# How much water should you drink a day? What the guidelines say

Eight glasses a day is the rule everyone knows, and nobody can find the science behind it. Here’s what health bodies actually say, and how to tell if you’re getting enough.

Source: https://nourishandstrong.com/how-much-water-should-you-drink
Published: 2026-11-06
Updated: 2026-11-06
By: the Nourish & Strong team
General information, not medical advice.

**The short answer:** most healthy adults do well on about 9 cups of fluids a day for women and 13 for men, counting water, coffee, tea and milk. Food supplies about 20% of the water you take in. Harvard says that can mean just 4 to 6 cups of plain water. Your best daily guide is thirst, plus pale yellow pee.

You need more in the heat, when you exercise and when you’re sick. You may need less if you’re small, or if a doctor has given you a fluid limit.

## How much water should you drink a day?

Three major sources give three different answers, partly because they measure different things. Some count all the water you take in, including from food. The NHS counts only what you drink.

| Source | What it counts | Women | Men |
| --- | --- | --- | --- |
| US National Academies | All water, from drinks and food | 2.7 liters (91 oz) | 3.7 liters (125 oz) |
| US National Academies, as reported by Harvard | Fluids you drink | About 9 cups | About 13 cups |
| European Food Safety Authority | All water, from drinks and food | 2.0 liters (about 68 oz) | 2.5 liters (about 85 oz) |
| NHS (UK) | Fluids you drink | 6 to 8 cups or glasses |

The US figures are adequate intakes taken from what healthy, well-hydrated people consume. They aren’t targets you must hit. Harvard calls them a general guide, and says drinking less won’t necessarily harm the average person. The National Academies’ panel said most healthy people meet their needs simply by letting thirst guide them.

Your own number moves around. A 2022 study in *Science* tracked water use in 5,604 people from 23 countries and linked it to body size, age, activity, climate and pregnancy, among other things. That’s why one number can’t fit everybody.

## Do you really need 8 glasses of water a day?

No, and the science for it is missing. A 2002 review in the *American Journal of Physiology* went looking for the evidence behind "at least eight 8-ounce glasses a day" and found no scientific studies supporting it. It also found that caffeinated drinks can count toward the total.

The National Academies’ panel declined to offer any rule of thumb for glasses, because water comes from so many sources.

Eight glasses is still a reasonable amount for many people, and it matches the top of the NHS range. The weak parts of the rule are "at least" and the idea that only plain water counts. The review also stressed that some people do need more, including those doing hard work or exercise, especially in hot weather, and people treating or preventing certain diseases.

## Does coffee count as water intake?

Yes. Harvard, the NHS and the National Academies’ report all count coffee and tea. Caffeine can make you pee a little more in the short term, but Harvard calls the idea that it leaves you dehydrated a myth, because the water in the drink more than makes up for it.

A 2014 trial backs this up. Fifty men who regularly drank coffee spent three days on four cups of coffee a day and three days on the same amount of water. Their hydration markers didn’t differ. It was a small study of male coffee drinkers, so it can’t speak for everyone.

Two limits apply. Coffee can make you jittery and keep you awake, so see our guide to [how much caffeine is too much](https://nourishandstrong.com/are-energy-drinks-bad-for-your-heart). And if you’re already dehydrated, the NHS says to avoid caffeine and alcohol. [Alcohol](https://nourishandstrong.com/how-much-alcohol-is-safe) needs more care in general, since Harvard says more than a couple of drinks in a short time can raise your risk of dehydration.

## Which foods and drinks have the most water?

About 20% of the water you take in comes from food. These ranges come from a major review using USDA data.

| Water content | Foods and drinks |
| --- | --- |
| 90 to 99% | Fat-free milk, cantaloupe, strawberries, watermelon, lettuce, cabbage, celery, spinach |
| 80 to 89% | Fruit juice, yogurt, apples, grapes, oranges, carrots, pears, pineapple |
| 70 to 79% | Bananas, avocados, cottage cheese, baked potato, cooked corn, shrimp |
| 60 to 69% | Pasta, legumes, salmon, chicken breast |
| 30 to 39% | Bread, bagels, cheddar cheese |
| 1 to 9% | Walnuts, peanuts, crackers, cereals, pretzels, peanut butter |

A diet heavy in fruit, vegetables and soup brings a lot of water with it. If you’re eating more fiber, drink a little more as well, as our [high-fiber foods guide](https://nourishandstrong.com/high-fiber-foods) explains. Sugary drinks count as fluid too, but the NHS advises keeping juice and smoothies to one small glass a day, about 5 ounces.

## What are the signs of dehydration?

The NHS and MedlinePlus list these signs in adults:

- Feeling thirsty
- Dark yellow, strong-smelling pee
- Peeing less often than usual
- Dry mouth, lips and tongue
- Headache, dizziness or feeling lightheaded
- Feeling tired
- Sunken eyes

Harvard says symptoms such as fatigue, mood changes and confusion can appear with as little as a 2% water deficit. If you feel worn out often, a drink is an easy first step, and our guide to [why you’re always tired](https://nourishandstrong.com/why-am-i-always-tired) covers what to check next. The link with [leg cramps at night](https://nourishandstrong.com/leg-cramps-at-night) is less clear, because sources disagree.

Pee color is a rough check. Foods, medicines and vitamins can change it. Thirst is less reliable in older adults, because the sense of thirst fades, and Harvard notes that thirst and pee color alone shouldn’t be relied on in this group. Babies, children and older adults are all at higher risk, so offer them drinks regularly.

## Can you drink too much water?

Yes, though it’s rare for healthy people. There’s no official upper limit, because your kidneys usually clear extra water. Trouble comes when you drink a large amount in a short time, faster than your kidneys can keep up. Blood sodium then falls too low, a condition called hyponatremia. Cleveland Clinic counts anything below 135 mEq/L as low.

The signs are nausea, vomiting, headache, confusion, muscle spasms and seizures. Severe cases can cause brain swelling, coma or death, so this needs urgent care.

The people most at risk are endurance athletes, people working in extreme heat, and people with kidney problems. Harvard also lists thyroid, liver and heart conditions, and medicines that make you hold on to water, such as NSAID pain relievers, opiate pain medicines and some antidepressants. Keep taking what you’ve been prescribed, and ask your doctor how much to drink.

Cleveland Clinic suggests letting thirst guide you, unless your provider says otherwise, and choosing drinks with electrolytes during long events.

## How can you get enough water every day?

- **Drink with routine.** Harvard suggests a drink at each meal, when you’re with people, and when you take medicine.
- **Sip through the day.** Don’t try to catch up all at once.
- **Check your pee.** Aim for clear pale yellow.
- **Use a bottle if thirst lets you down.** Harvard’s example is a 20-ounce bottle filled four times a day, or a large glass with each meal and snack.
- **Make it taste better.** The NHS suggests sparkling water or a slice of lemon or lime.
- **Drink more when you lose more.** That means heat, exercise, fever, vomiting or diarrhea.
- **Eat your water.** Add fruit, salad or soup to a meal.

Harvard lists keeping blood pressure normal among water’s many jobs in the body. If your readings run high, our guide on [how to lower blood pressure](https://nourishandstrong.com/how-to-lower-blood-pressure) covers the habits with the best evidence.

## When should you see a doctor?

See your doctor promptly if you feel dizzy when you stand and it doesn’t go away, if you’re peeing much less than normal, or if you’re unusually tired. For vomiting or diarrhea, start with small sips. A pharmacist can recommend a rehydration solution.

**Call 911 or your local emergency number** if someone with signs of dehydration is confused, unusually sleepy or hard to wake, has blue, gray, pale or blotchy skin, has skin that feels cold, or is struggling to breathe. These can be signs of shock.

If you’re thirsty all the time and peeing a lot even though you’re drinking plenty, ask for a blood test, because those are common [signs of diabetes](https://nourishandstrong.com/diabetes-symptoms).

A piece of fruit is one of the easiest ways to eat a little more water. If you’d like a simple daily habit to build on, start with [what a banana a day does](https://nourishandstrong.com/banana-every-day).

## Quick questions

### Does tea count as water intake?

Yes. Tea counts toward your daily fluids, according to the NHS and Harvard. Harvard says caffeinated drinks can add to your total water intake, even though more than about 180 mg of caffeine a day may make some people pee more for a short time.

### How much water is too much in a day?

There’s no official daily upper limit, because healthy kidneys usually clear extra water. The danger comes from drinking a large amount in a short time, which can dilute your blood sodium. Your safe amount depends on your health, so ask your doctor if you have kidney, liver or heart problems.

### Is 8 glasses of water a day enough?

For most healthy adults, yes, particularly when you add the water in food and other drinks. It matches the top of the NHS range of 6 to 8 glasses. You may need more in hot weather, during long exercise, when you’re ill, or while pregnant or breastfeeding.

### How do you know if you are dehydrated?

The easiest signs are thirst, dark yellow pee and peeing less often than usual. You may also have a dry mouth, a headache, tiredness or dizziness. Confusion, fainting, a racing heart or fast breathing mean you need medical help right away, so call 911 or your local emergency number.

### What happens if you don’t drink enough water?

You become dehydrated, which can cause tiredness, dizziness and dark pee. Harvard says dehydration can raise your risk of urinary tract infections, kidney stones, gallstones and constipation. If it isn’t treated, the NHS warns, it can become serious and lead to shock, a medical emergency.

### How much water should a woman drink a day?

About 9 cups of fluids a day is the usual guide for healthy women, based on the National Academies’ figures, with water from food on top. The NHS says you may need more if you’re pregnant, breastfeeding, active for long periods, ill or somewhere hot.

This article is general information, not medical advice. If you have kidney, liver or heart problems, have been given a fluid limit or take medication, talk with your doctor.

## Sources

- [National Academies: Report Sets Dietary Intake Levels for Water, Salt, and Potassium (2004)](https://www.nationalacademies.org/news/2004/02/report-sets-dietary-intake-levels-for-water-salt-and-potassium-to-maintain-health-and-reduce-chronic-disease-risk)
- [EFSA Panel on Dietetic Products, Nutrition, and Allergies. Scientific Opinion on Dietary Reference Values for water. EFSA Journal, 2010](https://doi.org/10.2903/j.efsa.2010.1459)
- [NHS: Water, drinks and hydration](https://www.nhs.uk/live-well/eat-well/food-guidelines-and-food-labels/water-drinks-nutrition/)
- [NHS: Dehydration](https://www.nhs.uk/conditions/dehydration/)
- [MedlinePlus: Dehydration](https://medlineplus.gov/dehydration.html)
- [Harvard Health Publishing: How much water should you drink?](https://www.health.harvard.edu/staying-healthy/how-much-water-should-you-drink)
- [Harvard T.H. Chan School of Public Health: Water](https://nutritionsource.hsph.harvard.edu/water/)
- [Valtin H. "Drink at least eight glasses of water a day." Really? Am J Physiol Regul Integr Comp Physiol, 2002](https://doi.org/10.1152/ajpregu.00365.2002)
- [Yamada Y et al. Variation in human water turnover associated with environmental and lifestyle factors. Science, 2022](https://doi.org/10.1126/science.abm8668)
- [Killer SC et al. No evidence of dehydration with moderate daily coffee intake. PLoS One, 2014](https://doi.org/10.1371/journal.pone.0084154)
- [Popkin BM et al. Water, hydration, and health. Nutrition Reviews, 2010](https://pmc.ncbi.nlm.nih.gov/articles/PMC2908954/)
- [Cleveland Clinic: Hyponatremia](https://my.clevelandclinic.org/health/diseases/17762-hyponatremia)
- [Cleveland Clinic: Dehydration](https://my.clevelandclinic.org/health/diseases/9013-dehydration)