How much water you should drink a day
Recommended litres based on your weight, exercise and climate.
Calculator inputs
Results
Enter your values and press “Calculate” to see the result.
In short
- What it calculates
- Recommended litres based on your weight, exercise and climate.
- Formula used
Base = weight × 0.033 L- Example
- A 70 kg person, 45 minutes of exercise, temperate climate:
Daily water intake calculator
The two-litre rule is a simplification: someone weighing 55 kg and someone weighing 95 kg do not need the same, and exercise and heat shift the figure considerably.
How it works
It starts from roughly 33 ml per kilo of body weight, adds 350 ml for every half hour of exercise and adjusts for climate. Water from food adds an extra amount not counted here.
Formula
Base = weight × 0.033 L
Exercise extra = minutes ÷ 30 × 0.35 L
Total = (base + extra) × climate factor
Worked example
A 70 kg person, 45 minutes of exercise, temperate climate:
Base: 2.31 L
Extra: 0.53 L
Total: 2.84 L (about 11 glasses)
Explanation
The eight glasses come from no study
The popular recommendation of eight glasses a day has no clear scientific origin. It is attributed to a misreading of a 1945 American report that did mention around 2.5 litres a day, but went on to say that most of it comes from food. That second sentence got lost along the way.
Food contributes a good deal
Between twenty and thirty per cent of daily water arrives through food. A watermelon is over ninety per cent water, and soups, yoghurts, fruit and vegetables all contribute. That is why agency recommendations speak of total fluid intake, not just water drunk.
Urine colour is a better guide
A pale yellow tone indicates adequate hydration. Dark yellow suggests drinking more. It is an immediate, free indicator and more useful than any weight-based formula, although certain supplements and medicines alter the colour.
Drinking too much carries risk too
Taking in very large amounts in a short time can dilute blood sodium and cause hyponatraemia, an uncommon but serious condition. It occurs mainly in endurance events. Thirst and urine colour are better guides than a fixed figure.
Frequently asked questions
Do coffee and tea count?
Yes, they contribute to hydration. Food also provides 20% to 30% of your daily water.
Can you drink too much water?
In extreme amounts over a short time, yes, because it dilutes blood sodium. It is rare, but spreading intake through the day is sensible.
Where does the eight glasses idea come from?
From an incomplete reading of a 1945 American report that mentioned 2.5 litres a day but clarified that much of it comes from food.
How do I know if I am well hydrated?
Urine colour is the most practical indicator: pale yellow means good hydration and a dark tone suggests drinking more.
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