How Much Water Should You Drink a Day?
There is no single magic number for daily water, but a few simple rules get you close. Your needs shift with body size, activity, heat and diet. Here is how to estimate a sensible target.
The common rules of thumb
Two popular starting points: the "8 glasses a day" rule (about 2 litres) and a body-weight estimate of roughly 30–35 ml of water per kilogram. For a 70 kg adult that is about 2.1–2.45 litres a day.
These are starting points, not strict limits — thirst and urine colour (pale straw is the target) are good day-to-day guides.
What raises your needs
- Exercise — add roughly 350–700 ml per hour of activity, more in heat
- Hot or dry climates and high altitude
- Pregnancy and breastfeeding
- Fever, vomiting or diarrhoea (you lose extra fluid)
Food counts too
You do not have to drink every drop. Roughly 20% of daily fluid comes from food — fruit, vegetables, soup — and other drinks count as well. The numbers above are total fluid, not only plain water.
Estimate your target
The Water Intake Calculator turns your weight and activity into a daily target in litres. Pair it with the BMI Calculator if you are also tracking general health markers.
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Last updated: July 6, 2026