How much water do adults need?
There is no single number that fits everyone. Health authorities publish reference values for total water, which includes water from food (roughly 20% for most diets) as well as all drinks, including tea, coffee and milk:
| Source | Women | Men |
|---|---|---|
| European Food Safety Authority (EFSA, 2010) | 2.0 L / day | 2.5 L / day |
| US National Academies (2004) | 2.7 L / day | 3.7 L / day |
Because body size matters, many clinicians use a simple weight-based estimate for fluids: about 30 to 35 ml per kilogram of body weight per day for a healthy adult. This calculator uses that range and adds water for exercise, hot weather, pregnancy and breastfeeding.
Base fluid = weight (kg) × 30 to 35 ml
+ about 500 ml per hour of exercise
+ about 500 ml in hot or humid weather
+ 300 ml if pregnant, + 700 ml if breastfeeding (EFSA)
Worked examples
An average day
A 70 kg adult: 70 × 30 = 2,100 ml and 70 × 35 = 2,450 ml, so about 2.1 to 2.5 litres a day. That is roughly 71 to 83 US fluid ounces, or 9 to 10 cups of 8 oz.
With a workout
The same person exercises for 45 minutes: 45 ÷ 60 × 500 = 375 ml extra, giving about 2.5 to 2.8 litres. On a hot day, add another half litre.
Signs you are drinking enough
- Urine colour: pale straw-coloured urine usually means you are well hydrated; dark yellow suggests you need more.
- Thirst: for most healthy adults thirst is a good guide, but it becomes less reliable in older age and during hard exercise.
- Weight before and after exercise: every kilogram lost during a workout is roughly a litre of sweat to replace.
Exercise and heat
Sweat losses vary enormously, from less than half a litre to more than two litres per hour, depending on intensity, temperature and the person. Sports-medicine guidance is to drink enough to avoid losing more than about 2% of body weight during exercise, without drinking so much that you gain weight. For long sessions in the heat, drinks containing sodium (sports drinks or oral rehydration solutions) help replace salt lost in sweat.
Can you drink too much?
Yes. Drinking very large amounts in a short time, far beyond what you lose, can dilute the sodium in your blood (hyponatraemia). It is rare in everyday life but has happened in endurance events and water-drinking challenges. Spread your drinks through the day and drink to thirst during long events.
When needs are different
- Heart, kidney or liver disease: some people are told to limit fluids. Always follow your doctor's advice over any calculator.
- Fever, vomiting or diarrhoea: needs rise; oral rehydration solutions replace both water and salts.
- Older adults: the sense of thirst weakens with age, so regular drinking habits matter more.
- Children need different amounts; this calculator is for adults.
Common mistakes
- Forgetting that all drinks count. Tea, coffee and milk contribute to hydration; a moderate caffeine intake does not cause net fluid loss.
- Forcing a fixed "8 glasses" target. It is a reasonable rough guide for some people but too little for others and more than enough for small adults.
These are general estimates for healthy adults, not medical advice.
Water from food
About a fifth of daily water typically comes from food. Fruit and vegetables such as cucumber, watermelon, oranges and lettuce are mostly water, and soups, yoghurt and cooked grains add more. People who eat plenty of these foods may need to drink a little less than the estimate, while a diet of dry, salty food increases thirst.
A simple daily routine
- A glass of water on waking and one with each meal covers roughly a litre.
- Keep a bottle within reach at work and refill it once or twice.
- Drink before, during and after exercise, more in hot weather.
- Use the colour of your urine as a check: pale straw is the target.
Older adults
The sense of thirst becomes weaker with age, and some medicines increase fluid loss. Older adults can benefit from regular drinking habits rather than waiting until they feel thirsty, unless a doctor has advised a fluid limit.
Frequently asked questions
How much water should I drink a day?
For a healthy adult, roughly 30–35 ml per kg of body weight from drinks, about 2.1–2.5 litres for a 70 kg person, plus more for exercise and hot weather.
Do coffee and tea count towards water intake?
Yes. All drinks contribute, and moderate amounts of caffeine do not cause a net loss of fluid.
Is 8 glasses a day right?
Eight 8-oz glasses (about 1.9 litres) is a rough guide. Larger or more active people usually need more, and food provides some water too.
How much extra water do I need when exercising?
A common starting point is about 500 ml per hour, but sweat rates vary widely. Weighing yourself before and after exercise shows your own loss.
Can drinking too much water be dangerous?
Very large amounts in a short time can lower blood sodium (hyponatraemia). Spread intake through the day and drink to thirst during long events.
Does food count towards water intake?
Yes. Roughly 20% of daily water usually comes from food, especially fruit, vegetables and soups.
Is it better to drink water all at once or spread out?
Spread it through the day. Large amounts in a short time are not more useful and, in extreme cases, can be harmful.
Sources
- European Food Safety Authority (EFSA). Scientific Opinion on Dietary Reference Values for water. EFSA Journal 2010; 8(3): 1459.
- Institute of Medicine (US National Academies). Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (2004).
- American College of Sports Medicine. Exercise and fluid replacement, position stand (2007).
Last reviewed 5 October 2026. Results are estimates; see our disclaimer.