The '8 glasses a day' rule is a myth with no single source — real needs vary with body size, weather, activity and even what you eat.
A common working estimate is 30–35 ml per kilogram of body weight daily. For a 65 kg adult that's roughly 2–2.3 litres from all sources — including water in food, which typically supplies about a fifth of intake.
Hot and humid weather, outdoor work, and exercise can add 500 ml to well over a litre of extra need. If you're sweating visibly, plain water plus a little salt-and-lemon or ORS-style hydration serves better than water alone in long heat exposure.
Pale-yellow urine through the day generally signals adequate hydration; dark, scanty urine suggests drinking more. Thirst is a late signal — older adults especially should drink on schedule rather than waiting for it.
Forcing enormous volumes in short windows can dilute blood sodium (hyponatremia) — a real risk during endurance events. Steady sipping beats chugging. People with kidney or heart conditions should follow their doctor's fluid guidance specifically.
This article is educational and general. Your situation has its own context — for symptoms that are severe, sudden, persistent or worrying, consult a qualified medical practitioner.
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