The advice to drink eight glasses of water daily is repeated everywhere and has no clear origin in research. Tracing it leads to a decades-old recommendation that also noted most of that water comes from food, a detail that was dropped.
What the body actually needs
Enough to replace what is lost, which varies enormously with body size, activity, temperature and diet. There is no single number that applies to everyone.
Food contributes substantially – fruit, vegetables, soup, and most cooked food contain a great deal of water. So do tea, coffee and other drinks, despite the persistent belief that caffeinated drinks dehydrate you. At normal consumption they do not.
The simple indicator
Urine colour. Pale straw suggests adequate hydration. Dark yellow suggests drinking more. Completely clear suggests drinking more than necessary, which is harmless in healthy people but not beneficial.
Thirst is also a functioning mechanism in healthy adults, though it becomes less reliable with age.
When more attention is warranted
Heat, exercise, illness with fever or fluid loss, and older age where thirst signalling weakens.
The overhydration point
Drinking far more than needed is not virtuous and in extreme cases is dangerous, because it dilutes blood sodium. This is rare and has occurred in endurance events and in people following aggressive intake targets.
The practical version
Drink when thirsty, drink more when hot or active, glance at the colour occasionally. There is no need to track it and no benefit to forcing more.
General information rather than advice. Anyone with kidney or heart conditions should follow their doctor’s guidance instead.