For those of us who don't have much faith in formulae about how much liquid or water to drink, there is an all natural test that any doctor will admit (okay, you may have to push them a little bit before they fess up) is the best indicator of hydration:
the color of your urine. The darker it gets, and the more yellow or orange the urine is, is an indication of dehydration.
The lighter the urine is, the more hydrated.
This assumes a normal kidney, not having jaundice, etc or having just eaten beets.
If your urine is pale yellow, you are most likely fine.
Charm