Water Intake Calculator

Two and a half litres, and where that number comes from

A moderately active person weighing 70 kg has an estimated daily water requirement of about 2.45 litres — 2,450 ml, or roughly 10 glasses of 250 ml.

Water = weight (kg) × factor for activity level

The factor sits around 35 ml per kilogram for moderate activity, rising with training volume and heat exposure. It is a population-level heuristic, not a physiological measurement, and individual requirements vary considerably.

Where "eight glasses a day" came from

The familiar advice has remarkably thin origins. It traces largely to a 1945 US Food and Nutrition Board recommendation of about 2.5 litres daily — which, in a sentence usually omitted from the retelling, added that most of this quantity is contained in prepared foods.

The food contribution is not trivial. Fruit, vegetables, dal, curd, rice and most cooked meals are substantially water, and food typically supplies 20–30% of total intake. Tea, coffee, milk and buttermilk all count towards fluid intake too — the belief that caffeinated drinks are net dehydrating is not supported at ordinary consumption levels, since the fluid volume far exceeds the mild diuretic effect.

So the figure this calculator produces is closer to total water than to water you must drink from a glass.

Indian conditions

Requirements rise considerably in circumstances common across much of India for much of the year:

Heat and humidity. Sweat losses in a hot, humid climate can exceed a litre an hour during exertion, and humidity impairs evaporative cooling so more sweat is produced for the same cooling effect.

Outdoor work. Construction, agriculture, delivery and field roles in summer carry genuine heat-illness risk, and fluid replacement alone is insufficient — electrolytes matter, because sweat contains sodium. ORS, nimbu pani with salt, buttermilk and lassi replace both.

Fever, vomiting and diarrhoea raise requirements sharply, and this is where ORS specifically — not plain water — is the correct response.

Pregnancy and breastfeeding both increase requirements meaningfully.

Thirst, urine colour, and which to trust

For most healthy adults, thirst is a reliable guide. The mechanism is well calibrated and responds before meaningful dehydration occurs. The main exceptions are older adults, in whom thirst sensation genuinely declines with age, and athletes during prolonged exertion, where losses can outpace the thirst response.

Urine colour is the practical check: pale straw suggests adequate hydration, dark amber suggests drinking more. Note that B vitamins turn urine bright yellow regardless of hydration, and some medications and foods also affect colour.

Overhydration is real

Drinking far beyond requirement is not harmless. Hyponatraemia — dangerously diluted blood sodium — occurs when water intake overwhelms the kidneys' excretion capacity, roughly 0.8–1.0 litres per hour. It has caused deaths in endurance events and in water-drinking challenges.

The practical guidance: there is no benefit to forcing intake well beyond thirst and requirement, and during prolonged exercise, replacing electrolytes alongside fluid matters more than volume alone.

This is general information, not medical advice. Anyone with kidney disease, heart failure, liver disease, or on diuretics or lithium may have fluid restrictions or requirements that differ substantially, and should follow clinical guidance rather than a formula.

Common mistakes

Counting only plain water. Food, tea, milk and curd all contribute.

Ignoring electrolytes when sweating heavily. Water alone can worsen the imbalance.

Using plain water for dehydration from illness. ORS is formulated for it.

Forcing intake well beyond thirst. There is no benefit and a real risk.

Applying one figure year-round. Summer requirements differ substantially from winter.