Most healthy adults don't need to count glasses. The National Academies' adequate intakes are about 2.7 liters a day for women and 3.7 for men. They count all the water from food and drinks, and drinks make up about four fifths of it (NASEM). Drinking when you're thirsty, plus drinks with meals, is enough for most people (MedlinePlus).
Key takeaways
The 2.7 and 3.7 liter figures are total water from food and drinks, not glasses of plain water (NASEM).
"8 glasses a day" has no studies behind it (Valtin, 2002).
Coffee and tea count toward your water (NASEM).
For most healthy adults, thirst is a good guide, even during exercise (NASEM, 2004; Hew-Butler et al., 2015).
Drinking far past thirst during long exercise can cause dangerously low blood sodium (Hew-Butler et al., 2015).
How much water do you need a day?
About 2.7 liters of total water a day for women and 3.7 for men, by the National Academies' figures (NASEM):
Who | Total water a day | In 8-ounce cups |
|---|---|---|
Women | 2.7 liters | about 11 |
Men | 3.7 liters | about 16 |
Pregnancy | 3.0 liters | about 13 |
Breastfeeding | 3.8 liters | about 16 |
The totals include the water in food. The cups are worked out from the liters. Of those totals, drinks supplied about 2.2 liters (about 9 cups) for women and 3.0 liters (about 13 cups) for men (NASEM).
These are adequate intakes, set from typical U.S. intakes and meant to cover healthy, somewhat inactive adults in mild climates. People stay well hydrated across a wide range of intakes, so there's no single target. The same values apply to adults over 70 (NASEM).
Europe's food safety authority sets lower figures: 2.0 liters a day for women and 2.5 for men (EFSA, 2010). The two bodies landed on different numbers, so read any figure as a reference point, not an exact requirement.
Is 8 glasses a day a myth?
As a rule, yes. A 2002 review found no scientific studies supporting it. The two possible origins it found both counted more than plain water. One, a 1945 allowance, said most of the water "is contained in prepared foods" (Valtin, 2002).
Eight 8-ounce glasses come to about 1.9 liters. That's less than the 2.2 and 3.0 liters of drinks in the National Academies' figures.
Does the water in food count?
Yes. Food supplies about a fifth of the water most U.S. adults take in: 19% for adults aged 19 to 30 (NASEM). A review put it at about 22% in the U.S., and higher where people eat more fruit and vegetables. That review was funded in part by a bottled water company (Popkin et al., 2010).
Iceberg lettuce, cooked squash, cantaloupe, cooked broccoli, peaches, raw carrots and oranges are all more than 85% water (NASEM).
Do coffee and tea count?
Yes. The National Academies say caffeinated drinks add to your daily water much like other drinks (NASEM).
In one trial testing 13 drinks, 72 men each drank a liter of water and of three other drinks, on separate days. Coffee, tea, cola and sparkling water kept them as hydrated as still water over four hours, and milk did a little better. A hydration institute funded it (Maughan et al., 2016). In a trial funded by a coffee institute, 50 men who drank coffee daily showed no difference in body water after days of coffee or water (Killer et al., 2014).
Alcohol is different. MedlinePlus says it doesn't add to your body's water (MedlinePlus).
Who needs more water?
Activity, heat, pregnancy and breastfeeding all raise how much you need.
Active people and hot weather: very active people who are in hot weather day after day often need 6 liters or more of total water (NASEM, 2004).
Pregnancy and breastfeeding: about 3.0 and 3.8 liters a day of total water (NASEM).
Older adults, a weaker signal: needs are the same as for younger adults. But thirst can be weaker after exercise in heat or a stretch without fluids, and fluid balance comes back more slowly (NASEM; Kenney and Chiu, 2001).
Can you drink too much water?
Yes, though it's rare in everyday life. Healthy kidneys can clear only about 0.7 to 1.0 liters (about 3 to 4 cups) an hour (NASEM).
During long exercise, drinking far beyond what you lose can drop the sodium in your blood. This has been linked to at least 14 athlete deaths since 1981 (Hew-Butler et al., 2015). In one Boston Marathon study, 13% of 488 runners finished with low blood sodium (Almond et al., 2005).
Gaining weight during a long event is a warning sign, and sports drinks don't prevent it. The experts' advice is to drink to thirst (Hew-Butler et al., 2015). The American College of Sports Medicine suggests weighing yourself before and after exercise to learn how much you sweat (Sawka et al., 2007).
Who should drink less, or ask first?
If you have heart failure or kidney disease, or take water pills, your doctor's fluid advice comes before any general figure.
Heart failure: your provider may change how much fluid you drink (MedlinePlus).
Kidney disease: damaged kidneys can't remove extra fluid, so you may need a limit. Ask how much (NIDDK). On dialysis, soup and other foods that are liquid at room temperature count toward it (NIDDK).
Diuretics (water pills): they can lead to dehydration, and they're one cause of low blood sodium (MedlinePlus; MedlinePlus).
If you've been told to limit fluids, ask your doctor what to do in very hot weather (NIA).
How do you know you're getting enough?
Your body tells you. Thirst, a dry mouth, little urine and darker yellow urine are signs of mild to moderate dehydration (MedlinePlus). If you rarely notice those signs, you're likely getting enough.
Water shifts move the bathroom scale from day to day too, as reading weight trends without the noise explains.
Frequently asked questions
Does sparkling water count? Yes. In the trial above, sparkling water kept the men as hydrated as still water (Maughan et al., 2016).
Does salt change how much water I need? Somewhat. Your kidneys need water to clear salt, so salt intake is one of the things that changes your water needs (NASEM). There's more on sodium in sodium and potassium.
Does water change my smart scale's body readings? Yes. Body-composition scales read through your body water, so lean mass and body fat move with it (Holmes and Racette, 2021). See lean body mass.
How Knit shows your water
When you log water in an app connected to Apple Health, Knit shows it in the Record tab, under Nutrition. The chart has no target line, since needs vary so much. A few weeks of logging shows your own typical day (why Knit compares you to you).
