An average person loses 3-4 liters of water in a day, that's all we need to replace with fluid intake. Since the food also contains liquid, we should also consume another 2-3 liters on average. Of course, this amount can vary depending on whether you play sports or do hard physical work, as you can lose a lot of fluids by sweating. Many would not think so, but only by breathing do we lose 1-2 liters of fluid a day.