nutrients (e.g. glucose to working muscles, oxygen via red blood cells), remove waste products, support the immune system, hydrate your tissues and organs, maintain blood pressure, and maintain an appropriate body temperature.
As for the electrolytes, sodium, chloride, and potassium all help maintain fluid balance within the body, while magnesium and calcium promote optimal muscle function and aid in energy metabolism. You lose electrolytes through sweat and urine, and lose sodium followed by chloride in the highest concentration through sweat.
If you're not replacing your losses adequately, you can develop electrolyte imbalances that disrupt normal bodily functions. This leads to symptoms like dizziness, headache, fatigue, irregular heartbeat, muscle cramps, mental confusion, or nausea and vomiting.
It's easy to get dehydrated, especially if you're exercising for many hours. In fact, even mild dehydration (i.e. >2% loss in body weight) during exercise can lead to undesirable outcomes, such as increased perceived effort, reduced endurance, and higher risk of GI distress.
What makes this even more challenging is that sweat rate changes based on many factors. For instance, heat, high exercise intensity, and altitude all increase fluid losses.
It's important that you learn how to test your sweat rate so that you can measure it in different conditions.
How to Test Your Sweat Rate: Instructions
The first step to testing your sweat rate is to pick which exercise session you plan to test. Target a session around 60 minutes, but anywhere between 45 and 90min works.
Once you've fueled, gone to the bathroom, and you're ready to go, here's how to test your sweat rate:
Weigh yourself (naked) before your workout
Start your session and make note of the following:
How much fluid you drink (in ounces) during exercise, if any
Duration of your session
Exercise intensity / session type (e.g. easy run vs speed session)
Temperature and humidity
Avoid using the bathroom during exercise, if possible
Once you're back, remove sweaty clothes, towel off any excess sweat, and weigh yourself again (naked)
Do the math using the equation below
For every 1 lb that you lose, that equates to 16oz of sweat lost
Sweat rate = [(pre-exercise weight - post exercise weight in lbs) x 16 oz of sweat loss + fluid consumed (oz)] / exercise time (hr)
Example: If you run 60min, drink 16oz during the run, and lose 1 lb, then your sweat rate is (1 x 16 + 16) / 1 = 32oz/hr.
If you are a multi-sport athlete, you may also want to do one test for each sport (e.g. one on the bike and another on the run).
How to Interpret your Sweat Test Results
I see athletes with sweat rates all over the map. I've had athletes lose well above 2L/hr of sweat, and as little as 8oz/hr. This is why it's important to test and individualize fluid strategies!
Note that the goal here isn't to replace 100% of your losses during exercise. You will aim to replace 50-90% of losses, with adjustments made based on how heavy of a sweater you are, what conditions you're in, and what intensity you're aiming for.
In shorter sessions (i.e. 1-2.5hr), replacing 50-75% of losses may be adequate. In longer sessions (>2.5hr), try to replace 75-90%. Always first identify your baseline fluid intake during exercise, and gradually work up to your goal range.
Very heavy sweaters (if you lose >1.5L per hour) may not be able to tolerate much above 1L/hr, especially over extended periods of time. This is a great example of why you need to also start all exercise well-hydrated!
Learn more about what to do with your sweat test results, and how to begin all sessions hydrated and rehydrate afterwards in Hydration for Endurance Athletes.
How to Estimate Hydration Goals Without a Scale
If you don't have access to a scale or you find the scale to be triggering, you do NOT have to do this test. You can estimate fluid needs in other ways.
First, ask yourself if you think you are a heavy sweater. Is your clothing soaked after exercise, for instance? Do you struggle in the heat?
Then, you can use a general recommendation of 12-28oz of fluid per hour during exercise, making adjustments based on how you feel, weather, and intensity. Hydration for Endurance Athletes will help you make a plan!
Additional Nutrition Resources for Active People
If you enjoyed this post, I encourage you to check out my other online nutrition resources, many of which are free!
I currently offer 7 FREE nutrition downloads on a variety of topics
I am over 100 episodes deep into The Eat for Endurance Podcast
I offer a library full of low-cost nutrition mini guides, as well as a race-nutrition masterclass and a comprehensive sports nutrition course
Interested in Online Nutrition Coaching?
I offer a variety of online nutrition services to support your everyday and athletic performance needs.
My monthly membership, Fuel for Life Crew, gets you direct access to me and a supportive community of active people like you for all your nutrition questions. You also get live monthly masterclasses on a variety of nutrition topics and access to my full online library of resources, for just $30/month (cancel anytime).
If you're ready to go all-in to transform your nutrition, health, and performance and get more personalized support, 1:1 nutrition coaching is for you. To get started, book a discovery call here.
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