This site has a good write up in the Library section explaining why being properly hydrated is important. Its easier to understand than a lot of other writings I have found. You can find it here: http://www.calorie-count.com/calories/article /water.html
If you think about it, there is no way that a 260 lb man and a 120 lb woman should be drinking the same amount of water.
8 8-oz glasses is a good place to start (since most of us don't even drink that much)... once you know you can handle that, then you can get into the rough measurements and using the calculators.
Most calculators for me estimate that I should be drinking between 3 and 3.5 L of water per day. So I've settled on 3L as it is a manageable number.
Also, for those of us who eat when bored *raises her hand* or to stay awake, drinking water is a good way of replacing the bad habit. Zero cals and it can't hurt you. Or rather, it can, but it's terribly terribly unlikely. Just my two cents.
My cardiologist (two of them actually) says that 2 liters is more than enough to replace what you've used.
I've tried curbing hunger with water but it doesn't work for me.
A good estimate is to take your body weight in pounds and divide that number in half. That gives you the number of ounces of water per day that you need to drink. For example, if you weigh 160 pounds, you should drink at least 80 ounces of water per day. If you exercise you should drink another 8 ounce glass of water for every 20 minutes you are active. If you drink coffee or alcohol, you should drink at least an equal amount of water. If you live in an arid climate, you should add another 2 servings per day. As you can see, your daily need for water can add up to quite a lot.
A calculator to help you figure out how much water you should drink:
http://www.muscletech.com/CALCULATORS/WATER/W ater_Calculator .shtml
Do you walk or run and want to know how much water to carry with you: Check out this calculator: http://walking.about.com/library/cal/ucwaterc alc.htm
Can you drink too much water? http://www.drmirkin.com/fitness/hyponatremia. html
And if you're getting lots of fiber in your diet, you need lots of water to speed up your metabolism.
Water is soooo good for you... ;) Drink on (not more than 10 liters a day though)..
A lot of things have to be taken into account. Temperature, activity level (obviously somebody working construction on a hot summer day needs more water than somebody sitting at a desk in an airconditioned office), body size, salt intake , general state of health.
When I was pregnant with my son and had bladder kidney infections my ob/gyn prescribed a minimum of a gallon of water a day to keep my urine going and flush out my system. I had a hard time drinking that much at the time (severe hyperemesis), and ended up in the hospital with among other things severe dehydration, in part because I got so sick I couldn't even keep water in me.
Every system in the human body requires water to function. A person can live weeks without eating, but barely a few days with out moisture.
The rules above are a goood guideline. But the best way to make sure you have enough fluid is to check your urine, if it is pale yellow practically clear you are getting enough water, if if is bright yellow (assuming you aren' taking b vitamins), you need more water. If it looks like apple juice, you need to see a doctor.
I usually drink a minimum of 3-5 liters a day, depending on what I'm doing that day. I talk for a living, so I probably need more than many office workers.
While it is possible to drink too much water (water intoxication), it is not very common and ususally involves marathon runners who are water loading, or frat pledges being forced to drink large amounts of water. Most people usually know when they need to stop as thier body will tell them, enough.
Personally, I'm also in the "I drink when I'm thirsty" camp--works fine for me.
Please ensure that you intake enough Sodium in a day, remembering that if you dilute the sodium content of your blood too much it can cause problems with concentration, balance, it can cause cardiac arrhythmias. When exercising it is recommended to restore electrolytes as well as hydrating; make sure that you drink something along the lines of powerade/gatorade and such like.