Just by breathing our body looses around 800ml of water!!
As this water topic intake is so complex, it is quite impossible to estimate a good amount you should drink every day (depends on your size, metabolism, food intake, activity, country you live in, etc.)
The World Health Organization has estimated that on average we loose 30ml per body weight. So if you weight around 70 kg, you body uses around 2 Litres of water just for basic functioning.
When you are thirsty, your already lost 1% of your water body weight.
When you mouth gets dry, you already lost 3%.
At 5% you have difficulty concentrating, headache and sleepiness.
When your extremities start feeling numb and tingling you are at 6% loss and at 7% you will collapse.
Just to set things straight: good hydration is through water, not coffee, sodas, juices or beer! Although they have water in it, the other things they bring along into your body trigger other activities that again need H2O and lots of energy to digest.
Before you reach out to some medicine when you feel a headache, drink, drink, drink!
Learn to love water and drink before you feel thirsty. As simple as that!