How much water do you need?
Water makes up more than half of an adult's body weight and is involved in almost everything it does, from regulating temperature to carrying nutrients and removing waste. The familiar “eight glasses a day” rule is easy to remember, but real needs depend on body size, activity and climate. A larger, active person in a hot country needs considerably more than a smaller, sedentary person in a cool one.
This calculator starts from a widely used guideline of about 33 ml per kilogram of body weight and adds water for exercise, heat, pregnancy and breastfeeding. For reference, the European Food Safety Authority suggests total water intakes of about 2.0 liters a day for women and 2.5 liters for men from all drinks and food combined.
How to use it
- Enter your body weight and choose kilograms or pounds.
- Enter the minutes of exercise you do on a typical day.
- Tick the box if you live in a hot or humid climate, and select pregnancy or breastfeeding if it applies.
- Read your daily target in liters, glasses and fluid ounces.
How the estimate is calculated
Water (ml) = weight (kg) × 33 + 350 per 30 min of exercise + 500 if hot + 300 if pregnant or 700 if breastfeeding
The exercise allowance replaces fluid lost through sweat during moderate activity. Heavy sweaters and endurance athletes may need more, and should also replace electrolytes. The pregnancy and breastfeeding amounts follow EFSA's reference values for additional intake.
About 20% of daily water typically comes from food, especially fruit, vegetables and soups, so drinking slightly less than the target can still be enough.
Example
A person weighing 70 kg needs about 70 × 33 = 2,310 ml as a baseline. Adding 30 minutes of exercise (350 ml) brings the total to about 2.7 liters, or roughly 11 glasses of 250 ml. On a hot day, add another half liter.
More worked examples
In pounds: a 180 lb person needs about 2.7 liters, or roughly 91 US fluid ounces, before exercise.
Active in the heat: a 60 kg person who exercises 45 minutes in a hot climate needs about 3.0 liters a day.
Common mistakes to avoid
- Ignoring water from food. Around a fifth of daily water comes from food such as fruit and soup.
- Drinking it all at once. Spread intake through the day and follow your thirst.
- Applying it with a medical condition. Some heart and kidney conditions need fluid limits; follow your doctor's advice. For energy needs, use the calorie calculator.
Sources and methodology
The calculations on this page follow these published standards and references. The code is covered by automated tests against known values.