Water Intake Calculator
Estimate a daily fluid target from bodyweight, activity and climate.
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Daily fluid target
Where the “eight glasses” figure came from
The familiar advice to drink eight glasses a day has no clear evidential origin, and it ignores the two variables that matter most: your size and what you are doing. A weight-based estimate is more defensible — roughly 35 ml per kilogram for a healthy adult at rest, which puts a 70 kg person near 2.4 litres and a 100 kg person near 3.5.
Exercise adds to that substantially, because sweat losses scale with duration and intensity rather than bodyweight. A common allowance is around 400 ml per 30 minutes of activity, and hot or humid conditions raise it further. These are planning figures: actual sweat rates vary several-fold between individuals, and the only precise method is weighing yourself before and after a session.
Two corrections worth knowing. Food contributes roughly 20% of total fluid intake— fruit, vegetables and soup especially — so the drinking target is lower than the total figure. And tea and coffee do count: the diuretic effect of moderate caffeine is far smaller than the fluid volume delivered. Thirst is a reasonable guide for most healthy adults, though it becomes less reliable with age. Urine colour is the simplest practical check.
Medical disclaimer. This calculator is for general information and education only. It is not medical advice, not a diagnosis, and not a substitute for professional care. Fluid needs change substantially with kidney disease, heart failure, pregnancy, and some medications, and a few conditions require fluid restriction rather than encouragement.Speak to a doctor or another qualified clinician about any health decision.Last reviewed .
Frequently asked questions
How much water should I drink a day?
A weight-based estimate of roughly 35 ml per kilogram is a better starting point than a fixed number of glasses — about 2.4 litres for a 70 kg adult at rest. Add fluid for exercise and hot weather, and remember that around 20% of intake typically comes from food.
Do tea and coffee count towards my intake?
Yes. The diuretic effect of moderate caffeine is much smaller than the fluid the drink delivers, so both contribute net fluid. Very high caffeine intakes are a different matter, but a few cups a day count normally.
Can you drink too much water?
Yes, though it is rare. Drinking far more than the kidneys can excrete dilutes blood sodium, which is called hyponatraemia and can be serious. It occurs mainly in endurance events where people drink heavily without replacing electrolytes.
Is thirst a reliable guide?
For most healthy adults, largely yes. Thirst becomes less reliable with age, during intense exercise, and in illness. Urine colour is a useful supplementary check — pale straw suggests adequate hydration, dark amber suggests more is needed.
Does the calculator account for medical conditions?
No. Kidney disease, heart failure and some medications change fluid requirements substantially, and a few conditions require restriction rather than encouragement. Follow clinical advice over any general estimate.