"Drink eight glasses of water a day" is one of the most repeated pieces of health advice, and also one of the least precisely sourced — here's what the actual guidance says.
Where "8 glasses" actually came from
The origin is widely traced to a 1945 U.S. Food and Nutrition Board recommendation that suggested roughly 2.5 liters of water daily — but that same guidance explicitly noted that most of this amount is normally already contained in prepared food, a detail that got dropped as the "8 glasses" simplification spread over the following decades.
What more recent guidance actually recommends
Bodies like the U.S. National Academies of Sciences set total daily water intake recommendations that include water from all sources — beverages and food combined, not just plain water — and the figures differ by sex, with adult recommendations commonly cited in the range of roughly 2.7 liters for women and 3.7 liters for men from all sources, though official guidance varies by country and continues to be refined.
What actually changes your real need
Activity level, climate and heat, individual body size, and even diet (high-fiber, high-produce diets already contribute meaningful water) all shift the number meaningfully in either direction — which is exactly why a single fixed number was always an oversimplification for the population as a whole.
Get a more personalized estimate
A water intake calculator that factors in your weight and activity level gives a more tailored estimate than a flat "eight glasses" rule, though it's still an approximation rather than a personalized medical recommendation.
The bottom line
"8 glasses" is a rough, oversimplified rule of thumb rather than a precise scientific target — your actual need depends on body size, activity, climate and diet, and reasonable official guidance varies accordingly.
Try it yourself
Put this into practice with our water intake calculator.