Water makes up about 60% of your body weight and is involved in virtually every physiological process. Yet most hydration advice is oversimplified or just wrong.
This general guideline has no strong scientific basis. Your actual needs depend on body size, activity level, climate, and diet.
Adequate intake is about 3.7 liters/day for men and 2.7 liters/day for women — from all sources, including food (fruits and vegetables contribute significantly).
Pale yellow urine is the simplest indicator. Dark yellow or amber means drink more. Clear urine may indicate overhydration.
For exercise lasting over 60 minutes, sodium and potassium become important. Sports drinks or electrolyte tablets can help, though whole foods like bananas and salted nuts work equally well.