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 issued dietary recommendations that included a figure for water intake: approximately one milliliter per calorie of food consumed, which translated to about two to two-and-a-half quarts daily for most adults. The guidance came with an essential caveat, printed directly in the text. Most of this water, the Board explained, was already present in the food people ate. The recommendation described total water consumption from all sources, the moisture in fruits, vegetables, soups, coffee, and every other component of a normal diet, not an instruction to drink that quantity of plain water separately.

That qualifier disappeared in translation. By the 1970s, the advice had condensed into a simpler rule: drink eight glasses of water a day, sixty-four ounces, about two liters, in addition to whatever else you consumed. The eight-glass prescription became a fixture of health education, repeated in classrooms, textbooks, and public health campaigns as if it were settled medical knowledge backed by research. The number itself was real, it matched the total water figure from 1945, but the context that gave it meaning had been stripped away.

Heinz Valtin, a kidney specialist at Dartmouth Medical School and the author of standard physiology textbooks, decided in the early 2000s to track down the scientific basis for the eight-glasses rule. He searched medical journals, clinical trial databases, and older literature not indexed electronically. He consulted colleagues who specialized in fluid balance and thirst physiology. Valtin was looking for a study, any study, that demonstrated a health benefit or physiological requirement for sixty-four ounces of plain water per day in healthy adults. He found nothing. His 2002 review in the American Journal of Physiology reported that no scientific evidence supported the claim and that its origin could not be traced to any published research.

The body's water regulation system, Valtin explained, operates through thirst, which functions reliably under normal conditions. When blood becomes more concentrated, thirst signals the need for fluid. When balance is restored, thirst subsides. The kidneys continuously adjust urine concentration, producing dilute urine when fluid intake is high and concentrated urine when intake is low, maintaining equilibrium without requiring conscious tracking. For healthy adults in temperate climates, not engaged in strenuous activity, the body's regulatory mechanisms were sufficient. Counting glasses added nothing.

The eight-glass rule persisted not because following it caused obvious harm, drinking sixty-four ounces of water is generally safe, but because it rarely produced visible problems. The damage it inflicted was subtler. It created a market for bottled water, hydration apps, and wellness products premised on the idea that water consumption required deliberate intervention and constant monitoring. It turned a basic physiological process into a health task demanding conscious management.

Valtin noted the genuine exceptions. Athletes, manual laborers, people in hot climates, and individuals with certain medical conditions have legitimately higher water needs. Elderly people may experience diminished thirst signals. But these were specific cases, not grounds for a universal rule applied to the general healthy population. For that population, the recommendation had no physiological foundation. It was a misunderstood fragment of a 1945 guideline, detached from its original meaning, simplified into a memorable number, and repeated until it acquired the weight of fact.