How many liters of water should an adult drink in a day?

Generally you should drink 1 to 1.5 litres per day for every 50 Kg that you weigh. This is only a rule of thumb and there is no exact answer. Here is what I do. I drink 1/2 litre every time I feel thirsty. I drink 1/2 litre with every meal. I drink 1/2 litre for every 30 minutes of strenuous exercise. This program works really well. Hope this helps.