Where the number comes from
A common clinical starting point is about 30–40 ml of fluid per kilogram of bodyweight per day, with roughly 12 ml added per minute of exercise to replace sweat losses. This calculator uses 35 ml/kg and adjusts for climate.
Food contributes meaningfully — typically 20–30% of total fluid intake — so the drinking target is somewhat lower than the total-fluid requirement. Fruit, vegetables and soups all count.
The "eight glasses a day" myth
The familiar advice to drink eight glasses of water daily has no strong research behind it. It appears to trace back to a 1945 US recommendation of about 2.5 litres of fluid per day — a document that also noted most of that comes from food, a caveat that was quietly dropped as the advice spread.
Bodyweight-based estimates like the one above are more defensible, but they are still starting points. Kidney function, medication, altitude and diet all shift the real requirement.
Reading your own hydration
Urine colour is the most practical everyday indicator: pale straw suggests adequate hydration, dark amber suggests you are behind. Thirst is a reliable signal for most healthy adults, though it blunts with age, which is why older people are more prone to dehydration.
Overhydration is a real if uncommon risk. Drinking far more than the kidneys can excrete — roughly a litre per hour — dilutes blood sodium and can cause hyponatraemia, which has killed endurance athletes who over-drank plain water during long events.
Frequently asked questions
Do coffee and tea count? +
Yes. The diuretic effect of moderate caffeine is smaller than the fluid the drink provides, so they contribute net positively to hydration.
Does coffee dehydrate you? +
Not at typical intakes. The mild diuretic effect of caffeine is more than offset by the water in the drink, so coffee and tea contribute net positively to daily fluid.
Should I drink more in hot weather? +
Yes. Sweat losses can exceed a litre an hour in heat and humidity, and electrolytes matter as well as water once sessions run beyond an hour.