In short: The eight-glasses-a-day rule survives because it is easy to remember, not because it fits anyone. A 55 kg person and a 110 kg person do not need the same bottle, and neither accounts for a run at noon or a tropical August.
Updated August 28, 2026, after a fresh review, with updated visuals and links.
This calculator starts from your body weight (about 35 ml per kilogram), adds what you sweat out during exercise, and bumps the total for hot or humid weather.
Where the number comes from
The base guideline used by many clinicians works out to roughly 30-35 ml per kg. At 70 kg that is about 2.3 to 2.5 liters a day, which conveniently matches the '8 to 10 cups' advice most people actually meet.
Exercise adds 350 ml per 30 minutes of moderate sweat. A 45-minute workout means about half a liter extra, and the hot-climate switch adds another 350 ml for the invisible sweat you never notice.
The result arrives in liters, cups and ounces because nobody thinks in milliliters. If your bottle is 750 ml and your target is 2.8 L, the useful takeaway is simply: about four bottles spread across the day.
Worth remembering
- Front-load the morning. Two glasses right after waking replaces what you lost overnight and makes the rest of the quota easy.
- Thirst is a lagging indicator; pale yellow urine is the reliable one. Clear all day often means you are overdoing it.
Frequently asked
Do coffee and tea count toward the total?
Yes. The mild diuretic effect of caffeine does not cancel the water in the cup. Juice, milk and even water-rich food count too; plain water is just the cheapest source.
Can you drink too much water?
You can, though it is rare. Drinking several liters in a short window can dilute blood sodium (hyponatremia). Spreading intake across the day, as this result assumes, keeps you far from that zone.
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