The well-known “eight glasses a day” is an easy reminder, not a scientific rule. How much you need depends on your size, activity, climate, diet and health. Most healthy people do well by drinking when they are thirsty and with meals.
The reference numbers
The US National Academies of Sciences set “adequate intakes” for total water, meaning water from all drinks and food:
- Men: about 3.7 liters (125 oz) a day
- Women: about 2.7 liters (91 oz) a day
Food usually supplies around 20% of that, from fruit, vegetables, soups and more. That leaves roughly 3 liters (about 13 cups) from drinks for men and 2.2 liters (about 9 cups) for women. Coffee, tea, milk and other drinks count. Caffeine has a mild diuretic effect but still adds to your fluid intake overall.
Simple ways to tell if you are drinking enough
- Urine color: pale yellow is a good sign; dark yellow suggests you need more.
- Thirst: in healthy adults, thirst is a reliable signal. Older adults feel thirst less, so they should drink regularly.
- Few headaches, steady energy: mild dehydration can cause headaches, fatigue and trouble concentrating.
When you need more
- Heat and humidity, and high altitude.
- Exercise: drink before, during and after. For long, sweaty sessions, sports drinks or salty snacks replace sodium.
- Fever, vomiting or diarrhea: oral rehydration solutions help more than plain water.
- Pregnancy and breastfeeding: the National Academies suggest about 3 liters of total water a day in pregnancy and 3.8 liters while breastfeeding.
When you may need less
Some conditions, such as heart failure, advanced kidney disease or some liver problems, require limiting fluids. Follow your doctor’s advice rather than general targets. It is also possible to drink too much: see can you drink too much water.
Easy habits
- Keep a glass or bottle within sight.
- Drink a glass with each meal.
- Eat water-rich foods: watermelon, cucumbers, oranges, soups.
- Choose water over sugary drinks most of the time.
A simple daily routine
Aim for regular drinking through the day, more in heat and exercise, and use urine color and thirst as your guides. The CDC’s page on water and healthier drinks has more tips.
