The most basic formula we use to keep animals hydrated (if they refuse to eat or drink anything but don't lose higher amounts due to diarrhea or vomiting) is 48 ml/kg/d.
I see no reason not to apply this to human beings. The only thing needed to put into acount is the amount of fluid in the things you eat and drink. I seem to remember something about about thes containing about 1-2 litres depending on the kind of food you eat.