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Why and How to Calculate Your Athlete’s Sweat Rate

When it comes to proper hydration, calculating sweat rate is one of the easiest ways to ensure that your young athlete is drinking enough to stay hydrated during sport. And understanding sweat rate—how fast an athlete's body is sweating out fluids—is important for both performance and safety.

"People's hydration needs are really individual. If an athlete is even two percent dehydrated, that can decrease their performance by around 20 percent,” explains TrueSport Expert Kristen Ziesmer, a registered dietitian and board-certified specialist in sports dietetics. "Calculating your sweat rate is one of the best ways to tailor your hydration plan. It shows you how much fluid you're actually using, and how much you need to drink—and it's one of the cheapest and simplest tests to do."

Here, Ziesmer explains how to help your athlete calculate sweat rate, and what you need to know to optimize their hydration plan.

Sweat rate is a moving target

The ounces of fluid an athlete loses per hour will change with different conditions. You may need to calculate it a few different times so your athlete knows how much to drink in different temperatures and weather situations. Of course, the hotter it gets, the more your athlete will sweat.

But is it a dry heat? Humidity is an important consideration in addition to general temperature. "The way that your body cools itself is by sweating and having that sweat evaporate," explains Ziesmer. "But when you are in a humid environment, sweat never has the chance to evaporate. So your body's just getting more hot, more sweaty, and more dehydrated."

How to calculate sweat rate

Before calculating sweat rate, have your athlete step on the scale before and after each practice session, and during practice, measure the amount of fluid they drink. Ziesmer suggests repeating this calculation several times to get an average, since there is plenty of room for error—your athlete might forget that cup of sports drink that she chugged mid-practice. (Note: Make sure your athlete empties their bladder before getting on the scale to weigh in.)

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Athlete Health TrueSport