See how much water to drink each day for your weight.
The familiar "eight glasses a day" has no strong scientific basis — it appears to trace back to a 1945 recommendation that also noted most of that water comes from food, a caveat that dropped out of the retelling.
Current guidance from health authorities is expressed as total water from all sources, and it is higher than eight glasses: roughly 3.7 litres daily for men and 2.7 litres for women. But around 20 percent arrives through food, so the fluid you actually need to drink is lower.
This calculator scales the estimate by body weight and activity, which is a more useful starting point than a fixed figure, since a 55 kg office worker and a 95 kg construction worker plainly do not need the same amount.
For healthy adults with normal kidney function, thirst regulates hydration effectively. The idea that you should drink continuously to pre-empt thirst is not well supported.
A more practical indicator is urine colour: pale straw suggests adequate hydration, dark amber suggests you need more, and completely colourless can indicate you are drinking more than necessary. Frequency matters too — most people urinate six to eight times daily when adequately hydrated.
Older adults are a genuine exception. Thirst sensation diminishes with age, so relying on it alone becomes less reliable and deliberate intake matters more.
Nearly all fluids count toward intake, including tea and coffee. The diuretic effect of moderate caffeine does not offset the water consumed with it. Food contributes meaningfully as well — fruit, vegetables, soups, and yoghurt are largely water.
Requirements rise with heat, humidity, altitude, fever, and exercise. During prolonged or intense activity, losses through sweat include electrolytes as well as water, which is why endurance athletes use electrolyte drinks — replacing large volumes with plain water alone can dilute blood sodium dangerously.
Overhydration is rare but real. Drinking far beyond need, particularly during endurance events, can cause hyponatraemia, which is a medical emergency. More is not automatically better.