The Elemental Guide to Water

How Much Water Do You Really Need to Drink?

Scientists have a totally new understanding of thirst

This story is part of The Elemental Guide to Water, a five-part special report on the health benefits of water, the science behind seltzer, the truth about fancy H2O, the safety of tap water, and how much water you really need to drink.

The well-known “8 x 8” rule — you should drink eight 8-ounce glasses of water a day — is not only daunting, it’s unfounded. In fact, nobody is sure where the idea came from, and science doesn’t support it. “It has no basis in fact,” says Michael Farrell, a professor at Monash University in Australia, who studies how the brain responds to thirst and other sensations. Likewise, the old advice to “drink before you’re thirsty” is countered by the latest research, as scientists finally figure out how the brain knows when you’re thirsty, and when you’ve had enough.

The human body is 55–60% water, varying by individual (muscle has more water than fat). Blood is 83% water, and 70% of your brain is all wet. Water aids digestion, clears toxins from the liver and kidneys, removes excess sodium from the bloodstream, regulates body temperature and blood pressure, protects skin and other tissues, and keep joints lubricated.

A person can survive weeks without food, but seldom more than a few days without water. Even mild dehydration, within hours, can affect mood, cognitive function, and physical performance, studies show.

We lose water constantly, by breathing, sweating, and using the toilet. But water loss is highly variable. On a cool day when a person isn’t active, eight glasses of water could be “well in excess of need, in which case a lot of water will be excreted” along with vital substances like sodium, Farrell says. Alternately, a person exercising on a hot day might need more than eight.

Fluid figures

There is no official U.S. government recommendation for how much water to drink. But there are guidelines for total fluid intake from independent groups. The average adult woman should consume about 11.4 cups of fluid per day (a cup equals 8 ounces) and men should consume 15.6 — be it straight from the…