Health experts often recommend people to drink eight 8-ounce glasses of water per day. This is commonly known as the “8×8” rule. It may not apply to everyone, though.
Your body is about 60 percent water.
The body constantly loses water throughout the day, mostly through urine and sweat but also from regular body functions like breathing. To prevent dehydration, you need to get plenty of water from drink and food every day.
There are many different opinions on just how much water you should be drinking every day.
Health experts commonly recommend eight 8-ounce glasses, which equals about 2 liters, or half a gallon a day. This is called the 8×8 rule and is very easy to remember.
However, some experts believe that you need to sip on water constantly throughout the day, even when you’re not thirsty.
As with most things, this depends on the individual. Many factors (both internal and external) ultimately affect how much water you need.
This article takes a look at some water intake studies to separate fact from fiction and explains how to easily stay well hydrated for your individual needs.
How much water you need depends on a lot of things and varies from person to person. For adults, the general recommendation from The U.S. National Academies of Sciences, Engineering, and Medicine is about:
- 11.5 cups (2.7 liters) a day for women
- 15.5 cups (3.7 liters) a day for men
This includes fluids from water, beverages like teas and juice, and from food. You get an average of 20 percent of your water from the foods you eat (1, 2).
You might need more water than someone else. How much water you need also depends on:
- Where you live. You will need more water in hot, humid, or dry areas. You’ll also need more water if you live in the mountains or at a high altitude (
- Your diet. If you drink a lot of coffee and other caffeinated beverages you might lose more water through extra urination. You will likely also need to drink more water if your diet is high in salty, spicy, or sugary foods. Or, more water is necessary if you don’t eat a lot of hydrating foods that are high in water like fresh or cooked fruits and vegetables.
- The temperature or season. You may need more water in warmer months than cooler ones due to perspiration.
- Your environment. If you spend more time outdoors in the sun or hot temperatures or in a heated room, you might feel thirstier faster.
- How active you are. If you are active during the day or walk or stand a lot, you’ll need more water than someone who’s sitting at a desk. If you exercise or do any intense activity, you will need to drink more to cover water loss.
- Your health. If you have an infection or a fever, or if you lose fluids through vomiting or diarrhea, you will need to drink more water. If you have a health condition like diabetes you will also need more water. Some medications like diuretics can also make you lose water.
- Pregnant or breastfeeding. If you’re pregnant or nursing your baby, you’ll need to drink extra water to stay hydrated. Your body is doing the work for two (or more), after all.
Many factors affect how much water you need to stay healthy such as your health, activity, and environment.
Many people claim that if you don’t stay hydrated throughout the day, your energy levels and brain function start to suffer.
There are plenty of studies to support this.
One study in women showed that a fluid loss of 1.36 percent after exercise impaired mood and concentration and increased the frequency of headaches (
Another study in China that followed 12 men in university found that not drinking water for 36 hours had noticeable effects on fatigue, attention and focus, reaction speed, and short-term memory (5).
Even mild dehydration can reduce physical performance. A clinical study on older, healthy men reported that just a 1 percent loss of body water reduced their muscle strength, power, and endurance (6).
Losing 1 percent of body weight might not seem like a lot, but it’s a significant amount of water to lose. This usually happens when you’re sweating a lot or in a very warm room and not drinking enough water.
Mild dehydration caused by exercise or heat can have negative effects on both your physical and mental performance.
There are many claims that drinking more water may reduce body weight by increasing your metabolism and curbing appetite.
According to a study, drinking more water than usual correlated to a decrease in body weight and body composition scores. (
Another review of studies found that chronic dehydration was associated with obesity, diabetes, cancer, and cardiovascular disease (
Researchers in another older study estimated that drinking 68 ounces (2 liters) in one day increased energy expenditure by about 23 calories per day due to a thermogenic response, or a faster metabolism (
Drinking water about a half hour before meals can also reduce the number of calories you end up consuming (
One study showed that people who drank 17 ounces (500 mL) of water before each meal lost 44% more weight over 12 weeks, compared to those who didn’t (
Overall, it seems that drinking adequate amounts of water, particularly before meals, may give you a boost in managing appetite and maintaining a healthy body weight, especially when combined with a healthy eating plan.
What’s more, drinking plenty of water has a number of other health benefits.
Drinking water can cause slight, temporary increases in metabolism, and drinking it about a half hour before each meal can help you eat fewer calories.
Both of these effects can contribute to weight loss in some people.
Drinking enough water is required for your body to function in general. Several health problems may also respond well to increased water intake:
- Constipation. Increasing water intake can help with constipation, a very common problem (12, 13).
- Urinary tract infections. Recent studies have shown that increasing water consumption may help prevent recurring urinary tract and bladder infections (
- Kidney stones. An older study concluded that high fluid intake decreased the risk of kidney stones, though more research is needed (
- Skin hydration. Studies show that more water leads to better skin hydration, though more research is needed on improved clarity and effects on acne (
Drinking more water and staying adequately hydrated may help with some health problems, such as constipation, urinary and bladder infections, kidney stones, and skin dehydration.
Plain water is not the only drink that contributes to your fluid balance. Other drinks and foods can have a significant effect.
One myth is that caffeinated drinks, such as coffee or tea, don’t help you hydrate because caffeine is a diuretic.
In fact, studies show that the diuretic effect of these beverages is weak, but they can cause extra urination in some people (
Most foods contain water in varying levels. Meat, fish, eggs, and especially fruits and vegetables all contain water.
Together, coffee or tea and water-rich foods can help maintain your fluid balance.
Other beverages can contribute to fluid balance, including coffee and tea. Most foods also contain water.
Maintaining water balance is essential for your survival.
For this reason, your body has a sophisticated system for controlling when and how much you drink. When your total water content goes below a certain level, thirst kicks in.
This is carefully balanced by mechanisms similar to breathing — you don’t need to consciously think about it.
Your body knows how to balance its water levels and when to signal you to drink more.
While thirst may be a reliable indicator of dehydration, relying on feeling thirsty may not be adequate for optimal health or exercise performance (
At the time thirst strikes, you may be already feeling the effects of too little hydration such as fatigue or headaches.
Using your urine color as your guide can be more helpful to know if you’re drinking enough (21). Aim for pale, clear urine.
There really is no science behind the 8×8 rule. It is completely arbitrary (1,
The most important one may be during times of increased sweating. This includes exercise and hot weather, especially in a dry climate.
If you’re sweating a lot, make sure to replenish the lost fluid with water. Athletes doing long, intense exercises may also need to replenish electrolytes, like sodium and other minerals, along with water.
Your water need increases during pregnancy and breastfeeding.
You also need more water when you have a fever and when you’re vomiting or have diarrhea. If you desire to lose weight, consider upping your water intake too.
Furthermore, older people may need to consciously watch their water intake because the thirst mechanisms can start to malfunction with aging. Studies show that adults over 65 years old are at a higher risk for dehydration (23).
Most people don’t need to focus too much on their water intake, as the body has an automatic thirst signal.
However, certain circumstances do call for increased attention to how much water you’re drinking.
At the end of the day, no one can tell you exactly how much water you need. This depends on many factors.
Try experimenting to see what works best for you. Some people may function better with more water than usual, while for others it only results in more frequent trips to the bathroom.
If you want to keep things simple, these guidelines should apply to the majority of people:
- Drink often enough throughout the day for clear, pale urine.
- When you’re thirsty, drink.
- During high heat and exercise and other mentioned indications, make sure to drink enough to compensate for the lost or extra needed fluids.
- That’s it!