"Drink 8 glasses of water a day" is one of the most repeated health rules there is, and also one of the least individualized. It works out to roughly 2 liters, a reasonable amount for many sedentary adults in a mild climate, but it was never derived from a formula that accounts for the things that actually change how much water a specific person needs: body size, activity level, temperature, and diet.
Where the 8-glasses figure actually came from
The rule traces back loosely to a 1945 recommendation that suggested about 2.5 liters of water per day for adults, but that original guidance also noted that much of that amount is typically obtained from food, not just drinking water, a detail that got dropped as the simplified "8 glasses" version spread. Because it's a single flat number, it doesn't scale with body weight, a person who weighs 50 kg and a person who weighs 100 kg have genuinely different fluid needs, yet the 8-glasses rule gives them identical advice.
A more individualized estimate
A commonly used formula scales water need to body weight: roughly 30-35 ml per kilogram of body weight per day. For a 70 kg adult, that works out to about 2.1-2.45 liters, in the same ballpark as the 8-glasses rule for an average-sized adult, but it adjusts automatically for people who weigh significantly more or less. The Water Intake Calculator applies this kind of weight-based formula and then adjusts upward for factors like exercise duration and hot climates, since sweating increases fluid loss that needs to be replaced beyond the baseline.
Diet also contributes meaningfully, roughly 20% of daily fluid intake for many people comes from food, particularly water-rich fruits and vegetables, which is part of why the original 1945 guidance included food as a source and why relying purely on "glasses of water" undercounts total hydration from all sources combined.
Signs that generic rules miss
Beyond formulas, urine color remains one of the simplest practical indicators, pale yellow generally suggests adequate hydration, while dark yellow suggests a need for more fluids, though certain vitamins and medications can also affect color. Thirst itself is a reasonably reliable signal for most healthy adults, despite the persistent myth that thirst means you're "already dehydrated." People with certain medical conditions (kidney disease, heart failure) may need to follow fluid guidance from a doctor rather than general formulas, since both too little and too much water intake can carry health risks in those situations.