Water in an Indian summer

A better starting estimate

A common clinical estimate is 30–35 ml of total fluid per kg of body weight per day. For a 60 kg adult that is 1.8–2.1 litres; for an 80 kg adult, 2.4–2.8. European reference values land in the same neighbourhood, at roughly 2.0 litres a day of total water for women and 2.5 for men under temperate conditions.

The word doing the work in all of those figures is total. Roughly 20–30% of it normally arrives as food — dal, sabzi, curd, dahi, fruit, and the chai nobody counts. The number you need to drink is meaningfully lower than the number you need to consume. Full weight-by-weight figures are on the water intake calculator.

Where "eight glasses" came from

A 1945 US recommendation suggested about 2.5 litres of water a day for adults — and, in a sentence almost universally dropped when quoted, noted that most of that is contained in prepared food. The surviving fragment became "drink eight glasses of water," which is a plausible-enough target and has never had trial evidence behind it as a threshold.

What heat actually changes

Sweat rates vary enormously — from a few hundred millilitres an hour in mild conditions to well over a litre an hour for someone working or training in 40 °C heat with high humidity. Humidity matters as much as temperature, because sweat that does not evaporate does not cool you and you simply lose more of it. A Chennai afternoon and a Delhi afternoon at the same temperature are not the same hydration problem.

Practical adjustment: add roughly 500 ml to 1 litre on hot days, and more if you are outdoors or physically working. If you are sweating heavily for hours, plain water alone eventually becomes insufficient — you are losing sodium too, which is what nimbu-pani with salt, chaas and ORS are for.

Total fluid including that obtained from food. These are planning estimates, not requirements — individual sweat rates differ by a factor of three or more.

The test that beats every formula

Urine colour. Pale straw means you are fine; dark yellow means drink more; completely colourless all day usually means you are drinking more than you need. Thirst is a reasonably good signal for healthy adults under normal conditions — it becomes less reliable in older adults, during intense exercise, and in extreme heat, which are precisely the situations where deliberate drinking matters.