Aim for about 30 ml of fluid per kilogram of body weight a day, plus 12 ml for each minute of exercise, and 10–20% more in hot weather. A 70 kg adult at rest in a temperate climate needs about 2.1 litres, or 9 glasses of 250 ml.
The formula
Daily fluid
litres = (30 ml × kg + 12 ml × exercise minutes) × climate factor ÷ 1,000Climate factor
temperate 1.0 · hot 1.1 · very hot or humid 1.2Glasses
glasses = ml ÷ glass size, rounded upThe weight and exercise terms are common rules of thumb, and the climate factors are an allowance chosen for this page. For scale, the European Food Safety Authority's adequate intake for adults in a moderate climate is 2.0 litres a day for women and 2.5 for men, from drinks and food together.
Worked example
The same person at rest in a temperate climate needs 2,100 ml, or 9 glasses.
When to use it, and the mistakes to avoid
Use it to set a daily target, to adjust for a heatwave or a training block, or to turn a litre goal into a number of glasses for a habit tracker.
- Treating it as medical advice. It is an estimate. Illness, medicine and pregnancy change needs.
- Forgetting the rest of your fluids. Tea, coffee, milk and food count.
- Drinking it all at once. Spread it through the day.
- Ignoring thirst. Thirst and pale-yellow urine are good guides.
- Using it for children. The rule of thumb is for adults.
FAQ
How much water should I drink a day?
It depends on your size, how active you are and where you live. This calculator starts from 30 ml per kilogram of body weight, so 70 kg gives 2.1 litres, and adds to it for exercise and heat. For comparison, the European Food Safety Authority's adequate intake is 2.0 litres a day for women and 2.5 for men, from drinks and food together, in a moderate climate.
Does tea, coffee or food count?
Yes. All fluids count toward your intake, including tea, coffee, milk and juice, and food supplies roughly a fifth of the water most people take in. You do not need to drink the whole figure as plain water.
How much extra water do I need when I exercise?
This calculator adds 12 ml for each minute of exercise, which is about 0.7 litres an hour and sits inside the usual range for sweat loss of 0.5 to 1 litre an hour. A long, hard session in the heat can cost more, so drink to thirst and the colour of your urine as well.
Where do the climate multipliers come from?
They are an allowance chosen for this page, not a published standard: 10% more for a hot climate and 20% more for a very hot or humid one. Heat raises needs, but by how much depends on how much you sweat, so treat the figure as a planning aid.
Can you drink too much water?
Yes, though it is rare. Drinking far more than you lose, especially in a short time, can dilute the sodium in your blood. Spread your water through the day and do not force down the target. If you have kidney, heart or liver disease, or are on a fluid limit, follow your doctor's advice instead of this estimate.