Water intake calculator
Roughly a fifth of daily fluid comes from food — fruit, vegetables, soup and even bread carry water. The figure here is total fluid intake, so the amount you actually need to drink is about eighty per cent of it. That is one reason the famous "eight glasses a day" advice has never had much evidence behind it: it counts only what goes in a glass.
General guidance for healthy adults. Some conditions and medications require fluid restriction or a much higher intake — follow medical advice over any calculator.
A common clinical baseline is 35 ml of fluid per kilogram of bodyweight, so a 70 kg adult needs about 2.45 litres a day in temperate conditions. Add roughly 400–800 ml per hour of exercise, and more again in heat.
How much water to drink
The "eight glasses a day" advice has no clear origin in evidence and takes no account of body size, activity or climate. Weight-based figures are better but still approximate, because losses vary enormously with sweat rate. The most reliable everyday indicator remains urine colour: pale straw suggests adequate hydration, dark amber suggests more is needed, and completely clear usually means rather more than necessary. Thirst is a reasonable guide for most healthy adults, and becomes less reliable with age.
Questions
Around 35 ml per kg of bodyweight as a baseline — about 2.4 litres for a 70 kg adult — plus more for exercise and heat.