You’ve probably heard that you need two litres of water each day. This is a rough guideline, based on the average man. Hydration needs vary from person to person, depending on height, weight, life style, diet and climate. The average person loses roughly one litre of water per day through breathing and one litre through urinating. If you exercise, you’ll sweat more than usual – you can work out how much more you’ll need to drink by using a sweat-rate calculator.