Most adults need about 11.5 to 15.5 cups of total water per day, depending on sex, body size, and activity level. That number sounds high, but roughly 20% of it comes from food, so the amount you actually need to drink is closer to 9 to 12 cups. The old “eight glasses a day” rule is a reasonable starting point, but your real target depends on several personal factors.
General Daily Targets
The National Academies of Sciences recommends about 15.5 cups (3.7 liters) of total daily water for men and 11.5 cups (2.7 liters) for women. “Total water” includes everything: plain water, other beverages, and the water naturally present in food. Since food covers about 20% of that total, the drinking portion works out to roughly 12.5 cups for men and 9 cups for women.
These are population-level averages for healthy adults living in temperate climates. They’re a useful baseline, but not a prescription. Your actual needs shift based on how much you sweat, where you live, and what you eat. Someone who eats a lot of fruits, soups, and vegetables gets more water from food than someone eating mostly dry, processed meals.
Factors That Change Your Needs
Exercise and Heat
Physical activity increases your water needs substantially. Sweat rates during exercise range from about half a liter to 4 liters per hour, depending on intensity, temperature, humidity, and your body size. A general guideline for active people is to drink about 200 mL (roughly 7 ounces) every 15 to 20 minutes during exercise. That said, the best approach is individualized: weigh yourself before and after a workout, and aim to replace the lost weight with an equivalent volume of fluid.
Hot or humid weather raises your fluid needs even when you’re not exercising. If you’re sweating noticeably just from being outside, you need to drink more than your usual baseline.
Body Size
Larger bodies need more water. A commonly cited rough formula is to drink half your body weight (in pounds) in ounces of water. So a 180-pound person would aim for about 90 ounces, or roughly 11 cups. This is a ballpark, not a medical calculation, but it’s a practical way to personalize the general guidelines.
Pregnancy and Breastfeeding
Pregnant women generally need about 12.5 cups of fluids daily, and breastfeeding women need closer to 16 cups. The extra fluid supports blood volume, amniotic fluid, and milk production.
What Counts Toward Your Total
Plain water is the simplest choice, but it’s not the only one. Coffee, tea, juice, milk, sparkling water, and even soft drinks all contribute to your daily fluid total. Caffeinated drinks do count: while caffeine has a mild diuretic effect, the fluid in a cup of coffee or tea more than offsets the small increase in urine output. You don’t need to “subtract” your coffee from your water goal.
Foods with high water content also add up. Watermelon, cucumbers, oranges, strawberries, lettuce, and soups are all roughly 85% to 95% water by weight. A diet rich in fruits and vegetables can cover a meaningful portion of your hydration needs without you drinking a single extra glass.
How to Tell If You’re Drinking Enough
Rather than obsessing over a specific cup count, use your body’s own signals. Urine color is the most reliable everyday indicator. Pale, straw-colored urine (think light lemonade) means you’re well hydrated. Medium yellow means you could use a glass or two. Dark yellow or amber urine, especially if it’s strong-smelling and low in volume, signals dehydration and calls for immediate rehydration.
Thirst is generally a good guide for younger adults, but it becomes less reliable with age. As people get older, the brain’s thirst signaling weakens. By the time an older adult feels thirsty, early dehydration may already be underway. For people in their 70s and beyond, building a habit of sipping water throughout the day is more effective than waiting for thirst to kick in. Small, frequent sips also tend to work better than trying to drink a full glass at once, which can cause bloating and discomfort in older adults.
Can You Drink Too Much?
Yes. Drinking too much water too quickly can cause a condition called water intoxication, where excess fluid dilutes the sodium in your blood to dangerous levels. Symptoms include nausea, headache, confusion, and in severe cases, seizures. This typically becomes a risk when someone drinks more than about a liter (32 ounces) of water per hour. In some people, symptoms develop after drinking 3 to 4 liters within just an hour or two.
Water intoxication is rare in everyday life but occasionally occurs during endurance sports, hazing rituals, or water-drinking contests. The practical takeaway: spread your intake throughout the day rather than chugging large volumes at once. Your kidneys can handle about a liter per hour under normal conditions, so pacing your drinking well below that threshold keeps you safe.
Special Considerations for Health Conditions
Some medical conditions require fluid restriction rather than the standard recommendations. People with heart failure, for instance, may be advised to limit total fluid intake to around 50 ounces (about 6 cups) per day, including water-rich foods like fruit. This is because the heart’s reduced pumping ability makes it harder for the body to process excess fluid, worsening swelling and other symptoms. Kidney disease can similarly require careful fluid management. If you have a chronic condition affecting your heart or kidneys, your fluid target will be set by your care team and may look very different from the general guidelines.
A Simple Daily Approach
For most healthy adults, a practical routine looks like this: drink a glass of water when you wake up, keep a water bottle accessible during the day, have a glass with each meal, and drink extra before, during, and after exercise. If your urine stays pale yellow and you rarely feel thirsty, you’re almost certainly getting enough.
There’s no need to track ounces precisely. The “right” amount varies from day to day based on temperature, activity, what you ate, and even altitude. Paying attention to your urine color and energy levels gives you a more accurate, real-time picture of your hydration than any fixed number on a chart.