The familiar advice to drink eight glasses a day has no clear origin in research. The most-cited candidate is a 1945 US Food and Nutrition Board recommendation of about 2.5 litres a day — which included, in the very next sentence, the note that most of this quantity is contained in prepared foods. The qualifier was dropped and the number survived.
The National Academies of Sciences, Engineering, and Medicine set adequate intakes of roughly 3.7 litres of total water a day for men and 2.7 litres for women, from all sources including food. Around 20 per cent typically comes from what you eat, which is why this calculator shows a figure for drinks only. Those are population averages set to avoid deficiency, not personal targets: a small, sedentary person in a cool climate needs far less than a large, active one in the heat.
How to judge it properly
Two everyday signals beat any calculation. Thirst is a well-calibrated mechanism in healthy adults under about 65, and drinking to thirst keeps almost everyone adequately hydrated. Urine colour is the second: pale straw suggests you are fine, and consistently dark yellow suggests you are behind. Both signals weaken with age — older adults often feel less thirst even when they need fluid — and some medicines and vitamin supplements change urine colour independently of hydration.
What changes it
Heat, humidity, exercise duration and intensity, altitude, fever, vomiting and diarrhoea all raise needs, sometimes steeply. Caffeinated drinks count towards intake: the mild diuretic effect does not cancel out the fluid they contain. Alcohol is the exception worth noting, since it suppresses vasopressin and increases fluid loss. Drinking far beyond need is not harmless either — exercise-associated hyponatraemia, where blood sodium falls dangerously, happens in endurance athletes who drink heavily during long events.