How much water do you need a day? What the US values say

For healthy adults in the US, the National Academies set an Adequate Intake of about 3.7 liters of total water a day for men and 2.7 liters for women. That total counts plain water, every other beverage, and the water already inside food. Drinks alone account for roughly 3.0 liters (about 13 cups) for men and 2.2 liters (about 9 cups) for women. These are not minimums. They are the typical intakes of healthy Americans, and many people stay well hydrated on less.

The rest of this article covers where those numbers came from, why the old "eight glasses a day" rule does not match them, and when your own need moves up or down.

What the 3.7 and 2.7 liter figures are

The figures come from the 2005 Dietary Reference Intakes report on water and electrolytes, published by what is now the National Academies of Sciences, Engineering, and Medicine. The committee looked for a way to set a true requirement and did not find one. Healthy people keep their blood concentration in a normal range across a wide spread of intakes, so there was no single cutoff below which harm clearly begins.

Instead, the committee set an Adequate Intake (AI) at the median total water intake reported in a large US national survey. The report says plainly that intake below the AI does not necessarily raise risk for a healthy person and that the AI should not be read as a specific requirement. The values have not been revised since; the 2019 update to the electrolyte DRIs covered sodium and potassium only.

The adult values are the same from age 19 through over 70. The report notes that the ability of the kidneys to concentrate urine declines with age, but it did not set a different number for older adults.

About a fifth of your water comes from food

In the survey data behind the AI, food supplied about 19 percent of total water and drinks supplied about 81 percent. The DRI report includes a sample 2,200-calorie day to show how this adds up. Its meals, from cereal with milk to salmon with rice and salad, provided about 890 mL of water. The coffee, tea, milk, juice, and plain water in that same day added about 2,850 mL, for a total near 3.7 liters.

Some foods are mostly water. USDA FoodData Central lists raw watermelon at about 91 grams of water per 100 grams. Cooked grains, soups, fruit, and vegetables all add to the total, while crackers, nuts, and dried fruit add very little.

The 19 percent figure may also be low. A 2022 analysis in Science pointed out that self-reported food surveys tend to undercount what people eat, so they probably undercount water from food too. That fits with a broader pattern in diet records, covered in how far off calorie logs tend to be.

Where "eight glasses a day" came from

Eight 8-ounce glasses is 64 fluid ounces, or just under 2 liters. In 2002, Heinz Valtin, a physiologist at Dartmouth Medical School, published a review in the American Journal of Physiology looking for the scientific basis of that advice. He did not find any studies supporting it. He did find intake surveys of thousands of healthy adults who drank less and showed no sign of being unwell.

His conclusion was limited to healthy adults in a temperate climate who are mostly sedentary. He also noted that larger intakes make sense in some medical situations and during hard physical work or exercise in the heat. So the rule is not wrong for everyone. It just is not a measured requirement, and the "at least" part of it has no evidence behind it.

What measured water turnover shows

Surveys ask people what they drank. A different approach is to measure how much water actually moves through the body. In the 2022 Science study, Yamada and colleagues used isotope-labeled water to measure daily water turnover in 5,604 people aged 8 days to 96 years from 23 countries.

Turnover rose with age until about 30 and averaged about 4.3 liters a day in men and 3.4 liters in women at that peak. In people over 70, the averages were about 3.1 and 2.8 liters. Body size, physical activity, pregnancy, air temperature, humidity, and altitude were all linked to turnover. The spread was wide: the low end for adults was around 1 to 1.5 liters a day and the high end around 6.

Turnover is not the same as what you need to drink. It includes water from food and water your body makes when it burns fuel. The authors could not say how much of each person's turnover came from drinks. Their main point was that one fixed number does not fit everyone.

Coffee, tea, and the other drinks you already have

All beverages count toward total water. The DRI committee reviewed whether caffeinated drinks cause a net water loss. In studies of healthy, free-living men, coffee and other caffeinated drinks did not raise 24-hour urine output compared with other beverages. A large single dose given to regular coffee drinkers after five days without caffeine did raise urine output, so the effect seems to depend on habit and dose. Valtin reached a similar view and added that moderate amounts of mild alcoholic drinks such as beer can also count, to a lesser degree.

Juice, soda, and sweetened coffee drinks count as water too, but they also bring sugar and calories. If you are tracking those, the US and WHO limits are in how much added sugar per day.

When you need more, and when more is too much

The AI describes typical intake, not hot or hard days. The DRI report says physically active people and people exposed to heat need more, and that sweat losses during work in the heat can push fluid needs to several times the usual amount. Day to day, though, the report found that thirst plus drinking with meals keeps healthy people adequately hydrated.

There is no Tolerable Upper Intake Level for water, because healthy kidneys can clear a lot of it. The limit is speed. The report cites a maximum excretion rate of roughly 0.7 to 1.0 liter per hour, and cases of water toxicity have come from drinking large volumes much faster than that. Spreading fluids across the day avoids the problem.

Urine color is sometimes used as a quick check. The DRI report calls it a rough indicator only, since diet, medications, and vitamins can change the color.

Where CaRoot fits

CaRoot is a food logging app that estimates calories and macros (protein, fat, and carbs) from a photo of your meal. It also has water and weight logging, so you can see fluid intake in the same place as your meals. You can switch between metric and imperial units, which helps if you think in fluid ounces but read reference values in liters. The weekly report shows your average calories, macro balance, and how consistently you logged.

Keep in mind that a water log covers what you drink, not the roughly one fifth that comes from food. CaRoot is free on iOS through the App Store, with ads, and you can start without an account. The Android version is in development, and you can sign up for a launch notice on the waitlist.

Sources

• Institute of Medicine. Chapter 4: Water. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. The National Academies Press, 2005. doi:10.17226/10925.
• National Academies of Sciences, Engineering, and Medicine. Dietary Reference Intakes for Sodium and Potassium. The National Academies Press, 2019.
• Valtin H. "Drink at least eight glasses of water a day." Really? Is there scientific evidence for "8 x 8"? American Journal of Physiology: Regulatory, Integrative and Comparative Physiology, 2002.
• Yamada Y, Zhang X, Henderson MET, et al. Variation in human water turnover associated with environmental and lifestyle factors. Science, 2022.
• USDA Agricultural Research Service. Watermelon, raw (SR Legacy, FDC ID 167765). FoodData Central.

This article is general nutrition information, not medical advice; if you have a health condition, especially one affecting your heart or kidneys, talk with your doctor or a registered dietitian about how much fluid is right for you.