Ask your thirst. You don't need us.

The idea that we need at least 8 glasses a day is a myth.

That number was derived decades ago by studying comatose patients and estimating the amount of fluid they need in their drip feed! Translating that to daily requirement for healthy people can be very risky. For instance:

• The original formula had estimated about 8 cups of water. That's just two quarts a day. In popular media that 8 cups became glasses which became 'tall glasses' which got raised again to a gallon a day - all based on no data.

• We get water from ALL the food and liquid we consume. Meat, veggies, wine, coffee, candy, nuts - ALL contain some water and all of it counts. The idea that any normal foods are dehydrating is mostly a myth. Naturally there are some exceptions - very salty foods for example. Or sea water...

• Another myth is that salt is 'hard' on the body but water ("it's pure and clear, after all!") is 'easy'. But the kidneys have to regulate and filter and "work" either way. If you drink a lot of water the body still works to maintain balance according to its needs, has to pull water out of the blood and discard the extra.

• People are VERY good at self-regulation. Obviously we can be thrown out of balance (e.g., the whole carb thing), but people really hate to be thirsty and will drink when they are. It's almost impossible to stop someone from drinking when they need to!

• It's become a common idea that humans are 'at risk' for dehydration - but does that really make sense based on what we've learned? Could our ancestors really have been at risk if they 'forgot' to drink extra water every day? If so those folks would have joined the Darwin Club in a generation or two!

• Where in the world would our ancestors have gotten nice big jugs of clean pure water?? How 'paleo' is it to be carrying tall bottles of distilled water around? Not that I'm an extremist, but it almost implies we should be drinking from puddles and tree trunks - not the faucet! :-)

I suspect that if 'dehydration' is a real daily concern (I'm talking about normal people here, not nomads in the Sahara for example), it's probably a byproduct of the SAD diet. The real question isn't 'how much water should I drink?' but 'how does a lifetime of Cheerios and soda impact my water regulation?'

To really go against 'common wisdom' here (I'm on a roll!), I wonder if occasional dehydration might be good for you once in a while. Fasting, for instance, is a Good Thing. Has anyone tried intentional dehydration, just to see what it's like??

Edited to add: All the above is the long version! The short version has already been said by Anonymous Coward ("ask your thirst") and baconbitch ("listen to your thirst") and others.

In a German study by Franz Volhard Clinical Research Center, Michael Boschmann MD tracked energy expenditures in men and women who were healthy. After drinking 17 oz of water the patients metabolic rates increased 30%. The increase began ten minutes after drinking and maxed out at 30 to 40 minutes post drinking. Rates did differ in men and women however. In men they burned more fat while in women they tended to break down carbohydrate substrate better with water. It also appears the colder the water is the better the effect.

in 2002 Journal of Epidemiology found drinking more than 5 glasses of water a day reduced fatal heart attack by 50%

Doing that math if you increase your water consumption 1.5 liters a day you will burn 17,400 calories and lose about 5 pounds.

Never rely on thirst because it lags the real effect. By the time thirst kicks in your serum osmolarity is already impaired. Dehydration is a major cause of day time fatigue as well and dehydration slows your metabolism by 2-3%. In fact a 2% drop in body water can cause neurologic changes. So drink that water......COLD!

It depends on 1. your environment and 2. your activity level.

If you work outdoors in the heat, then drink a lot of water. If you work a desk job, and only sweat for a workout, then your requirements aren't going to be the same. There are other factors to consider once you get a base, but don't stress it too much.

I don't like drinking water but like some of the posters said, listen to your thirst. Though generally I try to drink a glass of water or cup of green tea every 2 hours. If I don't take in enough water I get headaches. But really, its pretty intuitive. Eat when hungry, drink when thirsty, sleep when tired.

I believe women should consume about a gallon of water a day, men probably a bit more. Seriously. At first you'll have to pee all the time but your body adjusts within about a week to the increased intake. Being properly hydrated allows your cells to let go of stored water and toxins, and allows your kidneys to function at their peak. Drink throughout the day (you'll need to, to get through a gallon!). Don't forget that tea and coffee and any other liquid counts towards the total, too! If you do this you will notice your skin will be much more clear and glowing, you will have more energy, weight will go down, you will look less "puffy" (not that you do!).

I drink enough water so that I pee clear at least once a day (i.e. urine is transparent, either colorless or very light yellow).

If I am ill or recovering from an illness I might drink more in an intentional attempt to flush my system, but generally that is it.

If I am drinking a fair amount of a diuretic (caffeine, alcohol), I try to have one cup of water for each cup of the other.

The quantity of water then varies depending on what I'm doing, how hot and humid it is, etc. Could be one 8-12oz glass, or could be 5 or 6, or quarts if I'm hiking or playing sports or something.

I have been told by Coach's my entire life and doctors that by the time your thirsty you are already partially dehydrated.

If you are doing physical activity then you need to be hydrated BEFORE you start the activity, which can mean making sure you are hydrated days before a hard workout. As a runner I pay close attention to my hydration three days leading up to my long run, or to race day. The same can be said of nutrition.

I think it's easy to get busy and not notice you are thirsty. I think that if you are very active that relying on thirst only is a sure path to failure.

You need to be drinking filtered water as well...

I drink 1.5 liters at work and 0.8 liters during my workout. plus other amounts during the day...

I've been doing well on 2-3 liters per day usu closer to 2 than 3.

Well for those who like formulas and stuff. half your bodyweight in lbs = amount in ounces of water to consume.

For every serving of a diuretic drink (coffee/tea/etc.) add 10 ounces to above result PER drink

Also don't do the opposite and drink 5 gallons of water in a day, you could affect the electrolytes in your body. Check this link out: Woman Dies from too much water

I do agree with listen to the body, it will tell you that you are thirsty. BUT some people confuse hunger with thirst. Personally I think it is a good idea to drink throughout the day instead of trying to get all the water in one sitting. Also I recommend no drinking fluids 30 minutes prior to a meal, and at least two hours after a meal (especially if you have digestive issues), you can take a few sips during the meal but you don't want to down a ton of water, I don't think you want all that liquid while you're producing acid to break down food.

If you're doing a ketogenic paleo, then definitely the above equation should be the minimum you drink. I would do a little more since you urinate when in ketosis. maybe up your intake in salt (himalayan or celtic) as well.

Oh and make sure your water is not fluoridated! and filtered with reverse osmosis, you don't want all the pharmaceutical/chemical present in municipal tap water