Most adults need between 2,700 and 3,700 milliliters of total water per day. Women should aim for roughly 2,700 mL (about 11.5 cups), while men should aim for roughly 3,700 mL (about 15.5 cups). These figures from the National Academies of Sciences, Engineering, and Medicine represent total water from all sources, not just what you pour into a glass.
What “Total Water” Actually Means
That 2,700 to 3,700 mL target includes everything: plain water, coffee, tea, juice, milk, and the water naturally present in food. Food alone provides about 20% of the water you take in each day. A bowl of oatmeal, a serving of soup, fruits like watermelon and oranges, and vegetables like cucumbers and lettuce all contribute meaningful amounts of fluid.
So if you subtract that 20% food contribution, the amount you actually need to drink comes out to roughly 2,200 mL (about 9 cups) for women and 3,000 mL (about 13 cups) for men. That drinking total includes coffee and tea, which count toward your daily fluid intake despite old claims that caffeine cancels them out. Peer-reviewed research confirms that caffeinated beverages do contribute to hydration.
Where the “8 Glasses a Day” Rule Came From
The popular advice to drink eight 8-ounce glasses (about 1,900 mL) each day has no solid scientific backing. Heinz Valtin, a kidney specialist at Dartmouth Medical School, traced the idea to a 1945 recommendation from the Food and Nutrition Board suggesting “approximately 1 milliliter of water for each calorie of food.” For a 2,000-calorie diet, that works out to about 2,000 mL, close enough to eight glasses. The problem is that the very next sentence in the original recommendation noted “most of this quantity is contained in prepared foods.” That sentence was widely ignored, and the number took on a life of its own. Valtin’s review found no published scientific studies supporting the 8×8 rule as a standalone drinking target.
The rule isn’t dangerous, but it undersells what most men need and slightly oversells what many women need once food-based water is factored in. Your body is quite good at signaling when it needs more fluid, primarily through thirst.
A Simple Formula Based on Body Weight
If you want a more personalized number, a commonly used formula is 30 mL per kilogram of body weight. Here’s how that looks in practice:
- 60 kg (132 lbs): 1,800 mL per day
- 70 kg (154 lbs): 2,100 mL per day
- 80 kg (176 lbs): 2,400 mL per day
- 90 kg (198 lbs): 2,700 mL per day
- 100 kg (220 lbs): 3,000 mL per day
This formula gives you a baseline for drinking water specifically, not total water from all sources. It’s a practical starting point, though you’ll need more if you exercise heavily, live in a hot climate, or are pregnant or breastfeeding.
When You Need More Than the Baseline
Exercise increases your fluid needs substantially. Sweat rates vary from person to person, but a general guideline from the National Athletic Trainers’ Association is to replace every pound of body weight lost during exercise with 475 to 710 mL (16 to 24 fluid ounces) of water. If you don’t want to weigh yourself before and after a workout, drinking about 200 to 300 mL every 15 to 20 minutes during vigorous activity is a reasonable approach.
Hot or humid weather, high altitude, heated indoor air in winter, illness involving fever, vomiting, or diarrhea, and pregnancy or breastfeeding all push your needs higher. During breastfeeding, total daily water intake recommendations rise to about 3,800 mL. If you’re sick with a stomach bug, small frequent sips are more effective than trying to drink large amounts at once.
Hydration Needs After Age 65
Older adults face a specific challenge: the sense of thirst naturally weakens with age. One study found that when healthy older participants went without water for 24 hours, they reported less thirst and mouth dryness than younger participants in the same situation. This means relying on thirst alone becomes less reliable over time.
The fluid targets for adults 65 and older remain similar to those for younger adults, roughly 13 cups per day for men and 9 cups for women. But because thirst signals are blunted, building habits around regular water intake matters more. Keeping a water bottle visible, drinking a glass with each meal, and eating water-rich foods like soups and fruits can help bridge the gap that a weakened thirst signal leaves open.
Signs You’re Drinking Too Little or Too Much
The simplest hydration check is urine color. Pale yellow, like lemonade, signals good hydration. Dark yellow or amber means you need more fluid. Completely clear urine consistently throughout the day could mean you’re overdoing it.
Mild dehydration shows up as headache, fatigue, dry mouth, and difficulty concentrating. These symptoms often resolve within 30 to 60 minutes of drinking water. More severe dehydration causes dizziness, rapid heartbeat, and very low urine output.
Overhydration is far less common but can be serious. Drinking extremely large volumes in a short time can dilute sodium levels in your blood to dangerous levels, a condition called hyponatremia. This is most often seen in endurance athletes who drink aggressively during long events without replacing electrolytes. For most people going about a normal day, the kidneys can handle up to about 800 to 1,000 mL per hour without trouble. Spreading your intake throughout the day rather than chugging large amounts at once keeps you well within safe limits.