Water intake calculator
Between a fifth and a third of daily water comes from food. Fruit, vegetables, soup and even bread carry it. The figure here is total water intake, so what you need to drink is meaningfully less than the headline. The famous eight glasses a day counts only what goes in a glass, which is one of several reasons it has never had much evidence behind it.
General guidance for healthy adults. Some conditions and medications require fluid restriction or a much higher intake. Follow medical advice over any calculator.
The baseline here is 35 ml of total water per kilogram of bodyweight, a clinical rule of thumb, which puts a 70 kg adult at 2.45 litres a day before adjustments. The panel then adds 12 ml for every minute of exercise and up to a litre for heat, so the defaults of 70 kg and half an hour of exercise come out at 2.81 litres.
How much water to drink
The official reference values are not per-kilogram figures at all. EFSA sets an adequate intake for total water of 2.5 litres a day for adult men and 2.0 for adult women, flat numbers for moderate temperature and moderate activity, and it counts drinking water, all other drinks and the moisture in food together. Food supplies roughly 20 to 30 per cent of that. The 35 ml per kilogram used here is a clinical convention with a different lineage, and its virtue is that it scales with body size, which the flat figures do not. Its cost is that it produces confident-looking numbers for people at the extremes of size where nobody has validated it.
The exercise term is cruder still. It adds a flat 12 ml per minute, so 720 ml an hour regardless of what you are doing, how hard, or how much you sweat. Real sweat rates across athletes span roughly 0.5 to 2 litres an hour, and the only way to know yours is to weigh yourself before and after a session: each kilogram lost is about a litre of fluid. If you train seriously, that fifteen-minute experiment beats any figure this panel can produce.
Eight glasses a day, meanwhile, is memorable and nothing else. Nobody has traced it to a study, it ignores body size, activity and climate, and it counts only what arrives in a glass.
The practical check is downstream rather than upstream. Pale straw urine suggests adequate hydration; dark amber suggests more is needed; consistently clear usually means rather more than necessary. Thirst is a reasonable guide for most healthy adults and becomes less reliable with age, which is the case for older people drinking to a schedule rather than to a sensation.
What people use it for
- Setting a daily hydration target that accounts for bodyweight
- Planning fluid for a long training session
- Adjusting intake in hot weather
- Checking a bottle size against a daily goal
- Comparing a per-kilogram rule against the flat national reference values
- Separating total water from the amount you have to drink
Questions
This panel uses 35 ml per kilogram as a baseline, so about 2.45 litres for a 70 kg adult before exercise and climate. EFSA's flat reference values are 2.5 litres for men and 2.0 for women. Both figures are total water, not glasses of it.
Yes, and it is a larger share than most people expect: roughly 20 to 30 per cent of total water intake. Fruit, vegetables and soup carry most of it.
Yes. The diuretic effect of ordinary coffee and tea is small and does not offset the fluid they contain.
The panel adds 720 ml per hour, which is an average and no more than that. Individual sweat rates range from about half a litre to two litres an hour. Weigh yourself before and after a session; each kilogram lost is roughly a litre to replace.
Yes, and it is a real risk in endurance events. Drinking well beyond losses over a few hours can dilute blood sodium, which is dangerous. More is not automatically better.
Urine colour is the everyday check. Pale straw is about right, dark amber means drink more, and consistently clear usually means more than needed.
Nowhere traceable. It has no clear origin in evidence, ignores body size and climate entirely, and counts only what you drink.