The short answer
No. The "8×8" rule — eight 8-ounce glasses, about 2 litres — is not backed by scientific evidence as a universal requirement. After an extensive search for its origin and supporting research, Valtin (2002) reported finding no proof that every healthy person must drink that much. Published surveys of thousands of healthy adults show that many take in less and stay perfectly well-hydrated. The number is easy to remember, which is most of why it stuck.
Where the "8 glasses" rule came from
The exact origin of "8×8" has never been firmly confirmed. Valtin traced its likely root to a 1945 U.S. Food and Nutrition Board recommendation of roughly 1 millilitre of water per calorie of food — about 2.5 litres a day for a typical diet. Crucially, that same recommendation noted that much of this water is already contained in food. As the advice spread through health writers and advertising, the qualifier was dropped, and "about 2.5 litres total" hardened into "eight glasses of water" — counted separately from everything else you drink and eat.
What the evidence actually shows
Three findings stand out. First, no peer-reviewed study has demonstrated that healthy people need 8×8. Second, published intake surveys show average fluid intake below that level in healthy populations, with no sign of harm. Third, caffeinated drinks count: controlled experiments indicate coffee and tea contribute to daily fluid intake for most people, and a moderate, dilute drink such as beer contributes to a lesser extent. The belief that only plain water "counts" is not supported — it lines up with the beverage hydration index, where coffee and tea sit close to water at 1.00.
Valtin was careful about the other side, too. Larger fluid intakes are genuinely advisable in some situations: vigorous work or exercise, hot climates, and certain medical conditions such as a history of kidney stones. The conclusion is not "drink less" — it is that there is no universal magic number.
Is 8 glasses too much?
For most healthy people, no. Drinking around eight glasses is harmless, because the kidneys simply excrete what the body does not need. The concern exists only at the extreme: forcing very large volumes far beyond thirst over a short period can, in rare cases, dilute blood sodium (a condition called hyponatremia). For everyday life this is not something most people need to worry about; it mainly affects endurance athletes and a few specific situations.
So how much should you actually drink?
Reference bodies give ranges, not a single rule. EFSA (2010) estimates about 2.0 litres of total water per day for women and 2.5 litres for men; the IOM (2004) sets higher totals of 2.7 and 3.7 litres — both figures including the water in food. Your own target sits somewhere personal, scaled by body weight, activity, climate, and exercise. Rather than counting glasses, it is more useful to work out how much water you actually need and drink to thirst around it.
