| Adult men | About 3.7 L of total water per day | The figure includes water from drinking water, other beverages, and food. Individual needs vary with body size, activity, climate, and diet. | Use thirst, urine color, activity, and weather as practical indicators rather than treating the value as a rigid target. |
| Adult women | About 2.7 L of total water per day | Foods and beverages both contribute to total water intake, so the amount of plain water required differs between individuals. | Drink regularly throughout the day and increase intake when sweating, exercising, or exposed to hot conditions. |
| Pregnancy | About 3.0 L of total water per day | Additional fluid supports increased blood volume, fetal development, and other physiological changes. | Spread fluids across the day. Personal recommendations may differ, especially with swelling, vomiting, or medical conditions. |
| Breastfeeding | About 3.8 L of total water per day | Milk production increases fluid requirements, although the exact need depends on milk production, climate, diet, and activity. | Drink to thirst and keep a beverage available during feeding sessions; forcing excessive fluids does not reliably increase milk supply. |
| Physical activity | No single universal amount | Sweat loss depends on exercise intensity, duration, body size, clothing, fitness, and environmental conditions. | Drink during prolonged or intense activity, replace fluids afterward, and avoid drinking substantially more than sweat losses. |
| Hot or humid weather | Needs increase with sweating | Heat raises sweat production and can increase the risk of dehydration, particularly during physical work or exercise. | Take frequent drinks, rest in cool areas, and replace electrolytes when sweating heavily for long periods. |
| Illness, fever, vomiting, or diarrhea | Needs may rise substantially | Fever and gastrointestinal fluid losses can reduce body water and electrolytes more quickly than usual. | Use oral rehydration solutions when appropriate and seek medical advice for severe, persistent, or bloody symptoms. |
| High-fiber or high-protein diet | Needs vary with food choices | Fiber requires fluid to support normal bowel function, while protein metabolism produces nitrogenous waste that must be excreted. | Increase fluids gradually with fiber and monitor bowel comfort, thirst, and urine concentration. |
| Older adults | No separate universal target | Thirst sensation may become less reliable, while mobility limitations, medications, and chronic conditions can affect hydration. | Plan regular drinks and ask a healthcare professional about restrictions related to heart, kidney, or liver disease. |
| High altitude or dry air | Needs may increase modestly | Dry air can increase respiratory water loss, and altitude may increase breathing rate and urination during acclimatization. | Drink consistently, especially during travel and outdoor activity, but avoid force-drinking beyond thirst and normal losses. |