Sweat rate calculator can help block heat illness, boost athletic performance

For the first time, researchers have developed a validated tool that lets athletes predict their rate of whole-body sweat loss. The tool consists of precise equations that help people regulate the amount of fluids they drink over three hours while active in warm to hot weather.

The results of the multi-institution study are published in the Journal of Applied Physiology.

High sweat rates can result in dehydration, impaired performance and increased risk of heat stroke or heat exhaustion. It is well-documented that people significantly underestimate—by as much as 50%—the amount of fluid they need to replace from sweat loss. On the other end of the spectrum, some athletes drink too much to offset sweat loss, which can cause low blood sodium levels.

A total of 102 athletes (64 men and 38 women) participated in the study. The researchers collected data from two sites where volunteers completed 158 outdoor cycling trials and 182 outdoor running trials. Each trial lasted approximately 60 minutes and was conducted in weather warmer than 64°F. Whole-body sweat rates were calculated by body mass changes.

The sweat rate loss equations developed through this study will allow athletes to properly manage their hydration before they participate in outdoor cycling and running and reach optimal performance. To do so, they input information such as measured or estimated environmental conditions and exercise speed and duration. The resulting data keeps athletes from replacing too much or too little fluids.

These equations are the first for such activities based on the biophysics of heat exchange. Before now, accurate pre-exercise guidance for athletes from a validated source has not existed. The researchers have made their prediction models available to the public via an online sweat rate calculator.

The research team suggested these results are particularly significant for athletes competing at the 2024 Summer Olympics in Paris. “With global warming causing more frequent and extreme hot weather over more months, this information is increasingly important to preserve health and performance,” said Mike Sawka, Ph.D., FAPS, co-author of the study.