Sweat rate is KEY to understanding your body and how much fluid you require. Once you know your fluid loss, over DIFFERENT temperatures, then you know how much you need to drink per hour to stay hydrated. Different temperature on race day, and different humidity levels will affect your numbers so you need to baseline test multiple times and record the number so you have an average. We did this on the 'run', since it would be the easiest way to make the calculations and we basically run three times a week, so you'll get three baseline readings quickly. Try to do it on one hour or two hour runs or rides.
It's a basic formula that's very simple, and you can just create an Excel spreadsheet and plug in the numbers to get an average. You want to test yourself over hot, humid, cool, cloudy and sunny conditions so you begin to see a pattern or average.
The goal is to see exactly how much dehydration you incur during your workout and in turn, determine your hourly fluid replacement (how much you need to drink). When you are done with this homework, you will have a hydration target that you can use in both training and racing.
Here's how to calculate your sweat rate:
Here is a downloadable Excel Calculator:
Conditions that will increase sweat rate include heat, humidity, and elevated heart rate (high intensity training). Athletes should measure sweat rate across several workouts to determine their fluid replacement needs in various environmental conditions.
On race day, based on the forecast, the athlete than can go back to their log and know exactly what fluid they need to plan on ingesting to prevent performance declines associated with dehydration. Deaths have occurred when the air temperature was less than 75 degrees F (24 degrees C) but the relative humidity was above 95%. Humidity levels over 75% will contribute to an increased risk of heat injury.