How Much Water Do We Really Need?
Most of us have heard that we should drink eight glasses of water a day. It's an easy rule to remember—but it isn't a scientific requirement.
How much fluid we need varies with age, body size, activity, temperature, diet, medications, and health conditions. And importantly, not all of our water has to come from a glass of plain water.
Coffee, tea, milk, soups, fruits, vegetables and other foods and beverages all contribute to our daily fluid intake.
For older adults, however, hydration deserves particular attention. Our sense of thirst can become less reliable as we age, so waiting until we feel thirsty may not always be the best guide.
How Much Is Enough?
There isn't one amount of water that's right for everyone.
The National Academies recommends a total daily water intake of about 3.7 liters (approximately 125 ounces) for men and 2.7 liters (approximately 91 ounces) for women. But those numbers can be misleading because they include water from both beverages and food.
Typically, about 20% of our water comes from food, particularly fruits, vegetables, soups and other water-rich foods.
That means we don't need to drink 125 or 91 ounces of plain water every day. Coffee, tea, milk and other beverages also count.
Our individual needs can vary considerably. Exercise, hot weather, body size, diet and certain medications can all affect how much fluid we need.
Rather than aiming for an exact number of glasses every day, the more useful goal is to stay adequately hydrated throughout the day.
Does Coffee Count?
Yes. Coffee and tea count toward your daily fluid intake.
Caffeine does have a mild diuretic effect, meaning it can increase urine production. But with ordinary amounts of coffee or tea, the water in the beverage more than compensates for that effect.
So your morning coffee isn't dehydrating you. It contributes to hydration, just as tea, milk and other beverages do.
Very large amounts of caffeine are a different matter, but for most regular coffee and tea drinkers, there's no reason to exclude them when thinking about daily fluid intake.
How Can You Tell If You're Drinking Enough?
Thirst is useful, but it isn't a perfect guide—particularly as we get older, when our sense of thirst may become less reliable.
Urine color can provide another simple clue. Pale yellow urine generally suggests adequate hydration, while consistently dark yellow urine can be a sign that you need more fluid. Certain medications, vitamins and foods can also change urine color.
Other possible signs of dehydration include dry mouth, fatigue, headache, dizziness and less frequent urination.
There's no need to constantly measure every glass. For most people, drinking fluids regularly, paying attention to thirst, and occasionally noticing urine color provides a practical way to judge hydration.
Do You Need Electrolytes?
Electrolytes such as sodium, potassium and magnesium help regulate fluid balance, nerve function and muscle contraction.
For most people going about an ordinary day, water and a normal diet provide all the fluid and electrolytes they need. An electrolyte drink isn't necessary simply because you're trying to stay hydrated.
Electrolytes become more useful when we lose substantial amounts of both water and salt—for example, during prolonged exercise, heavy sweating, very hot weather, vomiting or diarrhea.
It's also worth looking at the label. Some electrolyte drinks contain considerable sodium or added sugar, while others contain relatively small amounts of electrolytes despite their marketing.
For everyday hydration, plain water is usually a good choice. Electrolytes are most useful when there's a reason to replace electrolytes, not simply because you're thirsty.
Filtered Water — Is It Better?
For most people in the United States with a regulated public water supply, tap water is generally safe to drink. But a filter can improve taste and reduce certain contaminants.
The important point is that not all water filters do the same thing. Common pitcher and refrigerator filters using activated carbon can reduce chlorine and some other substances, but they don't remove everything.
More extensive systems such as reverse osmosis remove a wider range of contaminants, but they are more expensive and may also remove minerals from the water.
If you're concerned about your drinking water, the best first step is to find out what is actually in your local water. Public water systems provide annual water-quality reports, and private well water should be tested periodically.
A filter can be worthwhile, but filtered water isn't automatically healthier than tap water. What matters is the quality of the water you're starting with and whether the filter is designed to remove the substances you're concerned about.
Can You Drink Too Much Water?
Yes. Although dehydration gets much more attention, drinking excessive amounts of water can also be dangerous.
The kidneys can eliminate a great deal of excess water, but drinking very large amounts too quickly can dilute the sodium in the blood. This condition, called hyponatremia, can cause headache, nausea, confusion and, in severe cases, seizures.
This is uncommon in everyday life. It is more likely during prolonged endurance exercise or when someone deliberately drinks unusually large quantities of water.
The lesson is simple: more water isn't always better. The goal is adequate hydration, not drinking as much water as possible.
Hydration as We Get Older
Hydration deserves more attention as we age because our sense of thirst may become less reliable. Older adults may not feel thirsty even when their bodies need more fluid.
Some medications, including diuretics, can also affect fluid balance. Illness, hot weather and reduced appetite can make dehydration more likely.
At the same time, some people with heart or kidney disease may need to limit fluids rather than increase them. Anyone who has been given a fluid restriction should follow the advice of their healthcare provider.
For most healthy older adults, a simple approach works well: drink fluids regularly throughout the day rather than relying entirely on thirst.
Bottom Line
There's no magic number of glasses of water that everyone needs each day.
Water, coffee, tea and water-rich foods all contribute to hydration. For most healthy people, drinking fluids regularly and paying attention to thirst and urine color is more useful than trying to reach a precise daily number.
As we get older, however, thirst may become a less reliable signal, making it particularly important to drink regularly throughout the day.
The goal isn't to drink as much water as possible. It's simply to stay adequately hydrated.
Learn More
National Academies — Dietary Reference Intakes for Water
The scientific basis for recommended water intake. It explains that total water includes water from food and all beverages, not just plain drinking water.
Harvard Health — How Much Water Should You Drink?
A practical discussion of how hydration needs vary with age, activity, medications, health conditions and other factors.
CDC — About Water and Healthier Drinks
An overview of hydration, including when we may need more water and how foods and beverages contribute to daily fluid intake.
Videos
The Science of Hydration: How Much Water You Really Need - 20 Minutes
A clear explanation of how the body regulates water and electrolytes, how much fluid we need, and how to recognize whether we're adequately hydrated.
Why Drinking Water Is NOT The Best Way To Stay Hydrated - 18 Minutes
Dr. Sten Ekberg explains the role of sodium, potassium and other electrolytes in hydration and why simply drinking more water isn't always the answer.