Two official bodies have set how much water adults need: the US National Academies (NASEM, 2004) and the European Food Safety Authority (EFSA, 2010). Both give a total for water from all sources; drinks usually supply about 80% of it and food the rest. Choose a guideline, tell the calculator about pregnancy, exercise and heat, and it gives what to drink in liters or fluid ounces.
Example: a woman who exercises 30 minutes a day, US guideline. Total water: 2.7 L, of which about 2.2 L from drinks. With a rough 0.25 L for the half-hour of exercise: about 2.5 liters a day to drink – 83 fl oz, or 10 cups of 8 fl oz.
The official numbers
| Adults | NASEM (US): total water | NASEM: from drinks | EFSA (EU): total water |
|---|---|---|---|
| Men | 3.7 L | 3.0 L (101 fl oz) | 2.5 L |
| Women | 2.7 L | 2.2 L (74 fl oz) | 2.0 L |
| Pregnant | 3.0 L | 2.3 L (78 fl oz) | 2.3 L |
| Breastfeeding | 3.8 L | 3.1 L (105 fl oz) | 2.7 L |
The two bodies arrived at different numbers because each based its figure on the intakes observed in its own population; neither is a minimum or a maximum. They are amounts that cover the needs of most healthy adults in a temperate climate with light activity.
In glasses, cups and bottles
| Liters | US fl oz | 8 fl oz cups | 250 mL glasses | 500 mL bottles |
|---|---|---|---|---|
| 1.5 | 51 | 6.3 | 6 | 3 |
| 2.0 | 68 | 8.5 | 8 | 4 |
| 2.5 | 85 | 10.6 | 10 | 5 |
| 3.0 | 101 | 12.7 | 12 | 6 |
| 3.5 | 118 | 14.8 | 14 | 7 |
The “half your body weight in ounces” rule
A popular rule says to drink half your weight in pounds as fluid ounces – 75 fl oz for a 150 lb person. The calculator shows it for comparison, but it is not an official guideline: for heavier people it gives far more than either reference, and it ignores food, climate and sex.
Signs you are drinking enough
- Your urine is pale yellow through the day.
- You rarely feel thirsty, and you are not dizzy or tired after exercise.
- Your weight is about the same before and after a workout.
Drinking far more than you need is rarely a problem for healthy people, but very large amounts in a short time – during endurance races, for example – can dilute the blood’s sodium (hyponatremia). People with kidney, heart or liver disease may have a fluid limit: follow the doctor’s advice.
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Frequently asked questions
How much water should I drink a day?
The US National Academies set an adequate intake of about 3.7 liters of water a day for men and 2.7 liters for women, from all drinks and food. About 80% usually comes from drinks: roughly 3.0 liters (101 fl oz, 13 cups) for men and 2.2 liters (74 fl oz, 9 cups) for women. Europe's EFSA sets lower figures: 2.5 and 2.0 liters in total.
Is the 8 glasses a day rule right?
It is a fair reminder for many women, but not a scientific rule: 8 cups of 8 fl oz is 1.9 liters, a little under the US figure for women's drinks and well under men's. Your need depends on your body, your activity and the weather, and food covers part of it.
Do coffee and tea count toward water intake?
Yes. All drinks count, including tea, coffee, milk and juice; the mild diuretic effect of caffeine does not cancel the water in a cup for people who drink it regularly. Water-rich foods – fruit, vegetables, soup – count too, which is why the guidelines give a total and a part from drinks.
How much more should I drink when I exercise?
Enough to replace what you sweat, which varies a lot: from about 0.3 to more than 2 liters an hour depending on the effort, the heat and the person. The calculator adds a rough 0.5 L per hour of exercise. Weighing yourself before and after a workout shows your own sweat loss: each kilogram lost is about a liter.
Sources
- National Academies: Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (2005)
- EFSA Panel on Dietetic Products, Nutrition and Allergies: Scientific Opinion on Dietary Reference Values for water, EFSA Journal 2010; 8(3): 1459
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