When thinking about drinking, we've probably all heard of the 8 x 8 rule — drink eight glasses of water per day, with 8 oz of water per glass. But this isn't actually scientifically accurate, as we get a lot of fluid from our meals, and other liquids actually count towards that quota. If you drink a lot of fruit squash, tea, low-fat milk, sugar-free drinks or even coffee, these all count towards your daily hydration goals.
Water requirements are also very personal: someone who lives a very active life will probably need to drink more water to keep their body hydrated than someone who lives a sedentary lifestyle. An article in Nederlands Tijdschrift voor Geneeskunde, a leading medical journal in the Netherlands explains that the body's fluid balance is regulated by a hormone called vasopressin and the kidneys. A healthy person should output about 500 ml/day of urine and intake between 2000-3000 ml of fluid to achieve this — note the article says fluid, not water — so this includes all liquids you consume, including from food.
Water intoxication is a risk when drinking excessive amounts of water, although drinking eight glasses a day probably won't put you at risk of this. An article in the Journal of Clinical Pathology indicates that symptoms of water intoxication include disorientation, nausea and sometimes vomiting. Having too much water in your body can lead to low sodium levels, known as hyponatraemia, which can lead to coma and even death if untreated.
Dr. Rebecca Breslow is a physician, researcher, and writer. A graduate of Yale University, she did her medical training at Harvard Medical School, Brigham and Women’s Hospital, and Boston Children’s Hospital. She was a practicing physician in academic medicine for 17 years, during which time she authored numerous publications for academic and lay audiences. Currently, she focuses on freelance medical writing and editing to help make medical, health, and wellness information accessible to a broad audience.
There's no dispute that water is crucial to a healthy life (or any life at all, for that matter). And yet, there's little scientific consensus about the exact amount of the stuff an individual should consume each day, as it is so personal and dependent on other lifestyle factors.
An article in the American Journal of Physiology soundly disagreed with the myth that 'you're already dehydrated once you feel thirsty' — our thirst response is there to prevent us from getting dehydrated in the first place, and while it shouldn't be ignored, mild thirst is just a reminder to drink something.
The article also explains that while urine color is a good way to judge your hydration levels at home, it varies from person to person. Your urine color can also be impacted by the volume of urine you've produced, so your urine may be darker not because you are dehydrated, but because there is a lot of it. Generally, urine is darker in the morning, but this isn't a cause for concern as for most healthy adults, this will be an accumulation of eight hours' worth of fluid, and our thirst response usually prompts us to drink upon waking. So, while you might want to monitor your urine color at home to judge how much you should be drinking, this wouldn't fly in a lab, where they would test your plasma osmolality to see whether or not you are dehydrated.
Food and drink
One such source is food. Everything you eat contains some water. Raw fruits and vegetables have a lot; fruits such as watermelons and strawberries, for example, are more than 90 percent water by weight, according to the U.S. Department of Agriculture. Different diets naturally contain different amounts of water, but it adds up. According to a 2004 report by the National Academies of Sciences, the average American, aged 19-30, gets about 20 percent of his or her daily water intake through food, and that counts toward healthy hydration.
The other key water sources that the "8 x 8" rule overlooks are other beverages. Non-alcoholic drinks such as coffee, tea, milk, juice and soda contain mostly water, and all contribute to your hydration. Contrary to another popular myth, studies show that coffee does not dehydrate you and is a suitable form of H2O intake. (Just remember that there can be adverse side effects of drinking too much caffeine, including headaches and disrupted sleep.)
So, between all the food, water, and other fluids you consume in a day, how much water should you aim to imbibe? The National Academies of Sciences suggests that women aged 19-30 consume a total of approximately 2.7 liters (91 ounces) of water from all beverages and foods each day and that men of the same age get approximately 3.7 liters (125 ounces) daily. But these are just general guidelines and are not supported by firm scientific studies.
The truth is, there is no magic formula for hydration — everyone's needs vary depending on their age, weight, level of physical activity, general health and even the climate they live in. The more water you lose to sweating, the more water you'll need to replace with food and drink. So, naturally, a person doing strenuous physical work in a hot, tropical climate would need to drink more water than a person of identical weight and height who spent the day sitting in an air-conditioned office.
If you are looking for concrete advice, though, the best place to look is within.
"The vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide," according to the National Academies of Sciences. Your body naturally feels thirsty when your hydration levels are dropping, and water is the best medicine. (On the other end of the digestive spectrum, your urine can also tell you whether you're getting enough to drink — dark yellow or orange urine usually indicates dehydration, while well-hydrated urine should look pale yellow or colorless.)
The bottom line: Drink up when you're thirsty, and drink more when you sweat more. Your body will take it from there.
Originally published on Live Science on Jan. 7, 2018 and updated on Aug. 25, 2022.
Live Science newsletter
Stay up to date on the latest science news by signing up for our Essentials newsletter.
Brandon is the space/physics editor at Live Science. His writing has appeared in The Washington Post, Reader's Digest, CBS.com, the Richard Dawkins Foundation website and other outlets. He holds a bachelor's degree in creative writing from the University of Arizona, with minors in journalism and media arts. He enjoys writing most about space, geoscience and the mysteries of the universe.