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.
Reviewed by Richard Ross · Last updated April 2026
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.
How much water do I need per day?
NHS guidelines recommend 6-8 glasses of fluid per day (approximately 1.5-2 litres). The European Food Safety Authority recommends 2.0 litres for women and 2.5 litres for men from all fluid sources (including food, which contributes about 20-30% of total intake). These are population averages — individual needs vary significantly with body size, activity level, climate, and diet. Thirst is a reliable guide for most healthy adults, but older adults' thirst mechanism is less reliable.
Do caffeinated drinks count towards hydration?
Yes. Despite a mild diuretic effect, caffeinated drinks such as coffee and tea do count towards daily fluid intake. Research shows that the fluid in a caffeinated drink more than offsets the mild diuretic effect, resulting in a net hydration benefit. Alcohol, however, is a stronger diuretic and should not be counted towards hydration — it increases urine production beyond the fluid consumed. Water, herbal teas, milk, fruit juices, and soft drinks all contribute to daily fluid intake.
What are the signs of dehydration?
Early signs of dehydration (1-2% body weight fluid loss) include thirst, dark urine, reduced urine frequency, dry mouth, and mild fatigue. At 3-4% loss: reduced performance, headache, and difficulty concentrating. At 5%+: heat exhaustion, muscle cramps, nausea. Urine colour is a practical indicator: pale straw yellow indicates good hydration; dark amber indicates dehydration; colourless urine may indicate overhydration. Older adults and athletes should monitor proactively as their thirst sensation is less reliable.
Does drinking more water help with weight loss?
Drinking water before meals can reduce calorie intake by increasing feelings of fullness. A clinical study found that drinking 500ml of water before each main meal led to significantly greater weight loss over 12 weeks compared to controls. Cold water also requires the body to expend energy warming it to body temperature, though this effect (approximately 8 kcal per 500ml glass) is small. Replacing sugary drinks with water reduces calorie intake substantially for those who regularly consume them.
Can you drink too much water?
Yes — hyponatraemia (water intoxication) occurs when excessive water intake dilutes sodium levels in the blood. It is rare in healthy adults drinking in response to thirst, but has occurred in endurance athletes drinking large volumes without electrolyte replacement, people taking MDMA (which causes compulsive water drinking), and those with kidney disease. Symptoms include nausea, headache, confusion, and in severe cases, seizures and coma. Drinking more than 1 litre per hour is the main risk scenario.
How does exercise increase water requirements?
During exercise, sweat rates range from 0.5 litres per hour in mild conditions to 2+ litres per hour in hot, humid conditions during intense exercise. Losing 2% of body weight as sweat impairs performance; losing 5%+ is dangerous. Athletes should aim to replace 150% of sweat losses in the 4-6 hours following exercise. Sports drinks with electrolytes (sodium, potassium) are beneficial for sessions over 60-90 minutes to replace electrolytes lost in sweat, not just fluid.
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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.