To know how much extra water you should drink, you should weight yourself before and after your workout. For every pound lost during exercising you should drink 500 ml of fluid. You should also drink water before (250 to 500ml 1 hour before exercise), during and after working out.
You can also have sports drinks but you will
only really benefit from those if you do intense workouts for longer hours such
as running for more than 1 hour. Sports drinks can help in that case cause they
also have carbohydrates that will give you energy and also sodium which will
help to absorb fluids. For the regular people on a light to moderate exercise
routine water is better cause it is quickly absorbed by the body and has zero
calories.
Remember to keep yourself hydrated throughout
the day. In case you have any alcohol or drinks with caffeine (hopefully you
will avoid them), try to drink at least one glass of water for every drink you
have.