You should drink eight 8-ounce glasses of water per day (64 ounces total).

In 1945, the Food and Nutrition Board of the National Research Council published dietary guidance that included a recommendation for water intake: roughly one milliliter per calorie of food consumed, which worked out for most adults to about two to two-and-a-half quarts per day. The recommendation was preceded by a crucial qualifier: most of this quantity, the Board noted, was already contained in prepared foods. The guidance was describing total water intake across all sources, the water in soups, vegetables, fruit, coffee, and everything else a person ate and drank, not a prescription for how many glasses of plain water to consume independently.

Somewhere in the passage of that recommendation through health education, the qualifier got lost. By the 1970s and 1980s, eight glasses of water per day, equivalent to sixty-four ounces, approximately two liters, had become a standard piece of health class advice, presented as a medical fact with the authority of official guidance behind it. The eight glasses represented the recommendation's total water figure, stripped of its context, and repackaged as a daily target for plain water consumption above and beyond everything else eaten or drunk.

Heinz Valtin, a physician and kidney specialist at Dartmouth Medical School and the author of two widely used textbooks on kidney physiology and water balance, set out in the early 2000s to trace the claim to its source. He searched the scientific and medical literature for a study, a clinical trial, or a physiological rationale that established sixty-four ounces of water per day as a requirement for healthy adults. He found nothing. The 2002 review he published in the American Journal of Physiology concluded that no scientific evidence supported the eight-glasses rule and that it had no identifiable origin in clinical research.

Valtin's review addressed the physiological question directly. The human body regulates its water balance through thirst, a mechanism that is accurate and responsive under ordinary conditions. Thirst sensation kicks in when blood osmolality rises, when the body's fluids become more concentrated, and it resolves when fluid balance is restored. The kidneys adjust urine concentration across a wide range, excreting dilute urine when fluid intake is high and concentrated urine when intake is low, maintaining equilibrium without the drinker needing to count glasses. For healthy adults living in temperate climates and not engaged in vigorous physical activity, Valtin found that the body's own regulatory systems were adequate guides to hydration.

The eight-glasses rule survived not because it was wrong in practice, drinking sixty-four ounces of water is generally harmless, but because it was sufficiently general and benign that it rarely caused visible problems. The harm it caused was more economic and psychological: it sustained a market for bottled water, hydration tracking apps, and health products built on the proposition that deliberate water consumption was a health intervention requiring conscious management.

Valtin was careful to note exceptions. People in hot climates, those doing physical labor, endurance athletes, and individuals with certain medical conditions have genuinely elevated water needs. Thirst is also less reliable in the elderly. But for the general healthy adult population that eight glasses had been prescribed to for decades, the recommendation was not grounded in physiology. It was a round number that had escaped its context, a footnote that had become a fact.