In this article
“Eight glasses a day” is the rule almost everyone knows, and it is not a bad guess for many adults. But it is a round number from a very old statement, not a measurement of you. Your need depends on your body size, how much you move, how hot it is, and how much water your food already gives you. This guide gives a practical way to work it out.
A weight-based estimate
Multiply your weight in kg by 30 and by 35 to get a range in millilitres. It is a starting point for a healthy adult in mild weather with light activity.
| Body weight | At 30 ml per kg | At 35 ml per kg | In 250 ml glasses |
|---|---|---|---|
| 50 kg | 1.50 L | 1.75 L | 6 to 7 |
| 60 kg | 1.80 L | 2.10 L | 7 to 8 |
| 70 kg | 2.10 L | 2.45 L | 8 to 10 |
| 80 kg | 2.40 L | 2.80 L | 10 to 11 |
| 90 kg | 2.70 L | 3.15 L | 11 to 13 |
This is total fluid, and includes tea, coffee and the water in food only if you count it that way. Many people use the figure for drinks alone and let food top up the rest.
The two lines make a band. Most healthy adults in everyday conditions fall inside it.
What the health bodies say
The US National Academies set an adequate intake of total water of about 3.7 litres a day for men and 2.7 litres for women. That figure includes water from food and every drink. About 20 percent normally comes from food, so the drinks part is about 3.0 litres for men and 2.2 litres for women.
- Water, tea, milk, juice and other drinksabout 2.2 L
- Food (fruit, vegetables, soup, curd, rice)about 0.5 L
For men the same split of 3.7 litres gives about 3.0 litres from drinks. These are averages for healthy adults, not targets to hit to the millilitre.
Do tea, coffee and food count?
- Tea and coffee count. The caffeine has a small, short effect. For regular drinkers the water in the cup more than makes up for it.
- Milk, buttermilk, juice and soup count. Sugary drinks add calories, so water should still be the main drink.
- Fruit and vegetables are mostly water: cucumber is about 95 percent, watermelon about 92 percent and an orange about 87 percent.
- Alcohol does not help. It makes you pass more urine.
More when you sweat
Sweat is the big variable. Light sweating loses a few hundred millilitres an hour and a hard session in heat can lose a litre or more. A practical rule is to add about 0.5 litre for each hour of moderate exercise, and more in heat or humidity.
Illustrative. Start from the weight-based figure and add for exercise and heat. Spread the drinking across the day.
The urine colour check
A simpler guide than any formula is the colour of your urine. Pale straw colour is about right. Darker means drink more. Completely colourless, all day, can mean you are drinking more than you need.
Some vitamins, foods and medicines change urine colour. Use the check as a hint, not a diagnosis.
Eight glasses: where it came from
- Easy to remember
- About 1.9 litres at 240 ml a glass
- Ignores size, heat and exercise
- Scales with your body
- Add for exercise and heat
- Check by thirst and urine colour
A 1945 US statement said adults need about 2.5 litres a day and added that most of it is already in prepared food. The second sentence was forgotten, and the first became ‘eight glasses’.
Can you drink too much?
Yes, but it is rare. Drinking several litres in a short time, far beyond thirst, can dilute the sodium in your blood, a condition called hyponatraemia. It is seen in some endurance events and in people forced to drink large amounts. Healthy adults who drink to thirst are not at risk.
Easy ways to drink enough
- Drink a glass when you wake up, before you reach for tea or coffee.
- Keep a bottle you like within sight, and finish it by lunch and again by evening.
- Add a glass with each meal, and one before and after a workout.
- On hot days, sip regularly, and add a pinch of salt or an ORS-style drink if you are sweating heavily for hours.
If you are managing your weight too, read BMR and TDEE and the heart rate zones guide for planning exercise.
See also: how many km 10,000 steps really is.
Questions people ask
How much water should I drink a day?
A common estimate is 30 to 35 ml per kg of body weight, so about 2.1 to 2.45 litres for a 70-kg adult. Health bodies give a total of about 2.7 litres for women and 3.7 litres for men from all drinks and food.
How many glasses is 2 litres of water?
About eight glasses of 250 ml. With 200 ml glasses it is ten.
Is 3 litres of water a day too much?
Not for most healthy adults, especially if you are large, active or in hot weather. It is too much if you force it down with no thirst, or if your doctor has set a fluid limit.
Does tea or coffee count as water?
Yes. They are mostly water, and for regular drinkers the mild diuretic effect of caffeine does not cancel it out.
How much water per kg of body weight should I drink?
About 30 to 35 ml per kg per day in everyday conditions. Add for exercise and heat.
How do I know if I am dehydrated?
Common signs are thirst, dark yellow urine, a dry mouth, headache and feeling tired or dizzy. Severe dehydration, with confusion, a very fast heartbeat or no urine, needs medical help.
Is the 8 glasses a day rule a myth?
It is not harmful, but it is not a scientific target either. It is a rough figure that fits many adults and ignores body size, activity, climate and the water in food.
Can drinking too much water be dangerous?
Rarely. Very large amounts in a short time can lower blood sodium. Healthy people who drink to thirst are very unlikely to reach that point.