If you want to figure out just how much you perspire, thus how much hydration you need to take in you can figure it out this way.
Your standard sweat check procedure is:
Check your weight before and after training, and calculate weight loss.
Convert any weight loss to ounces or ml of fluid.
Check/measure the amount of fluid consumed during training.
Add the amount of fluid lost to the amount of fluid consumed to get total fluid losses.
For example
You lost 2 pounds during a 2 hour run
You drank 32 ounces.
There are 16 ounces to a pound
2lb x 16oz = 32oz + 32oz you drank = 64oz used during a 2 hour run.
Your sweat rate is 32 ounces per hour.
Important: Your sweat rate will vary by season.
This is good to know so you can work on not getting dehydrated. If you lose 2% of your body weight, it will start to affect your performance.