Most adults need roughly 91 to 125 ounces of total water per day, depending on sex. The National Academies of Sciences, Engineering, and Medicine sets the reference intake at 91 ounces (2.7 liters) for women and 125 ounces (3.7 liters) for men. That number includes water from all sources: plain water, other drinks, and food. Since food typically provides about 20% of your daily water, the amount you actually need to drink comes out closer to 73 ounces for women and 100 ounces for men.
Why the “8 Glasses a Day” Rule Falls Short
The advice to drink eight 8-ounce glasses of water a day (64 ounces total) is one of the most repeated health tips in existence, but it has surprisingly little science behind it. A thorough review published in the American Journal of Physiology searched for the origin of this guideline and found no scientific studies supporting it. Surveys of thousands of healthy adults showed they functioned perfectly well without hitting that target, largely because the body’s built-in thirst mechanisms do a reliable job of maintaining fluid balance.
That said, 64 ounces isn’t a bad starting point. It just isn’t a universal rule. Some people need considerably more, others get by on less. The actual amount depends on your body size, how active you are, the climate you live in, and whether you’re pregnant or nursing.
A Simple Formula Based on Body Weight
A more personalized approach is to take your body weight in pounds and divide it by two. The result is a rough daily fluid target in ounces. A 160-pound person, for example, would aim for about 80 ounces. A 200-pound person would target 100 ounces. This formula, popularized by exercise physiologist L. Lee Coyne, gives you a baseline that scales to your body rather than treating everyone the same.
Keep in mind this is a starting point for a sedentary day in mild weather. You’ll need to adjust upward for exercise, heat, or other factors covered below.
When You Need More Water
Exercise
Physical activity increases your water needs substantially. You lose roughly one liter of fluid for every 2.2 pounds of body weight lost during a workout, and that fluid needs to be replaced. During exercise, aim to drink about 7 to 10 ounces every 15 minutes. If you’re exercising intensely for more than an hour, especially in heat, a drink with electrolytes helps replace the sodium and potassium you lose through sweat.
People with very high sweat rates (above 2 liters per hour) can’t fully replace fluids during exercise because the stomach can only absorb about 1.2 liters per hour. In that case, post-workout rehydration becomes critical. Sports medicine guidelines recommend drinking 150% of whatever weight you lost during the session. So if you dropped 2 pounds, you’d want to take in about 48 ounces over the next few hours.
Heat and Humidity
Hot weather above 85°F, combined with humidity above 65%, dramatically increases your risk of dehydration and heat illness. If you’re working or exercising outdoors in those conditions, drink 15 to 20 minutes before you start and continue drinking every 15 minutes throughout the activity. Even if you’re not exercising, sitting in extreme heat forces your body to sweat more to cool down, which means your baseline intake should go up.
Pregnancy and Breastfeeding
The American College of Obstetricians and Gynecologists recommends pregnant women drink 64 to 96 ounces of water daily (8 to 12 cups). Your blood volume increases significantly during pregnancy, and adequate water helps support amniotic fluid levels, nutrient transport, and kidney function. If you’re breastfeeding, your needs climb even higher since you’re producing fluid that leaves your body with every feeding.
How to Tell If You’re Drinking Enough
Rather than obsessing over a specific number, your urine color is one of the most reliable real-time indicators of hydration. Pale, nearly clear urine that flows freely means you’re well hydrated. Slightly darker yellow suggests you need to drink a bit more. Medium to dark yellow urine, especially if there isn’t much of it and it has a strong smell, signals dehydration that needs attention.
Other signs of mild dehydration include headaches, fatigue, dry mouth, and difficulty concentrating. By the time you feel genuinely thirsty, you may already be slightly behind on fluids, though for most healthy adults, thirst is a perfectly adequate guide throughout the day.
Can You Drink Too Much?
Yes. Drinking excessive amounts of water in a short period can cause a dangerous drop in blood sodium levels, a condition called hyponatremia (sometimes called water intoxication). According to Cleveland Clinic, symptoms can develop after drinking about a gallon of water over just one to two hours. A safe upper limit is roughly 32 ounces per hour. Spreading your intake throughout the day rather than chugging large amounts at once keeps your kidneys from being overwhelmed.
Water intoxication is rare in everyday life but does occasionally happen during endurance events like marathons, or in situations where people force large volumes of water in a short window. Sipping steadily throughout the day is both safer and more effective for staying hydrated.
What Counts Toward Your Daily Total
Plain water is the simplest choice, but it’s not the only fluid that counts. Coffee, tea, juice, milk, and even sparkling water all contribute to your daily intake. The old idea that caffeinated drinks dehydrate you has been largely debunked. Published research shows that caffeinated beverages do count toward your daily total, and even mild alcoholic drinks like beer in moderation contribute some hydration, though water and unsweetened drinks are still the best options.
Food contributes more than most people realize. Fruits and vegetables like watermelon, cucumbers, oranges, and strawberries are over 90% water by weight. Soups, yogurt, and cooked grains also add meaningful amounts. That 20% of daily water coming from food means a person eating lots of fresh produce may need fewer glasses of water than someone eating mostly dry, processed foods.