How much water you actually need

The "eight glasses a day" rule is repeated everywhere and traces back to no particular study. The guidelines that do exist say something more useful and more variable: your requirement depends on your size, your activity, the climate you are in, and what you eat — and a meaningful share of it does not come from drinking at all.

Body weight gives you a starting point

A common clinical rule of thumb is around 33 ml of water per kilogram of body weight per day. For a 70 kg adult that is 70 × 33 = 2,310 ml, roughly 2.3 litres.

Treat that as an opening estimate, not a target. It scales with the thing that matters most — body mass — which is why it beats a fixed number of glasses for anyone much larger or smaller than average. The published intake guidance from European and US bodies lands in a similar region for total water, around 2 litres a day for women and 2.5 for men.

Food is roughly a fifth of it

Those totals are total water, including what is in your food. Fruit, vegetables, soup, yoghurt and even bread carry water, and together they typically supply 20–30% of daily intake. So a 2.3-litre total is closer to 1.6–1.8 litres of actual drinking.

Tea, coffee and juice all count towards it too. The old claim that caffeine dehydrates you does not survive contact with the evidence at normal intakes — the fluid in a cup of coffee more than offsets its mild diuretic effect.

Activity and heat dominate everything else

Sweat rates during exercise range from about half a litre to two litres an hour depending on intensity, heat and the individual. That single variable can double a day's requirement, and it dwarfs the difference between any two baseline formulas.

Which is why weighing yourself before and after a long session is more informative than any calculator: each kilogram lost is roughly a litre of fluid. Heat, altitude, illness with fever, and pregnancy or breastfeeding all raise the requirement as well.

More is not automatically better

Drinking far beyond your needs is not harmless. Taking on large volumes of plain water quickly can dilute blood sodium — exercise-associated hyponatraemia — which is genuinely dangerous and has occurred in endurance events among slower participants who drank at every station. The advice that reversed it was to drink to thirst rather than to a schedule.

For most people, thirst plus pale-yellow urine is a better instrument than arithmetic. Use a calculator to sense-check whether you are in the right region, then drink to thirst and adjust for heat and exercise. And if you have kidney or heart conditions, or take medication affecting fluid balance, the number that matters is the one your clinician gives you.

Where the eight-glasses figure came from

The most credible explanation is a misreading. A 1945 US recommendation suggested roughly 2.5 litres of water a day for adults and noted, in the same breath, that much of it is contained in prepared food. The qualifier fell away, the number survived, and it became a target for drinking rather than a total including meals.

That is worth knowing because it explains why the rule feels demanding: it is close to right as a total and roughly a third too high as a drinking target.