How much water do we need?

Why do we need water?

Water makes up 50 to 70 per cent of an adult's total body weight and, without regular top-ups, our body's survival time is limited to a matter of days.
Water's essential for the body's growth and maintenance, as it's involved in a number of processes. For example, it helps get rid of waste and regulates temperature, and it provides a medium for biological reactions to occur in the body.
Water's lost from the body through urine and sweat, and must be replaced through the diet. If you don't consume enough you can become dehydrated, causing symptoms such as headaches, tiredness and loss of concentration. Chronic dehydration can contribute to a number of health problems such as constipation and kidney stones.

How much water do we need?

The body gets its fluid from three sources:
Drinks, either plain water or as part of other beverages including tea, coffee and squash
Solid foods, especially fruit and vegetables (even foods such as bread and cheese provide small amounts of fluid)
As a by-product of chemical reactions within the body
Most healthy adults need between one and a half to three litres a day, so aim to drink six to eight medium glasses of fluid daily. Beverages such as tea, coffee and fruit juices count towards fluid intake, and may bring with them other nutrients or benefits.
You may require more fluid if you're very physically active or during periods of hot weather.
You can judge whether you're drinking enough by the colour of your urine. If it's a pale straw colour then your fluid intake is probably fine. If your urine is dark yellow, you probably need to drink more.
How to maintain fluid levels
Start as you mean to go on, with a glass of water when you wake.
Find time to make yourself regular drinks during the day - don't forget that tea, coffee and juices can count. Just watch out for the amount of sugar consumed in some soft drinks.
Keep a bottle of water in your bag, as it's a convenient way of providing fluid if you're travelling or exercising.
Get into the habit of having a glass of water with every meal.
The sensation of thirst is not triggered until you're already dehydrated, so it's important to drink before you get thirsty.
Increase your intake of fresh fruit and vegetables, as they have a high water content.

As we can see, it's very important drinking water everyday in order to maintain our body healthy!!

Source: http://www.bbc.co.uk/health/treatments/healthy_living/nutrition/healthy_water.shtml