How much water should you drink?
The "8 glasses a day" rule ignores your size, how much you move, and the weather. Get a general daily target based on the factors that actually matter - then let thirst do the fine-tuning.
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Method & sources
We estimate total daily fluids at about 33 mL per kilogram of body weight (within the common 30–35 mL/kg clinical range), adjusted for your activity level and the day's climate, plus roughly 350 mL per 30 minutes of exercise. Because about 20% of your water normally comes from food, the "water to drink" figure is lower than the total. As a sanity check, population reference intakes are ~3.7 L/day total water for men and ~2.7 L for women (NAM).
- National Academies (IOM). Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (2004) - total water 3.7 L (men) / 2.7 L (women); ~80% from beverages.
- American College of Sports Medicine. Position Stand: Exercise and Fluid Replacement.
- EFSA. Scientific Opinion on Dietary Reference Values for Water (2010).
- Valtin H. "Drink at least eight glasses of water a day." Really? Am J Physiol Regul Integr Comp Physiol. 2002;283(5):R993–R1004.
Why "8 glasses" is a myth.
There is no single right number. Fluid needs scale with your body size, how much you sweat, and the temperature around you. A common starting estimate used in nutrition is roughly 30 to 35 mL of total fluid per kilogram of body weight per day, adjusted up for exercise and heat - which is what this tool does.
Food counts, and so does thirst
Around a fifth of most people's daily water comes from food - fruit, vegetables, soups, even coffee. So the glass count is lower than the total. The simplest real-world check is your body: if you rarely feel thirsty and your urine is pale yellow, you are generally well hydrated.
Water vs. electrolytes
When you sweat heavily - long or intense exercise, or hot weather - you lose electrolytes (sodium, potassium, magnesium), not just water. Replacing only plain water can leave you feeling flat. That is when electrolytes matter, alongside fluids.
What this tool isn't
It's a general wellness estimate, not medical advice. Kidney, heart, and certain other conditions - and some medications - change fluid needs, sometimes a lot. If any of those apply to you, your provider's guidance comes first.