How much water should you drink a day? The calculator
Estimate how much to drink based on your weight, today's exercise, heat and sweating. A rough estimate, not a quota.
Short answer
An adult needs about 2 to 2.5 litres of water a day in total, including 1.5 to 2 litres from drinks: the rest comes from food. European reference values are 2 L for women and 2.5 L for men. Exercise and heat increase this need.
Calculator
Validated inputsYour settings
Your guide for the day
To drink
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per day, from drinks
Equivalent to
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250 ml glasses
Total water
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L / day, including food
Calculation details
What the tool measures
The 8-glass rule is not based on any study
“You need to drink 2 litres of water a day”: the figure is everywhere, and often misunderstood. Official reference values refer to total water, including food. EFSA sets an adequate intake of 2.0 L per day for women and 2.5 L for men. Fruit, vegetables, soups and meals provide about 20 to 30% of this.
In practice, most sedentary adults in a temperate climate meet their needs with 1.5 to 2 litres of drinks. Coffee and tea count. Needs increase mainly with sweating: during exercise, you often lose more than half a litre per hour, and considerably more in the heat.
For a healthy person, thirst and urine colour are enough most of the time. Drinking far beyond this has no proven benefit for skin, energy or flushing out “toxins”.
Interpreting the result
Interpret without overpromising
The calculator displays three figures:
- To drink: your drinks target for the day, including exercise and heat.
- Glasses: the same amount in 250 ml glasses.
- Total water: drinks plus water from food, the figure comparable to official reference values.
Pale yellow urine and no persistent thirst generally indicate that you are drinking enough. Dark urine at the end of the day or after exercise suggests drinking a little more.
Calculation and assumptions
Methodology
- Baseline total water = weight × 30 ml. For 70 kg, this gives 2.1 L, consistent with EFSA reference values.
- Baseline drinks = 80% of total water, with the rest coming from food.
- Exercise = duration × sweat rate (0.5, 0.8 or 1.2 L/h depending on your answer) × heat factor (1, 1.25 or 1.5). These rates are average guides: actual sweating varies widely between people.
- Heat outside exercise: +300 ml in hot weather, +600 ml above 30 °C.
The result is rounded to the nearest tenth of a litre.
Sources consulted
Sources
- EFSA (2010) : Scientific Opinion on Dietary Reference Values for water
Adequate total water intake: 2.0 L per day for women and 2.5 L for men, including food.
- National Academies (2004) : Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate
US reference values: 2.7 L per day for women and 3.7 L for men in total water, with thirst as the main everyday guide.
- American College of Sports Medicine (2007) : Exercise and fluid replacement
Reference position statement on hydration during exercise: avoid water losses exceeding 2% of body weight and adjust intake to your own sweat rate.
Scope of validity
Limitations to know
- Actual sweat rate varies widely. To find yours, weigh yourself before and after a one-hour session: each kilo lost corresponds to about one litre.
- The calculation does not apply in cases of heart failure or renal impairment, prescribed fluid restriction, pregnancy or in children.
- Drinking very large amounts of water without salt during prolonged exercise can cause hyponatraemia, a dangerous drop in blood sodium.
Frequently asked questions
Before using the result
How much water should you drink per day?
About 1.5 to 2 litres of drinks for a sedentary adult in a temperate climate, or 2 to 2.5 litres of total water including food. Exercise and heat increase this need.
Do you really need to drink 2 litres of water a day?
Not necessarily as plain water. The 2 to 2.5 litre reference value refers to total water, including food and drinks. Coffee, tea, milk and soups count.
Does coffee dehydrate you?
No, not in usual amounts. Coffee has a mild diuretic effect, but it provides more water than it causes you to lose. It counts towards your drinks.
How do I know if I am drinking enough?
Pale yellow urine and no persistent thirst are good guides. Dark urine, a dry mouth or headaches after exercise in the heat indicate a shortfall.
Can you drink too much water?
Yes. Drinking several litres without salt in a short time, especially during prolonged exercise, can lower blood sodium. Drink to thirst and add sodium after more than an hour of intense exercise.