Daily water intake calculator
A practical daily water target based on your body weight, exercise and climate — with an honest account of how rough a guide this really is.
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This is a rule of thumb, not a clinical guideline. Actual needs vary with diet, medication, altitude and health, and roughly 20% of typical fluid intake comes from food. Thirst is a reliable guide for most healthy adults — there is no strong evidence that hitting a fixed target improves health.
How to use
- Enter your weight. The baseline is about 35 ml per kilogram.
- Add today’s exercise. Roughly 400 ml is added per 30 minutes of activity.
- Set the climate. Warm and hot conditions add 350 ml and 750 ml respectively.
Questions
Is the "eight glasses a day" rule real?
It has no clear scientific origin and no strong evidence behind it. It is often traced to a 1945 recommendation that included water from food, which was later misquoted as eight glasses of drinking water on top.
Does food count towards this?
Yes, and it is significant — roughly 20% of typical fluid intake comes from food, more if you eat a lot of fruit, vegetables or soup. Tea, coffee and milk all count too; caffeine’s diuretic effect is mild and does not put you at a net loss.
How do I know if I am drinking enough?
Pale straw-coloured urine and infrequent thirst are reliable signs for most healthy adults. Thirst is a well-calibrated mechanism — the idea that you are already dehydrated by the time you feel it is largely a myth.
Can I drink too much water?
Yes. Drinking far more than the kidneys can excrete dilutes blood sodium, causing hyponatraemia, which is dangerous and occasionally fatal. It has happened to endurance athletes over-drinking during long races. There is no benefit to forcing fluid past thirst.
Methodology & sources
How this is calculated
A baseline is estimated from body weight and adjusted for exercise duration and hot climates. For context, EFSA’s adequate intake for total water — counting both drinks and the water in food — is 2.0 litres a day for women and 2.5 litres for men in a temperate climate at moderate activity.
Assumptions
- Roughly 20% of total water intake typically comes from food, with about 80% from drinks; the target shown is for total water unless stated otherwise on the tool.
- Baseline figures assume a healthy adult in a temperate climate.
Limitations
- This is a rule of thumb, not a clinical guideline. Requirements vary with diet, medication, altitude, pregnancy, breastfeeding and kidney or heart conditions.
- For most healthy adults, thirst is a reliable guide, and there is no strong evidence that meeting a fixed numerical target improves health outcomes.
- Drinking very large volumes of water quickly can dilute blood sodium — a rare but genuine risk. Anyone advised by a clinician to restrict or increase fluids should follow that advice instead.
Sources
- Scientific Opinion on Dietary Reference Values for water (adequate intake: 2.0 L/day women, 2.5 L/day men, total water) European Food Safety Authority
Sources last checked 30 July 2026. Estimates for general information only — not medical advice, and no substitute for a qualified clinician.
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