The 8 glasses rule is only a starting point
"Eight glasses a day" (about 2 liters) is easy to remember, but real needs vary. The US National Academies suggest total daily fluid intake of about 3.7 liters for men and 2.7 liters for women from all drinks and foods, and roughly 20% of that usually comes from food.
A weight-based estimate
A popular rule of thumb is about 30 to 35 ml of water per kilogram of body weight per day. A 70 kg person would need roughly 2.1 to 2.5 liters. In pounds, that's about half an ounce to two-thirds of an ounce per pound. Our water intake calculator gives a quick personalized figure.
Add more for exercise and heat
Add around 350 to 700 ml for every hour of exercise, more in hot or humid weather. Sweat rates differ a lot between people; weighing yourself before and after a workout shows how much you lost, and each kilogram lost is roughly one liter of fluid to replace.
Signs you're drinking enough
Pale yellow urine is a simple indicator of good hydration. Dark urine, headache, fatigue and dry mouth can be signs you need more. Thirst is a reliable signal for most healthy adults, but older adults may feel less thirsty and should drink regularly.
Can you drink too much?
Yes. Drinking extreme amounts in a short time can dilute sodium in the blood (hyponatremia), which is dangerous. This mainly affects endurance athletes who drink large volumes without electrolytes. Spread your intake across the day.
Easy habits
Keep a refillable bottle visible, drink a glass with every meal, and flavor water with lemon, cucumber or mint. Coffee and tea count toward intake; moderate caffeine doesn't cause net dehydration in regular drinkers. Use our liters to gallons converter if your bottle is labeled in ounces or gallons.
Special cases
Pregnant and breastfeeding women need more fluid, and people with kidney or heart conditions may need limits set by their doctor.
Frequently asked questions
How much water should I drink per day?
Roughly 30–35 ml per kg of body weight, plus extra for exercise and heat.
Does coffee count toward water intake?
Yes, for regular coffee drinkers moderate amounts count toward daily fluids.
How can I tell if I'm hydrated?
Pale yellow urine and not feeling thirsty are good signs.
Is it possible to drink too much water?
Yes; extreme amounts in a short time can cause hyponatremia.