How much water do we really need to drink?

We all know people who have a water bottle practically surgically attached to their lips, and are acquainted with a few folks who are possibly camel-descendant, their liquid intake seems so sparing. But no matter how much they, or you, actually drink, chances are most of us long ago internalized the "Eight glasses a day" rule. That, we have been led to believe, is the magic number.

Or is it?

It turns out that we don't necessarily need that much water. At least, not all of us. And not all of the time. Instead of slavishly refilling your glass, consider:

• The history of the 8-glass rule: In the 1940s, the Food and Nutrition Board came up with a recommendation of 2.5 liters a day of water for adults -about 8 glasses. The part where they also stated that most of that intake was taken care of by food got ignored.
• Other sources of fluid: When researchers look at optimal intake, they're looking at all fluid intake. H2O gets all the glory, but even coffee, juice and soda count (calories, obviously, notwithstanding).
•  Whether you're actually thirsty. If you are - whoops! - you're probably already dehydrated. Get some water in you, stat.
• How you feel when you take a gulp. Researchers at Monash University found recently that when the body's had enough water, it's harder to swallow. Imagine that.
• What your urine looks like: If you pee in pale lemon tones, you're good. Or anyway, your kidneys are. Darker yellow urine is a sign your liquid-waste-elimination system is working too hard and you need more fluid.
•  That you can overindulge in water: There can be a dangerous outcome to overdoing the hydration (though it's not easy to do, frankly), called hyponatremia. Taking in too much water too quickly causes blood sodium levels to drop off a cliff, leading possibly to coma or death.

If you still want an ounce-a-day guide, go by this admittedly crude formula, says nutritionist Lauren Slayton, MS RD: Cut your weight in half, so a 140 lb. person should drink about 70 oz. of water per day, varied by circumstance. And pre-load H2O in certain scenarios, like airplane flights and hard workouts.