Sweat Rate Calculator
This online calculator allows you to find your sweat rate (fluid loss per hour) in a simple but effective way. Contrary to its title, it also takes fluid losses due to breathing into account. So we might have called it "Fluid Loss Rate Calculator", but this sounded catchier.
Weigh yourself before and after your workout, and add any fluids you consumed during your session. Works best for workouts that are short enough so that you don't have to go to the loo during them.