A widely used rule of thumb is to drink about half an ounce of water per pound of body weight per day (roughly 35 ml per kilogram). On top of that, add about 12 ounces (350 ml) for every 30 minutes of exercise to replace what you sweat out. The total counts all the water you need from drinks; food supplies roughly 20% of daily fluid, and coffee, tea and milk count too.
Example: at 160 lb the base need is 80 oz. With 30 minutes of exercise you add 12 oz, for 92 oz, which is about 2.7 liters or 11.5 cups.
No. The "8 glasses" figure is a simple average. Larger people, hot climates, high altitude, illness, pregnancy and breastfeeding all raise needs, so a weight-based estimate is a better starting point.
Yes. Tea, coffee, milk, juice and water-rich foods such as fruit and soup all contribute. Pale yellow urine is a simple sign you are drinking enough.
Yes, though it is rare. Drinking many liters in a short time can dilute blood sodium. Spread intake across the day and talk to a doctor if you have kidney, heart or other medical conditions.
This is a general estimate, not medical advice.
See also: BMI calculator · TDEE calculator · weight converter