Daily Water Intake Calculator

How the calculation works#

The baseline comes from body mass: 30–35 ml per kilogram a day, adjusted for age. The per-kilogram figure falls over the years as total body water declines and the kidneys concentrate urine less effectively.

Two allowances are added on top. Training, priced at average sweat rates: 400 ml per hour for light work, 700 for moderate, 1200 for hard. Heat, up to a litre above 32 °C.

The result is total water for the day. To get what you actually need to drink, the share arriving in food is subtracted: EFSA puts it at 20–30%, and the calculator uses 22%.

The eight glasses rule#

The most famous advice about water has no source. Heinz Valtin, a kidney physiologist, went looking in 2002 for the study it might have come from and found none.

The most plausible origin is a 1945 US National Research Council recommendation: about 2.5 litres of fluid a day for an adult. The next sentence in the same document said most of that quantity is contained in prepared food. In retelling, the second sentence disappeared and the first turned into eight glasses on top of meals.

Читайте также: How Much Water Per Day: The Real Numbers and the Myths

What actually counts#

Source	Contribution to balance	Notes
Water, tea, coffee	Fully	Caffeine at habitual doses does not break the balance
Soups, vegetables, fruit	20–30% of daily water	Cucumber and watermelon are over 90% water
Milk, kefir	Fully	Plus protein and calcium
Sweetened drinks	Technically yes	But they bring sugar you would rather not bring
Alcohol	Negative	Suppresses vasopressin and increases urine output
Metabolic water	250–350 ml	Produced by oxidising fats and carbohydrates

The signs worth watching#

The most practical is urine colour. It tracks concentration better than any subjective sensation except thirst: pale yellow through the day is normal, deep amber signals a deficit. The first sample of the morning is always darker and cannot be judged. B vitamins and beetroot colour urine regardless of hydration.

The second is thirst itself. It is often dismissed as a late signal, which overstates the case: hypothalamic osmoreceptors respond to a 1–2% shift in plasma osmolality, long before anything clinically meaningful. The exception is older people, in whom thirst genuinely blunts with age — there, drinking ahead of it is justified.

Headache, dry mouth and fatigue are unreliable markers: they have countless causes, and attributing them to dehydration by default is a mistake.

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When water becomes too much#

Hyponatraemia is the state in which blood sodium is diluted by excess fluid. Cells — including brain cells — take on water, and in severe cases cerebral oedema follows. Among Boston Marathon participants studied in 2005, 13% of finishers were hyponatraemic, and the leading risk factor was drinking too much along the course.

The practical conclusion: there is no need to drink beyond thirst during prolonged efforts, and past an hour in the heat sodium should go in alongside the water — via a sports drink, salt capsules, or simply salty food.

Читайте также: Salt: How Much You Need and Where It Hides

Who this calculation does not fit#

Heart failure, chronic kidney disease, cirrhosis, diuretic therapy, syndrome of inappropriate ADH secretion — in all of these fluid is deliberately restricted and the target comes from a doctor. It can be considerably lower than the calculated figure, sometimes 1–1.5 L a day.

Kidney stones are the opposite case: there the target urine volume is deliberately raised, usually above 2 L a day, and drinking becomes part of the treatment.