How much water should you drink a day?

A study conducted by the US National Academies of Sciences, Engineering, and Medicine recommends a daily intake of approximately 3.7 liters (a little less than a gallon or 16 cups) of water a day for men and 2.7 liters (0.7 gallons or 11 cups) for women. These findings came as a part of their study entitled Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate.

The Institute of Medicine (IOM) recommends a little less. They say the required intake is 3 liters (13 cups ) of water each day for men and a little over 2 liters (half a gallon or 9 cups) for women. Pregnant women should drink about 2.4 liters (10 cups) of water whereas those who breastfeed need 2.8 liters (12 cups).

This might seem like a departure from the 8x8 rule (drink eight 8 ounce glasses of water a day) but that is an outdated recommendation made by the Food and Nutrition Board in 1945, which suggested that a person consume one milliliter (ml) of water per calorie of food consumed. If the average American consumes 1,900 calories per day, it implies an intake of 1900 ml (64 fluid ounces) of water.

Furthermore, the recommendation did not account for water consumed in other beverages like teas and juice, or water in the foods one consumes.

On average, 20 percent of water comes from the foods you eat. At the same time, the body is constantly losing water in the form of urine and sweat. Regular bodily functions like breathing cause loss of water.

The body is approximately 60 percent water and needs it for every bodily function. It carries nutrients to your cells, removes toxins from organs, lubricates joints, and helps you digest the food you eat. Water maintains body temperature and is therefore extremely crucial for overall good health. This diagram shows the percentage of water in various parts of the body.

If you don't stay hydrated, you may notice a drop in energy levels and brain function. A study conducted in China by the Department of Nutrition and Food Hygiene at Peking University entitled “Effects of Dehydration and Rehydration” followed 12 men who abstained from drinking water for 36 hours. Tests on their mood and cognitive function were administered before (the baseline), following dehydration, and after they were given water to rehydrate. Their average results under these three conditions are plotted below. Researchers concluded that dehydration had considerable effects on energy levels, attention, focus, and even short-term memory.

To answer the question about how much water you should drink every day, experts generally agree between 2-4 liters.

What Affects Daily Water Needs?

Daily water needs are affected by where you live, the climate, diet, lifestyle choices, health conditions, pregnancy or breastfeeding, and age. Your body may need more water than others depending on the work it is doing.

• Where You Live: Where you live affects daily water needs. When humidity and temperatures increase, daily water needs increase. For example, a person in Saudi Arabia needs more daily water than a person in Germany. The same is true for people living in mountainous areas or higher altitudes. A person in Switzerland needs to drink more water than a person in the Netherlands.
• Climate: You need more water in warmer months due to perspiration and slightly less when the temperatures are cooler. If you spend time outdoors in the sun or even indoors in a heated room, you lose more water and need to adapt accordingly. Be mindful about staying hydrated during winters because you may not feel thirsty but you still need adequate water intake for the maintenance of bodily functions.
• Diet: Excessive consumption of coffee and other caffeinated beverages can cause loss of water through extra urination. Diet plays a significant role. If you eat foods that are high in salt, spices, or sugar, your body needs more water than with a diet rich in hydrating foods like fresh or cooked fruits and vegetables.
• Lifestyle: If you exercise or do intense physical work, increase your fluid intake to cover the increased water loss. If your job entails standing for long hours or being out and about, you need more water than someone sitting at a desk.
• Health Conditions: Intense diarrhea or vomiting causes dehydration. Certain medications increase needed water intake just as much as fever or infections. Medical conditions like diabetes call for higher intake whereas heart conditions or a particular type of kidney disease may need you to limit fluid intake because the body cannot process water correctly. It is important to consult with your doctor to know what is right for you. Dr. Julian Seifter, a kidney specialist and associate professor of medicine at Harvard Medical School believes that "Older people don't sense thirst as much as they did when they were younger. And that could be a problem if they're on a medication that may cause fluid loss, such as a diuretic".
• Pregnant or Breastfeeding: Remember your body is doing the work for two. Increase your fluid intake to stay hydrated.
• Age: As you grow older, the body’s ability to retain water declines while its hydration requirements go up. Older adults should increase their daily water intake unless they have specific conditions that limit it.

Your gender, metabolism, location, diet, physical activity, and age all factor into how much water you need. Use the numbers given earlier as a starting point. Drink more water during a heatwave than a blizzard. If you live in a dry climate, drink a little more than the daily recommendation. If you have just had a sweat-inducing run, hydrate yourself.

Be careful as you grow older. According to Dr. Nodar Janas, medical director of the Upper East Side Rehabilitation and Nursing Center in New York, “As we get older, our thirst center — which is located in the hypothalamus — isn’t as active as it used to be, so the brain doesn’t always give the signal that we need to drink. We need to make an extra effort to ensure that the elderly consume appropriate amounts of fluids, whether they’re thirsty or not.”

How Much Water Should Adults Drink?

According to studies by both the Mayo Clinic and the US National Academies of Sciences, Engineering, and Medicine, adult men should drink about 3.7 liters (125 fluid ounces) of water a day and adult women should drink about 2.7 liters (90 fluid ounces) daily.

Men tend to weigh more than women and have higher levels of fat. There is more water in lean muscle than there is in fatty tissue which means men need to drink more water to make up for the shortfall.

A study conducted in 2010 on Experimental Physiology concluded that men start sweating earlier than women during exercise. This is another factor that requires them to drink more water.

To account for body weight and exercise, physical therapist and clinical supervisor Jennifer Stone suggests two other very basic formulas, displayed below, for determining how much water to drink per day.

• If not exercising: Body Weight (pounds) / 2 = intake in fluid ounces. Multiply by 29.6 for number in milliliters.
• If exercising: Bodyweight (pounds) / 2 + Water Lost = intake in fluid ounces. Multiply by 29.6 for number in milliliters. To determine the water lost portion of the formula, weigh yourself before and after exercising. For each pound (0.45 kg) lost, drink around 16-20 oz (0.5-0.6 liters) of water.

This suggests a lower daily intake of water than the Mayo Clinic and US National Academies of Science studies say for most people, so it should definitely be taken at best as a lower bound. The portion of the Jennifer Stone formula for additional water intake based on exercise is used on top of the prior 3.7 liters (for men) and 2.7 liters (for women) recommendations from the Mayo Clinic.

How Much Water Should Older Adults Drink?

The European Society for Clinical Nutrition and Metabolism (ESPEN) recommends older adult men drink at least 2 liters (8 cups) and older adult women at least 1.6 liters (7 cups) of fluids per day in addition to fluids in food (unless otherwise medically indicated).

Hydration is crucial for maintaining body temperature but as the body ages, it struggles to adjust to the loss in temperature. You lose muscle tissue which is 80% water and the brain sensors for thirst aren't as sharp as they used to be.

The elderly are sensitive to dehydration and the presence of other chronic conditions as well as prescription medication, so people should speak with their doctor to determine what is best for their conditions.

According to Dr. Rand McClain, founder of Regenerative & Sports Medicine in Santa Monica, California, our bodies tend to “dry out” as we age. Senior citizens need to compensate for these lower levels by changing old habits.

How Much Water Should Boys Drink?

According to recommendations from the Institute of Medicine of the National Academies, boys between the ages of 4 and 8 should drink 1.7 liters of water per day (7 cups or 56 fluid ounces). This progresses to 2.4 liters (10 cups or 80 fluid ounces) for boys between 9 and 13, as displayed below.

As they grow towards early adulthood, boys aged 14 to 18 should drink 3.3 liters of water per day (14 cups or 112 fluid ounces) to keep up with the increased needs of their growing bodies.

These recommendations come from a study entitled Dietary Intake Levels For Water, Salt, And Potassium To Maintain Health And Reduce Chronic Disease Risk.

In general, boys weigh more than girls and hence require more water. They burn energy at a faster level than girls.

How Much Water Should Girls Drink?

The Institute of Medicine of the National Academies recommends that girls between the ages of 4 and 8 should drink 1.7 liters of water per day (7 cups or 56 fluid ounces). This progresses to 2.1 liters of water (9 cups or 72 fluid ounces) for girls between 9 and 13.

Recommended intake doesn't change as significantly for girls as boys as they move towards adulthood. From 14 to 18, daily water drinking of 2.4 liters (10 cups or 80 fluid ounces) is considered adequate. This data is displayed in the chart below.

Females are about 50% water compared to 59% for men according to a 2005 study from the US National Academy of Sciences. They weigh less than their male counterparts and hence have reduced water intake requirements. Females are more sensitive to dehydration.

A 2010 study at the University of Connecticut found that dehydration levels of even 1.5% in women can cause significant issues in concentration in performance tests.

How Much Water Should Models Drink?

According to an article in the Hollywood Reporter, a typical Victoria’s Secret model drinks up to a gallon a day to prepare for her show. Models understand that hydrated skin has a healthier appearance and to keep that glow they drink plenty of fluids each day. This is in the form of protein shakes, lemon water, green tea, or pure water. Models actively train their bodies to inculcate this habit.

Models adjust their water intake depending on their requirements. The article goes on to mention that a typical supermodel aims for a minimum of 2 liters a day but goes up to a gallon (3.8 liters) 10 days before the show. If deemed necessary, some reportedly go on no solid food diets and only consume liquids. The water intake tapers down a day or two before the show. Some models limit or stop drinking liquids completely 12 hours before the big day to eliminate the risk of bloating. They resume their routine after the show.

These dietary decisions are taken in consultation with personal trainers and health experts who monitor them diligently as they help them look their leanest selves before a show.

Male models drink up to a gallon a day (3.8 liters) for similar reasons. They increase or decrease the intake depending on their exercise regimen and the requirements of the body.

How Much Water Should Athletes Drink?

According to The American Council on Exercise, athletes should drink 17 to 20 ounces (0.5 to 0.6 liters) of water 2-3 hours before exercising. Athletes need to follow a strict regimen to prevent excessive fluid and electrolyte loss.

The National Strength and Conditioning Association (NSCA) has shared the following guidelines regarding drinking water for athletes during their exercise routines:

• Drink 16 to 24 ounces (0.5 to 0.7 liters) of water two hours before exercise.
• Drink another 8 ounces (about a quarter of a liter) 20 to 30 minutes before exercise.
• Another 8 ounces (quarter-liter) of water every 15 minutes during exercise.
• Weigh yourself immediately before and after exercising and drink 16 to 24 ounces (0.5 to 0.7 liters) of water for every pound of weight lost.

The suggested water intake for athletes is further discussed by Canadian professional bodybuilder Jeff Nippard in this video.

Models can alter the pattern on their off days but athletes should not. Even on their off days, because athletes can push their bodies to the limit they should ensure they hydrate adequately for their overall exercise regimen. The American Council of Exercise sets the standard at drinking 1.9 to 2.2 liters per day (8 to 9 cups) to keep the muscles lubricated and avoid muscle cramps.

How Much Water Should Teens Drink?

The Institute of Medicine recommends that teens should drink 9 to 14 cups (2.1 to 3.3 liters) of water a day. A preteen boy should drink 2.4 liters (81 ounces) of liquid daily, and then up the intake to 3.3 liters of fluid a day (112 ounces) beyond age 14.

Girls aged 9-13 should drink 2.1 liters (71 ounces) of fluid daily. This intake goes to 2.3 liters (78 ounces) of liquids once they turn 14. These guidelines are displayed in the chart below.

Most children hit puberty during their teen years, with girls generally earlier than boys. Puberty impacts lean body mass and protein metabolism. Hormones affect hydration levels, and girls are at a greater risk of dehydration around their period.

According to a study conducted by the Harvard T.H. Chan School of Public Health that looked at more than 4000 children and adolescents aged 6 to 19 years who participated in the National Health and Nutrition Examination Survey, more than half showed signs of minor dehydration. This does not need to be a health catastrophe. Besides encouraging more water drinking, improving teen diets with more hydrating foods like fresh fruits and vegetables can go a long way to ensure proper hydration.

How Much Water Should You Drink Based On Your Weight?

Physical therapist Jennifer Stand says you should drink half your weight (in pounds) in fluid ounces of water per day. If you weigh 200 pounds, you should drink 100 ounces (3 liters) of water. If you weigh 120 pounds, 60 ounces (1.8 liters) of fluid intake is considered adequate.

This should just be considered a baseline rule of thumb. Hydration needs are governed by factors mentioned earlier including lifestyle, geographic location, and climate.

What Do Experts Say About Ounces Of Water Needed Per Day?

According to the World Health Organization (WHO), an average adult needs to drink between 50 and 100 ounces (1.5 to 3 liters) of water per day. 64 ounces (1.8 liters) is considered ideal.

The US Food and Drug Administration (FDA) recommends 90-125 ounces (2.7 to 3.7 liters) per day. As discussed previously, the recommended amount of water that should be consumed daily varies by age and gender. The following chart shows these amounts in terms of standard 330 ml beverage cans.

A study by registered dietician Kim Chin emphasizes that everyone’s water intake needs are highly personal and urges individuals to listen to their bodies. The study suggests a baseline of 64 ounces a day, but to adjust by following your body’s thirst signals. Drink water when you feel thirsty. Stop when you are not thirsty. Be sure to make up for fluid loss when you work out or in hot weather by increasing water intake.

Dr. Courtenay Moore, a urologist and surgeon with the Cleveland Clinic suggests using the color of urine to gauge hydration levels. If you have clear pee or feel the urge to urinate frequently, cut back. If you notice infrequent urination or darker pee, you need to drink more water.

What Is The Daily Water Need Of A Toddler?

When asked “how much water should an infant drink a day?”, the American Academy of Pediatrics said the daily water need of a toddler 12 to 24 months old is 8 to 32 ounces (0.2 to 0.9 liters), and 40 ounces (1.2 liters) for children 2 and above. Younger infants 6 months to 1-year-old have a daily need of just 4 to 8 ounces (0.1 to 0.2 liters). These amounts are displayed in the chart below. The American Academy of Pediatrics strictly warns against giving water to infants under the age of 6 months because it can interfere with their ability to receive proper nutrition.

Dr. Janine Rethy, Division Chief of Community Pediatrics at MedStar Georgetown University Hospital has similar recommendations. Approximately 4 cups (1 liter) of daily fluid intake (including water or milk) for children 1 to 3 years old.

Babies 6 months and under should not be given water for their fluid needs because they get their primary nutrition from breastmilk or formula. The need for primary nutrition from milk is why infants 6 to 12 months should not exceed 4 to 8 ounces of water per day.

What Are The Signs Of Drinking Too Much Water Per Day?

The signs of drinking too much water per day include clear urine, going to the bathroom more than 7 times a day or frequently at night, swelling, fatigue, muscle spasms, or cramps.

Drinking too much water is referred to as Hyponatremia, a condition where the body is unable to remove excess water fast enough. The biggest indicator that you are drinking too much water is in the color of your pee. Pale-yellow urine indicates a healthy level of hydration. Clear or colorless urine is indicative of overhydration.

If you find yourself running to the bathroom more than the daily average of six or seven times every 24 hours, or wake up frequently at night to pee, you are probably wondering if it is bad to drink too much water. Other signs are swelling in the hands, feet, or lips. Diluted electrolytes can cause fatigue, muscle spasms, and even cramps, according to the Mayo Clinic.

Hyponatremia can occur when water intake exceeds the body’s capacity for normal removal. According to figures quoted in a study by the Department of Pediatrics at Chosun University School of Medicine in Korea entitled Hyponatremia Caused By Excessive Intake Of Water, kidneys can eliminate about 20 to 28 liters of water a day, but they can remove no more than 0.8 to 1.0 liters every hour. If this pace is exceeded, fluids will not clear out fast enough and an electrolyte imbalance in the body is created.

Overhydration and its associated symptoms are discussed in further detail here by Dr. Amit Varma.

You are unlikely to experience Hyponatremia in your normal routine but you need to be aware of it and avoid excessive drinking. Endurance athletes who drink lots of water before, during, and after events are at risk of suffering from water intoxication. If not addressed in a timely manner, the condition is life-threatening.

The condition can occur for individuals who set out to drink extremely large amounts of fluids, as in the case of models mentioned earlier.