There is no scientific basis for the '8×8' rule; food and other drinks count, and thirst is a decent guide.
What the evidence shows
The '8 glasses of 8 ounces a day' rule is repeated everywhere, but its origin is murky. Investigating it, physiologist Heinz Valtin (2002) traced the number to misreadings — including a 1945 guideline that said the body needs about that much water but added that most of it comes from food.
Valtin found no scientific studies supporting a rigid 8×8 target for healthy people in temperate climates doing normal activity. Water in food, coffee, tea, and other drinks all count toward hydration, and the body's thirst mechanism regulates intake well for most people. Needs genuinely rise with heat, exercise, illness, and pregnancy. The universal '8 glasses' mandate is a myth; hydration matters, but the specific rule is unsupported.
Sources
Valtin, H. (2002). “Drink at least eight glasses of water a day.” Really? Is there scientific evidence for “8 × 8”?.
American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 283(5), R993–R1004
Found no scientific evidence for the 8×8 rule; healthy people in temperate climates meet needs through varied intake and thirst.
DOI: 10.1152/ajpregu.00365.2002 →