People often hear that they should drink eight glasses of water per day. However, that may not be entirely true, and the amount of water a person should drink can vary depending on their age, activity level, and more.
Water is an essential part of life, but how much should a person really be drinking?
The commonly touted wisdom of eight glasses of water per day may be suitable for some people, but it is not a “one-size-fits-all” recommendation.
Some experts say there is a lack of scientific evidence supporting these claims. Others note that promoters include bottled water manufacturers.
Recommended daily water intake
Back in 1945, the U.S. Food and Nutrition Board advised people to consume 2.5 liters, or 84.5 fluid ounces (fl oz), of water per day, including fluid from prepared foods. Today, the Centers for Disease Control and Prevention (CDC) say there is no recommendation for how much plain water adults should consume daily.
The 2015–2020 U.S. Dietary Guidelines do not recommend a specific daily water or fluid intake, but they do recommend choosing plain rather than flavored water and juices.
In the United Kingdom, the National Health Service (NHS) recommends consuming 6–8 glasses of water a day, or 1.9 liters (almost 34 fl oz), including water that is in food. The NHS notes this amount is suitable for a temperate climate. It says more will be needed in hotter climates.
Recommended intake by age
There is no fixed amount of fluid recommended by age, but some patterns emerge among healthy individuals doing a moderate amount of activity in a temperate climate.
The following sections show the average water intake for people of different ages.
Experts do not recommend plain water for infants before the age of 6 months.
The CDC says that if infants over 6 months of age need additional fluid on hot days, they can consume water in a bottle. However, their primary form of fluid and calories should be breast milk or formula.
Children over 12 months of age
Children over 12 months of age should be encouraged to drink water in the following situations:
- as part of their daily routine (for example, after brushing their teeth and before, during, and after playtime at school)
- when the weather is warm
- as an alternative to sweetened drinks and juices
Children should limit their juice consumption to one glass per day.
Parents are advised to keep a pitcher handy to encourage healthy water-drinking habits, and schools should have water fountains or equivalent facilities.
Adults ages 19–30 years
The recommended adequate intakes of total water from all sources each day for most adults between 19 and 30 years of age are:
- 3.7 liters (or about 130 fl oz) for men
- 2.7 liters (about 95 fl oz) for women
People who are pregnant are likely to need an extra 0.3 liters (10 fl oz). Those who are breastfeeding will need an additional 0.7 to 1.1 liters (23–37 fl oz).
Older adults may be at risk of dehydration due to health conditions, medications, loss of muscle mass, reduction in kidney function, and other factors.
Older adults who are well hydrated have been found to have:
Dehydration has been linked to a higher frequency of:
- urinary tract infections
- kidney failure
- slower wound healing
People can consume water by:
- drinking water and other fluids
- eating foods high in water, such as fruits and vegetables
Some surveys suggest that around 20 percent of water intake comes from food, and the rest is from fluids. This depends on diet. A higher intake of fresh fruit and vegetables will mean a higher intake of water from foods.
Here are some examples of the water content of different foods and fluids:
|Water content as a percentage (%)||Food or drink|
|90–99%||fat-free milk, tea, coffee, juicy fruits (e.g., strawberries and cantaloupes), vegetables such as lettuce, celery, and spinach|
|80–89%||fruit juice, yogurt, fruits such as apples, pears, and oranges, vegetables such as carrots and cooked broccoli|
|70–79%||bananas, avocados, baked potatoes, cottage cheese|
|60–69%||pasta, beans, peas, fish, chicken breasts, ice cream|
|30–39%||bread, bagels, cheddar cheese|
|1–9%||nuts, chocolate, cookies, crackers, cereals|
During exercise, people may need to consume more water than usual. The amount they should drink depends on:
- the type and intensity of the activity
- environmental factors, such as temperature
- the size and muscle mass of the individual
Older guidance from the American College of Sports Medicine (ACSM) advises drinking water during activity to prevent dehydration that leads to a decline in body weight of more than 2 percent and changes in electrolyte balance.
These changes, researchers say, can lead to reduced performance.
However, a study published in the International Journal of Exercise Science concludes that a loss of up to 4 percent of water has no effect on real-world sports performance.
This means that, while it is important to be hydrated before a workout — and a person should aim to replace fluid lost after exercise — drinking water during a workout may not be essential. However, if people exercise for long periods, they may benefit from consuming water or an electrolyte beverage.
When should a person drink water?
Most of the time, the body’s sensitive natural mechanisms maintain appropriate fluid levels.
There are two main ways the body does this: (1) through thirst, which tells a person to drink more water; and (2) through urine output, in which the kidneys regulate the water we consume by either emptying it into the urinary bladder or holding onto it in the blood plasma.
The kidneys also regulate the balance of electrolytes, such as sodium and potassium, in the body fluids. Additionally, they receive hormonal signals to conserve or release water into the urine if the brain detects changes in the concentration of the solutes in the blood.
What about eight 8-ounce glasses of water?
It is often said that people need to drink at least eight 8-ounce (oz) glasses of water per day. However, this is an overly simplistic answer to a complicated question.
The body is good at regulating itself, and water is no exception. The body is constantly working to maintain a balance of water coming in and water going out. If a person drinks too much water, the body will excrete more. If they drink too little, it will excrete less.
In addition to body size and activity level, other everyday factors can play a role in determining how much water a person should drink.
For example, consuming more sodium and protein means a person may need to drink more water. Conversely, eating a lot of fruits and vegetables means they may not need to drink as much.
Most of the time, the body will give a person cues that tell them to drink more or less fluid. The body even has a water-regulating hormone — arginine vasopressin — that manages thirst, fluid excretion, and the body’s water balance.
Too much water?
Some people have raised concerns that consuming too much water could be dangerous. If a person drinks too much water, it could lead to hyponatremia, or water intoxication, which is when sodium levels in the blood plasma become too low.
Hyponatremia is rare. When it does occur, it usually affects endurance athletes, people with diabetes, and those taking certain medications.
The amount of water a person needs varies based on their age, size, activities, and the temperature.
Although many people follow the eight-by-eight rule, it may be outdated and overly simplistic. The body is incredibly good at maintaining its water balance, and it urges people to drink more by making them thirsty.
People who work outdoors in hot climates or exercise vigorously may need to consume more water. They can also get fluid from foods high in water, such as fruits and vegetables.