Remember the eight glasses of water per day rule? Unfortunately that turned out to be a total myth. Unfortunately there is no simple rule that applies to everybody, as your body's hydration needs depend on a huge range of factors such as where you live, how much you weigh, what you are doing and what you are eating. The rule of thumb is this: if it's hot and you are doing physical exercise, up your intake. You can even download apps to help remind you to drink water regularly, or simply keep tabs on a post-it or in a notes app on your phone.