How Much Water Should You Drink a Day?

You have heard it a thousand times: drink eight 8-ounce glasses of water a day. That is 64 ounces, or about 1.9 liters. It is easy to remember, it fits on a motivational poster, and it is not based on any scientific evidence. The rule traces back to a 1945 National Research Council recommendation that actually said adults need about 2.5 liters daily, including water from food. Everyone forgot the second half of the sentence.

The real answer is more specific and more useful. The U.S. National Academies of Sciences, Engineering, and Medicine set the official Adequate Intake at 3.7 liters per day for men and 2.7 liters per day for women, from all beverages and foods combined. About 20% of that comes from food, so the drinking target is about 3.0 liters for men and 2.2 liters for women. But your individual number depends on your weight, activity level, climate, and whether you are pregnant or breastfeeding.

What the Official Recommendations Say

The National Academies set Adequate Intake (AI) values for total water, which includes all beverages and the water content of food. These values have not been updated since 2005, and as of February 2026, no new water DRI has been published.

Group	Total water AI	From beverages	From food
Men, 19+	3.7 liters (125 oz)	3.0 liters (101 oz)	0.7 liters
Women, 19+	2.7 liters (91 oz)	2.2 liters (74 oz)	0.5 liters
Pregnant, 14 to 50	3.0 liters	2.3 liters	0.7 liters
Lactating, 14 to 50	3.8 liters	3.1 liters	0.7 liters

Food contributes about 19% of total water intake. Fruits and vegetables are 80% to 95% water. A cup of watermelon is about 140 grams of water. Coffee, tea, and other beverages all count toward your total, including those with caffeine.

The CDC updated its water and health guidance in March 2026, noting that daily water needs vary by age, sex, pregnancy status, activity level, and breastfeeding status. Dehydration can cause unclear thinking, mood changes, overheating, constipation, and kidney stones.

Use our Water Intake Calculator to get a personalized target based on your weight and activity level.

How to Calculate Your Personal Water Needs

The National Academies does not publish a per-kilogram formula, but clinical practice uses a weight-based rule:

Daily water intake (mL) = Body weight (kg) x 30 to 40 mL/kg

For different age groups:

Age range	mL per kg per day
16 to 30	35 to 40
31 to 54	30 to 35
55 to 65	30
Over 65	25

Examples

• 70 kg (154 lb) adult, age 30: 70 x 35 = 2,450 mL = 2.45 liters
• 80 kg (176 lb) adult, age 40: 80 x 32.5 = 2,600 mL = 2.6 liters
• 60 kg (132 lb) adult, age 70: 60 x 25 = 1,500 mL = 1.5 liters

A 2019 NHANES analysis found that the median total water intake associated with meeting hydration criteria was 42 mL/kg for men and 43 mL/kg for women. People with total water intake at or above 45 mL/kg/day were significantly more likely to meet hydration standards.

How Exercise and Climate Change Your Needs

Sweat loss ranges from 0.3 liters per hour at rest to 2.0 liters per hour during heavy activity in the heat. Under extreme conditions, daily water needs can rise to 6 liters or more.

Activity / condition	Additional water needed
30 minutes of moderate exercise	0.3 to 0.5 liters
1 hour of intense exercise	0.7 to 1.0 liters
Hot, humid climate	0.5 to 1.0 liters extra per day
High altitude (above 8,000 ft)	0.5 to 1.0 liters extra per day
Pregnancy	0.3 liters extra per day
Breastfeeding	1.1 liters extra per day

For exercise lasting over an hour, plain water may not be enough. You also lose sodium through sweat, and drinking large amounts of plain water without replacing electrolytes can lead to hyponatremia.

The Danger of Overhydration

Hyponatremia is a condition where blood sodium falls below 135 mmol/L because you have consumed more water than your kidneys can excrete. The kidneys can process about 0.8 to 1.0 liters of water per hour. Drinking much more than that, especially during endurance events, can be dangerous.

Symptoms of hyponatremia include nausea, headache, confusion, vomiting, seizures, and in severe cases, coma. Severe hyponatremia (below 120 mmol/L) can be life-threatening.

Exercise-associated hyponatremia is the most common form, seen in marathon runners and military recruits who drink too much plain water during prolonged exertion. The prevention guidance from sports medicine is simple: drink to thirst, do not force fluids, and use electrolyte-containing drinks during events lasting over an hour.

Example 1: The Office Worker

David is 35, weighs 180 pounds (81.6 kg), and works a desk job in an air-conditioned office. He walks about 30 minutes per day for his commute.

Weight-based target: 81.6 x 32.5 (age 31 to 54 range) = 2,652 mL = 2.65 liters Exercise addition: 0.3 liters for 30 minutes of walking Total: about 2.95 liters from all sources

Since about 20% comes from food, his drinking target is about 2.4 liters, or roughly 80 ounces. That is about 10 cups of beverages throughout the day, including coffee, tea, water, and milk.

Example 2: The Endurance Athlete

Maria is 28, weighs 140 pounds (63.5 kg), and is training for a marathon. She runs 90 minutes on Saturday mornings in 75-degree weather.

Weight-based target: 63.5 x 37.5 (age 16 to 30 range) = 2,381 mL = 2.38 liters Exercise addition: 1.0 to 1.5 liters for 90 minutes of running in warm weather Total: about 3.4 to 3.9 liters

She should drink about 0.5 to 0.7 liters per hour during the run, preferably with electrolytes. After the run, she continues drinking to replace the fluid deficit. She should not chug a liter of plain water immediately after, as that risks diluting her blood sodium.

Use our Water Intake Calculator and Caffeine Intake Calculator to track your daily fluid goals.

Hydration and Cognitive Performance

A 2026 randomized controlled trial by Zhang et al. in Frontiers in Nutrition tested 64 healthy students after 12 hours of water restriction. They were randomized to drink 500 mL, 200 mL, 100 mL, or nothing.

At baseline, 57.8% were already dehydrated. The 500 mL group showed improved mental arithmetic scores and increased arithmetic span compared to the control group. Brain MRI scans showed dose-dependent changes in the orbitofrontal cortex, hippocampus, and other regions.

The takeaway: even mild dehydration affects cognitive function. You do not need to be an athlete in the desert to experience the mental effects of insufficient water.

Common Mistakes to Avoid

1. Following the 8x8 rule blindly. Eight glasses is 1.9 liters, which is below the National Academies recommendation for both men (3.7 L) and women (2.7 L). And it ignores water from food, which accounts for about 20% of total intake.

2. Chugging water to hit a target. Your kidneys can only process about 1 liter per hour. Drinking much more than that in a short period risks hyponatremia. Spread intake across the day and drink to thirst.

3. Ignoring water from food and other beverages. Coffee, tea, milk, juice, fruits, and vegetables all count toward your total. You do not need to drink 3 liters of plain water if you are also eating soup, fruit, and drinking coffee.

4. Not adjusting for exercise and climate. A 2.5-liter target is fine for a sedentary day indoors. It is not enough for a 2-hour hike in 90-degree weather. Add 0.5 to 1.0 liters per hour of vigorous activity, and more in heat.

External Research and Resources

• National Academies: Dietary Reference Intakes for Water provides the official Adequate Intake values (3.7 L men, 2.7 L women) and the methodology behind them.
• CDC: Water and Healthier Drinks covers daily water needs by age and condition, updated March 2026.
• NIH StatPearls: Water Toxicity explains hyponatremia, its causes, symptoms, and prevention, including exercise-associated hyponatremia.

People Also Ask

How much water should I drink a day?

The National Academies recommend 3.7 liters per day for men and 2.7 liters per day for women from all beverages and foods combined. About 20% comes from food, so the drinking target is about 3.0 liters for men and 2.2 liters for women. A weight-based formula is 30 to 40 mL per kilogram of body weight.

Is the 8x8 water rule accurate?

No. The 8x8 rule (eight 8-ounce glasses, or 1.9 liters) has no scientific origin. It likely traces to a 1945 National Research Council recommendation that actually said adults need about 2.5 liters, including water from food. The current National Academies recommendation is higher: 3.7 liters for men and 2.7 liters for women from all sources.

Does coffee count toward daily water intake?

Yes. Coffee, tea, milk, juice, and other beverages all count toward your total water intake. While caffeine is a mild diuretic, the net fluid contribution of caffeinated beverages is still positive. The water content of food (fruits, vegetables, soups) also counts, making up about 20% of total intake.

Can you drink too much water?

Yes. Drinking more water than your kidneys can excrete (about 0.8 to 1.0 liters per hour) can cause hyponatremia, a dangerous drop in blood sodium. Symptoms include nausea, headache, confusion, and seizures. Endurance athletes are at highest risk. Drink to thirst and use electrolyte drinks during prolonged exercise.

How much water should I drink during exercise?

Drink 0.4 to 0.8 liters per hour during exercise, depending on intensity and heat. For sessions over an hour, use a drink containing electrolytes (sodium) to prevent hyponatremia. After exercise, drink to replace the fluid lost through sweat, roughly 1.5 times the weight lost during the session.

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