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.
Runs entirely in your browser — nothing is uploaded.
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.