How much water do you need to drink to prevent heat exhaustion?

I'm talking about physical labor or activity on a hot summer day in Florida. How much water is enough? I always feel like I'm drinking TOO much water but sometimes still feel a headache coming on. Is there a "rule" for how much water the body needs?How much water do you need to drink to prevent heat exhaustion?
If you are doing hard manual labor, then you should drink about a pint every hour. If you sweat a lot, then you should probably drink more.
In general about 2 litres a day should be good or half a gallon.
in intensive heat you sweat a lot =SO=water is the best thing and drink as much as you need so that you won't be dry
I don't live in florida. But drinking plenty so that you have to pee at least every two hours will keep you going. Experiment with how much you drink and the better you feel. You'll soon find what is perfect for your body. If headaches presist see your doctor. Wear lots of sunscreen. Tanning beds are not good for people who get headaches. Good Luck!