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).
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 fat loss.
How much water do you lose per day?
You cannot prevent a loss of certain amount of water from your body. 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 you believe you are well hydrated, weigh yourself after emptying your bowel and bladder and before eating or drinking anything. This is your morning morning weight and if it 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 symptoms and signs that suggest you may not drink enough water:
- Thirst. Note: you can be dehydrated without being thirsty or thirsty without being dehydrated.
- Less urination. An unusually small amount of strong yellow urine in the morning suggests 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 may be dehydrated.
Should you drink 8 cups of water per day?
The common recommendation is to drink 2 liters or 8 cups of water per day. This may be a good guess of an average water needs, but you personally may need less or more than that.
The 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.
If you get a lot of water from foods, like soups, milk and fruits, you will probably need to drink much less than 2 liters of water per day.
Which beverages are most 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 or for adults with severe diarrhea
- 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 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. Healthy infants 0-6 months of age on formula can get all the 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?
Drinking more than 1.5 liters of water per hour for several hours in a row can result in a drop of blood sodium levels (hyponatremia).
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 (4). 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
- BBC News Woman died from drinking too much water
- 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