The above is a screen shot of the water use calculator (available for download) I've written to simplify the dehydration page of my wiki. It simply shows how much water in fluid ounces, or pints, that a character needs to drink per day, based on their body weight, the ambient temperature and their food intake. This is a job half done ~ it doesn't take into account what a character would need if they had a disease that caused diarrhea or vomiting, but those things can be calculated on a case by case basis.
The table on the left, labeled in green, is not part of the calculator, but is a simplification chart for quick reference if the DM would rather not use a calculator all the time. It represents the typical amount of water a character drinks in cups (8 oz. or 237 ml) per half-day, when travelling or performing ordinary labor. Do note that the food eaten is taken into account, so that 60 lb. creatures can get their water intake, most of the time, from the food they eat.
For ease, halve the amount if the character is resting and double it if they have fought a battle that day. If the weather is hotter than sweltering, use the calculator.