How much water do you need per day?
A healthy sedentary adult living in a temperate climate (~21 °C or 70 °F) who sweats only a little may need about 2 liters (range: 1.2-3.7 liters) of water per day (from beverages and foods combined); the bigger the individual the more water he or she needs.
Active individuals and those living in hot climates who sweat a lot may need more than 5 liters of water per day and physical workers may need as much as 15 liters of water per day.
How do you know if you drink too little? For every liter of water you lose and you do not replace it, you lose 1 kilogram of body weight. If your body weight suddenly drops by more than 1 kilogram within a day, you can usually assume this is due to the body water loss (dehydration) and not due to body fat loss.
You need to drink only as much water per day as you lose it.
How much water do you lose per day?
We cannot prevent a loss of certain amount of water from our bodies. This obligatory water loss consists of water lost through breathing, insensible perspiration and urination (Chart 1).
Chart 1. Minimal (Obligatory) Water Loss
|THE WAY OF WATER LOSS||AMOUNT OF WATER LOSS (milliliters/day)|
|Metabolic water produced (water gain)||-250-350|
|Net water loss||1,050-3,200|
Chart 1 reference: (1)
- Healthy, sedentary, young to middle-aged adults who live in a temperate climate and sweat only minimally lose at least about one liter of water per day (1).
- Older people cannot concentrate urine efficiently, so they urinate more and therefore lose at least about 1,500 mL water per day (2). Old people often have decreased sense of thirst so it is important they drink regularly even when not thirsty.
To find out if your body weight loss is due to water loss, you need to know your usual morning body weight: once when you assume you are well hydrated, weigh yourself after emptying your bowel and bladder and before eating or drinking anything: this is then your morning body weight. If your morning weight is about the same each day, you can assume you are well hydrated, and if on some day is lower by more than 0.5 kg (about 1 lb), you can assume you are dehydrated.
Other simpler, but less reliable symptoms and signs that suggest you may not drink enough water:
- Thirst, which is not a reliable sign; you can be dehydrated without being thirsty or thirsty without being dehydrated.
- Less urination. An unusually small amount of urine (< 200 mL), especially if it is yellow, in the morning after a full night sleep is usually a symptoms of dehydration.
- Prolonged skin turgor. If you pinch and release the skin on the back of your hand and it takes more than 0.5 second to flatten, you are probably dehydrated (this does not work in older people who have wrinkled skin all the time).
Should you drink 8 cups of water per day?
The common recommendation about how much to drink is two liters or eight cups (8 x 237 mL) of water per day, because this is about what–statistically–many adults need. This does not mean that this amount is appropriate for you personally.
Te amount of water you need to consume increases with your body weight, sweating, salt (sodium) intake, pregnancy and breastfeeding and eventual health conditions, such as fever, diarrhea or vomiting.
The amount of water you need to drink can greatly decrease if you get a lot of water from foods, such as fruits and vegetables.
A doctor may recommend you to limit water intake if you have a certain heart, kidney or liver disease.
Which beverages are appropriate to replace lost fluid?
- Water, including tap, bottled, mineral and carbonated water
- Herbal tea
- Decaffeinated coffee
- Commercial or prescribed oral rehydration solutions (ORS) — for small children when they are dehydrated
- Sport drinks — during prolonged exercise, like marathon, or in severe dehydration, when no salted foods are available
Possibly less appropriate but still hydrating beverages:
- Fruit juices, ice tea, soft drinks (cola or other pop soda) contain about 10% of sugar, which may result in weight gain if you consume them regularly.
- Tea, coffee, cola and energy drinks contain caffeine, which, if you consume them in large amounts, can cause anxiety (10).
- Regular cow’s milk contains about 88% of water, and skim milk (0% fat) about 91% of water.
- Beer and other alcoholic beverages with up to 5% of alcohol can be used as a source of water when other beverages are not available (7,10).
Chart 2. How much water is in foods?
Foods with high water content
Foods with low water content
Chart 2. reference: USDA.gov (12)
Infants, Toddlers and Older Children
Infants need about 100 mL of water/kg body weight per day, so a 5 kg infant needs about 500 mL and a 10 kg infant needs about 1 liter (3). Healthy breastfed infants can get all the water they need with the mother’s milk– assuming their mothers produces enough milk, which all healthy mothers should. Healthy infants 0-6 months of age on formula can get all water they need from the formula.
A well hydrated infant should have more than 6 wet diapers per day (11).
Giving as little as one cup of water per day to an infant in addition to breastfeeding or formula can cause water poisoning.
Children 1-3 years old need about 100 mL of water per kg for the first 10 kg body weight, 50 mL/kg for the 2nd 10 kg of weight and 20 mL/kg for the remaining weight. A well hydrated toddler should have at least 1 wet diaper every eight hours (11).
Children older than 3 years need to drink enough to be well hydrated, which means they have clear urine and their day-to-day body weight does not fluctuate by more than 0.5 kg (1 lb).
In a hot environment, athletes may need more than 10 liters of water per day (8). Athletes should start the training well hydrated. They should drink about 500-600 ml of fluid about two hours before exercise and then again 200-300 mL about 15 minutes before exercise. During an exercise, an athlete may need about 250 ml of water every 15 minutes (9). For quick rehydration, athletes should drink about 50% more fluid than they have lost it; this means 1.5 liter fluid for every kilogram of weight loss; they should also eat salted carbohydrate fluids to enhance water retention (9). Athletes should now their sweating rate, which can range from 0.5 to more than 2.5 liters per hour, and drink according to that (9). After heat acclimatization, sweat rate increases (9).
The National Athletic Trainers’ Association in the United States recommends sport drinks only during or after prolonged exercise (> 4 hours) when salted foods are not available (9). Ideal sport drink should contain 6-8% of simple carbohydrates (sucrose, glucose) and 30-70 mg of sodium per 100 mL (9).
Is there any benefit of drinking a lot of water?
To our knowledge there is no proven benefit of drinking more water than you need to replace the water lost from your body. Drinking more water than you need does not necessary prevent kidney stones.
Can drinking too fast be harmful?
When you drink too fast, you might not be able to swallow the fluid properly and some of it may enter the trachea and lungs. Drinking several liters of water in few hours can result in a drop of your blood sodium levels (hyponatremia), which can be deadly.
Can drinking too much water kill you?
Yes, death due to excessive water drinking has been reported many times. For example, one woman on a low calorie diet has died after drinking 4 liters of water in less than 2 hours (13). This is called water intoxication. This amount of water would not likely kill an adult who eats regularly and his diet contains usual amounts of sodium, though.
- NAP.edu Obligatory water loss in young adults
- NAP.edu Decreased urine concentration ability in the elderyl
- Utmb.edu Water requirements in infants and children
- IOM.edu Dietary reference intakes for water and electrolytes
- NAP.edu Water requirements in hot weather
- Dtic.mil Dehydration and rehydration
- Physiology.org Is alcohol dehydrating?
- PubMed Central Dehydration and body temperature in athletes
- NATA.org Fluid replacement for athletes
- NAP.edu Caffeine, alcohol and dehydration
- Cleveland Clinic Dehydration and your child
- US Department of Agriculture List of foods high and low in water
- BBC News Woman died from drinking too much water