The gold standard has been 8 glasses, 8 ounces each, every day.
Dr. Henry Petry says a little less is fine - but it is important to stay hydrated, whether you're thirsty or not.
His patients don't always measure up.
"Are totally unaware they're dehydrated. They feel just fine, they do not think it necessary to even drink anything," Petry says.
Patients on a low sodium diet may feel less thirsty, due to the lack of salt. The urge is also lower in winter, as temperatures cool. And elderly patients will see a marked decline in thirst.
"For every 10 years, we lose 5% of a lot of our sensation of ability to do things. And one of those things may be thirst as well as hunger," Petry says.
Staying hydrated is important for overall health. And it can even help the performance of prescription drugs.
"But it's also important to drink adequate fluids so that the medications they are taking will actually work better," Petry says.