How much water do you need?
There is no single number that fits everyone. A bigger body needs more water, exercise and heat make you lose more through sweat, and pregnancy and breastfeeding raise your needs too. This water intake calculator turns those factors into an amount of water to drink per day, shown in cups, fluid ounces and liters, and spreads it over your waking hours. It is a starting point for good hydration, not an exact prescription.
How the calculator works
It uses a common rule of thumb: about half an ounce of water for every pound of body weight, which is roughly 33 ml per kilogram. On top of that it adds about 12 oz (350 ml) for every 30 minutes of exercise.
Example: someone who weighs 160 lb and exercises 30 minutes a day gets 160 × 0.5 = 80 oz, plus 12 oz for exercise, so 92 oz. That is 11.5 cups or about 2.7 liters.
The extra 16 oz for a hot or humid day is a rough allowance, not a measured value: with heavy sweating you can need a lot more. For pregnancy and breastfeeding the calculator adds the difference between the National Academies' adequate intakes: about 3 liters a day in pregnancy and 3.8 liters while breastfeeding, compared with 2.7 liters for women in general. That is about 10 oz (0.3 liters) more when pregnant and about 37 oz (1.1 liters) more when breastfeeding.
Signs of dehydration
- Thirst and a dry mouth. The first and most reliable signal for most healthy adults.
- Dark yellow urine, or going to the bathroom less often than usual. Pale yellow is a good sign.
- Headache, tiredness or dizziness, especially in heat or after exercise.
- Confusion, a fast heartbeat or fainting are signs of severe dehydration and need medical help right away.
Older adults feel thirst less strongly, and young children cannot always say they are thirsty, so keep an eye on them, especially in hot weather.
Why needs vary, and when to be careful
Two people of the same weight can need quite different amounts. Climate, altitude, diet, illness with fever, vomiting or diarrhea, and how much you sweat all change the picture. Food covers part of your water, and coffee, tea and milk count as fluid too. Ounces here are US fluid ounces, and a cup is 8 fl oz.
Be careful with any general rule if you have kidney disease, heart failure or another condition where your doctor limits fluids, or if you take medication that affects your water balance. In those cases, more is not better: ask your doctor how much you should drink. This tool is for information, not medical advice.
How to use the water intake calculator
- 1Pick US or metric units and enter your body weight.
- 2Add your daily minutes of exercise, choose the climate, and say if you are pregnant or breastfeeding.
- 3Read your daily amount in cups, ounces and liters, set your waking hours, and use the schedule as a guide. Copy the result or a link to it.
Frequently asked questions
How much water should I drink per day?
It depends on your size, how active you are and the weather. A common rule of thumb is about half an ounce of water per pound of body weight, plus more when you exercise. For a 160 lb person with 30 minutes of exercise, that is about 92 oz, or 11.5 cups. Thirst is a good guide for most healthy people.
Does coffee or tea count toward my water intake?
Yes. Coffee, tea, milk, juice and other drinks all add to your fluid intake. The mild diuretic effect of caffeine does not cancel out the water in a cup for people who drink it regularly. Water is still the best everyday choice, since it has no sugar or calories.
Does food count toward hydration?
It does. Fruit, vegetables, soup and many other foods contain a lot of water. The National Academies estimate that about 20% of the water people take in usually comes from food. Their adequate intake of about 3.7 liters for men and 2.7 liters for women covers drinks and food together.
Can you drink too much water?
Yes, though it is rare. Drinking very large amounts in a short time, for example during long endurance events, can lower the sodium in your blood to a dangerous level, called hyponatremia. Spread your water over the day, and during long exercise in heat consider drinks with electrolytes.
Should people with kidney or heart problems follow this?
No, not without asking a doctor. With kidney disease or heart failure, doctors often set a fluid limit, and drinking more can be harmful. The same goes for some medications. Follow the amount your doctor gives you, not a general rule of thumb.