Daily Water Intake Calculator
A daily fluid target from your weight, your training and the weather where you live.
The "eight glasses a day" rule has no particular evidence behind it. Fluid needs scale with body size, how much you sweat and how hot it is — and about a fifth of what you take in comes from food rather than drinks.
How this is worked out
The baseline here is about 33 ml per kilogram of body weight a day, which lands close to the EFSA adequate intakes of roughly 2.0 litres for women and 2.5 litres for men from all sources. Exercise adds around 350 ml for every 30 minutes; heat adds between 10 and 30 per cent; pregnancy adds about 300 ml and breastfeeding about 700 ml.
Thirst is a good guide for most healthy adults under about 65, and urine colour is a better daily check than any calculator — pale straw is the target. Older adults have a blunted thirst response and are better off drinking to a schedule.
It is possible to drink too much. Taking in several litres an hour, particularly during endurance events, can dilute blood sodium dangerously. If you are managing heart failure or kidney disease, follow the fluid limit your doctor has set rather than this number.
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Common questions
Do tea and coffee count?
Yes. At normal intakes the diuretic effect of caffeine is small and the fluid in the cup more than covers it.
How do I know if I am drinking enough?
Pale straw-coloured urine and no persistent thirst. Dark yellow urine, headaches and fatigue point the other way.