Dear A Moment of Science, If I were to stop drinking water right now, how long could I live? --Thirsty
There are different factors involved here. There are conditions under which a person could die of dehydration within a single day, and other conditions whereby a person might live for over a week. It's unlikely, though, anyone could go much beyond that.
Water comes into our systems not just from drinking but from eating as well. It goes out of us through urine, through sweat, and even through exhaling. Ever notice the fog you can make on a window by breathing on it? That's water vapor leaving your body.
The biggest reason it's bad to lose too much water is that the actual volume of your blood goes down. When blood volume goes down, blood pressure goes down. As blood pressure goes down, body heat goes up, because your blood helps regulate your core body temperature. Without enough blood, you can't cool off. Pretty soon you overheat.
So various factors, from eating to circulatory health to the ambient temperature, play a part in how long you could go without water. Probably the biggest factor, though, is simply age: Young people are much more likely to overheat faster than adults, which is why it's especially important to keep an eye on the little ones on hot summer days.
"How Long Can the Average Person Survive Without Water?" (Scientific American)