Water Intake Calculator
Find out how much water you should drink each day. Based on your weight, activity level, and climate, the calculator shows your recommended daily intake in litres and glasses.
How Water Intake Calculator works
How daily water requirements are estimated
A common evidence-based guideline is 35 ml of water per kilogram of body weight per day as a baseline. This is consistent with European Food Safety Authority (EFSA) recommendations. The EFSA reference value is 2 litres/day for women and 2.5 litres/day for men, which corresponds to a 57–71 kg reference weight — matching the 35 ml/kg formula. Total fluid intake includes all beverages, not just water.
Activity and heat increase fluid needs
Exercise significantly increases fluid requirements through sweat. Light to moderate exercise in temperate conditions adds approximately 500 ml of fluid need; intense exercise or a physical job adds approximately 1,000 ml or more, depending on intensity and duration. Hot and humid environments increase sweat rates further — in very hot conditions, hourly fluid losses during activity can exceed 1 litre.
Signs of dehydration
Mild dehydration (1–2% of body weight in fluid loss) causes thirst, darker urine, reduced concentration, and mild fatigue. Dehydration of 2–3% impairs physical performance and cognitive function noticeably. Urine colour is a practical indicator: pale yellow is well-hydrated, darker amber indicates you should drink more. Thirst is also a reliable indicator for most healthy adults.
Counting all fluids, not just water
Your daily fluid target can be met by all non-alcoholic beverages, including tea, coffee, juice, and milk. The NHS notes that caffeinated drinks like coffee and tea have a mild diuretic effect but still contribute net fluid — they do not dehydrate you at normal consumption levels. Food also contributes about 20% of daily fluid intake, particularly water-rich foods like fruits and vegetables.
Frequently asked questions
How much water should I drink per day?
The NHS recommends 6–8 glasses (approximately 1.2–2 litres) of fluid per day in a temperate climate. This can come from any non-alcoholic drink. Higher body weight, exercise, hot weather, and some medical conditions increase requirements. This calculator provides a personalised estimate based on weight and activity.
Can you drink too much water?
Yes — drinking excessive amounts of plain water in a short period can cause hyponatraemia (low blood sodium), which can be dangerous or fatal. This is rare and mainly occurs in endurance athletes who drink large volumes without replacing electrolytes, or in some psychiatric conditions. For most people, thirst regulation prevents overdrinking.
Does coffee count towards my water intake?
Yes. Despite containing caffeine, coffee and tea have a mild diuretic effect that is outweighed by their fluid content. Both contribute to daily fluid intake. The NHS explicitly includes tea, coffee, and soft drinks in the recommended 6–8 glasses per day.
Do I need to drink more water when exercising?
Yes. The general guidance is to drink 400–600 ml of fluid in the 2 hours before exercise, drink regularly during exercise (aiming to maintain hydration without overdrinking), and rehydrate after exercise. As a rough guide, weigh yourself before and after exercise — each kilogram of body weight lost corresponds to approximately 1 litre of fluid needed for replacement.
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This calculator provides estimates for informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider.