General adult guideline: about 2.7 liters/day for women and 3.7 liters/day for men (from all beverages and food)
Water-only guideline (common estimate): about 2.0 liters/day for women and 2.5 liters/day for men
If you prefer a simple body-weight rule: ~30–35 mL per kg per day
For exercise: add ~0.4–0.8 liters per hour of moderate activity (more for longer/hot/humid conditions)
For hot weather: increase intake by ~0.2–0.5 liters per hour depending on sweat rate
For dehydration signs: drink enough to produce pale-yellow urine and to maintain normal thirst
Pregnancy: typically ~2.3 liters/day (women) plus additional needs based on thirst and activity
Breastfeeding: typically ~3.1 liters/day (women) plus additional needs based on thirst and activity
Kidney, heart, or fluid-restriction conditions: follow clinician-specific limits
When to seek medical advice: inability to keep fluids down, severe weakness/confusion, very dark urine, or signs of heat illness