We love this calculator by Camelbak. The CamelBak Hydration Calculator guides you through a step-by-step experience intended to help you better understand the different factors that influence dehydration. It works by taking into account your physical parameters (age, body mass index, gender) and then looking at your current hydration status. They're essentially asking the question: are you beginning your activity with a water deficit, or are you properly hydrated? The Hydration Calculator then seeks to understand what type of activity you're about to start and how hard you're going after it. More than any other factor, your intensity will significantly influence your hydration requirements. Are you running close to top speed flat out for 30 minutes? Or are you running closer to a jogger’s pace for 30 minutes? The difference in sweat rate, and therefore hydration demands, could be significant.
Finally, the Hydration Calculator asks about the environmental conditions during your activity. Running in 95-degree heat will result in a greater demand for your body to cool itself than in temperatures around 80 degrees. Thus, environmental conditions affect your sweat rate.