Ignore the old adage that you have to drink 6-8 glasses or 1 litre of water a day. The new consensus is that you need to drink water according to your weight and level of physical activity. The exact calculation is your body weight divided by ½ and converted to ounces (i.e. 70 pounds / 2 = 45 oz (1.27litres)).
The Eatwell Guide recommends that people should aim to drink 6 to 8 cups or glasses of fluid a day. Water, lower-fat milk and sugar-free drinks, including tea and coffee, all count. You may need to drink more fluids if you're: pregnant or breastfeeding. in a hot environment. physically active for long periods.