Apparently, the eight-by-eight rule came out of a void, because there's no scientific evidence to back it up. It's just another one of those long-standing myths that people believe because that's what everyone believes. Drinking 64 ounces of water each day isn't a bad thing, but it could be too much or not enough for some people.
Other guidelines exist, but there's still no true consensus. There's no formal recommendation for how much water people should drink every day, perhaps because everyone needs different amounts of water.
An "adequate intake" of water for adult men and women does exist, but it can vary from person to person. This adequate intake includes water from nonwater beverages, such as milk, sports drinks, tea and yes, even coffee. It also includes water from fruits, vegetables and other foods (think of how much water goes into a bowl of oats or soup).
The adequate intake is 15.5 cups (3.7 liters or 125 ounces) for men and 11.5 cups (2.7 liters or 91 ounces) for women. How you choose to consume these 125 or 91 ounces of fluid is up to you. Although this is the closest figure we have to a recommended daily intake, even these numbers differ for each person based on their health condition.
How Much Water Do You Actually Need To Drink Every Day For Your Health?