How much water to drink a day?
Enter your age and sex to see the daily water intake the National Academies recommend, both from drinks and in total with the water in food.
- From drinks
- 2.2 L
Aim for about 2.2 L (9.3 cups) of drinks a day. With the water in food, that is 2.7 L of water in all.
- Cups of drinks
- 9.3
- Fluid ounces of drinks
- 74 fl oz
- Total water
- 2.7 L
From drinks: 2.2 L. Aim for about 2.2 L (9.3 cups) of drinks a day. With the water in food, that is 2.7 L of water in all.
This calculator gives general information only. It is not medical advice and does not replace a doctor or other health care provider. Talk with a professional about your results. Terms of use
How to calculate
Gives the daily Adequate Intake of water from the National Academies (Institute of Medicine, 2005) for an age and sex, and during pregnancy or breastfeeding: total water and the part from drinks, in litres, cups, and fluid ounces.
Example with the default inputs (Age 30, Sex Female, Pregnant or breastfeeding? Neither): Aim for about 2.2 L (9.3 cups) of drinks a day. With the water in food, that is 2.7 L of water in all.
Method: Look up the Institute of Medicine Adequate Intake for the age, sex, and life stage: total water and the part from drinks, in litres per day; cups = litres ÷ 0.2365882365.
- The intakes are the Institute of Medicine (2005) Adequate Intakes for healthy people, set from the median intakes of well-hydrated people in a US survey.
- Drinks include plain water and every other beverage, including coffee and tea; the rest of the total comes from food.
- People who are physically active or exposed to heat need more total water (IOM).
- Pregnancy and breastfeeding values apply to ages 14 to 50. This is general information, not medical advice.
Worked examples
Each example is checked against the calculator on every build.
- Age 30, Sex Female, Pregnant or breastfeeding? Neither gives Total water 2.7 L, From drinks 2.2 L, Cups of drinks 9.298856.Source: Institute of Medicine (2005), DRI for Water, chapter 4: women 19 to 30 years, 2.7 L/day total water, about 2.2 L (≈ 9 cups) from beverages
- Age 45, Sex Male gives Total water 3.7 L, From drinks 3 L, Cups of drinks 12.680259.Source: Institute of Medicine (2005), chapter 4: men 31 to 50 years, 3.7 L/day total water, about 3.0 L (≈ 13 cups) from beverages
- Age 28, Sex Female, Pregnant or breastfeeding? Pregnant gives Total water 3 L, From drinks 2.3 L.Source: Institute of Medicine (2005), chapter 4: pregnancy, 3.0 L/day total water, about 2.3 L (≈ 10 cups) from beverages
- Age 25, Sex Female, Pregnant or breastfeeding? Breastfeeding gives Total water 3.8 L, From drinks 3.1 L.Source: Institute of Medicine (2005), chapter 4: lactation, 3.8 L/day total water, about 3.1 L (≈ 13 cups) from beverages
- Age 15, Sex Male gives Total water 3.3 L, From drinks 2.6 L.Source: Institute of Medicine (2005), chapter 4: boys 14 to 18 years, 3.3 L/day total water, about 2.6 L from beverages
- Age 6, Sex Female gives Total water 1.7 L, From drinks 1.2 L.Source: Institute of Medicine (2005), chapter 4: children 4 to 8 years, 1.7 L/day total water, about 1.2 L from beverages
How the water intake is worked out
The calculator looks up the Adequate Intake (AI) for water from the National Academies' Institute of Medicine (2005). An AI is the median intake of healthy people who are well hydrated, from a US national survey. It is given as total water (drinks plus the water in food) and the part from drinks (all beverages, including drinking water).
The values, in litres a day (total water, then drinks):
- children 1 to 3 years: 1.3 and 0.9;
- children 4 to 8 years: 1.7 and 1.2;
- boys 9 to 13 years: 2.4 and 1.8; girls 9 to 13 years: 2.1 and 1.6;
- boys 14 to 18 years: 3.3 and 2.6; girls 14 to 18 years: 2.3 and 1.8;
- men 19 and older: 3.7 and 3.0; women 19 and older: 2.7 and 2.2;
- pregnancy, ages 14 to 50: 3.0 and 2.3;
- breastfeeding, ages 14 to 50: 3.8 and 3.1.
The age ranges include both ends (a 3-year-old uses the 1-to-3 row, a 4-year-old the 4-to-8 row). The pregnancy and breastfeeding question appears only for women aged 14 to 50; otherwise the calculator uses the age and sex row.
Cups and ounces. One US cup is 8 US fluid ounces, or 0.2365882365 litres exactly, so cups = litres ÷ 0.2365882365 and fluid ounces = cups × 8. The National Academies round these to whole cups (2.2 litres ≈ 9 cups); the calculator shows one decimal.
Assumptions
- The intakes are for healthy people in a temperate climate with typical activity. Physical activity and heat raise the need for water.
- Infants under 1 year get their water from breast milk or formula, so they are not covered.
- If a doctor has given you a fluid limit or target, follow it instead.
This page is for information only. It is not medical advice.
Worked examples by hand
A woman aged 30. The row for women 19 and older gives 2.7 litres of total water and 2.2 litres from drinks. In cups: 2.2 ÷ 0.2365882365 = 9.3 cups, or 9.3 × 8 = 74 fluid ounces.
A man aged 45. The row for men 19 and older gives 3.7 litres total and 3.0 litres from drinks: 3.0 ÷ 0.2365882365 = 12.7 cups.
Pregnant, aged 28. 3.0 litres total and 2.3 litres from drinks.
Breastfeeding, aged 25. 3.8 litres total and 3.1 litres from drinks.
A boy aged 15. 3.3 litres total and 2.6 litres from drinks.
A child aged 6. 1.7 litres total and 1.2 litres from drinks.
Other questions people ask
How much water should I drink a day?
For adults, the National Academies (Institute of Medicine, 2005) set an Adequate Intake of 2.7 litres of total water a day for women and 3.7 litres for men. About 80% comes from drinks: about 2.2 litres (9 cups) for women and 3.0 litres (13 cups) for men. The rest comes from food.
Do coffee and tea count toward my water intake?
Yes. The National Academies count all beverages, including caffeinated ones, toward total water. They found that the diuretic effect of caffeine may be short-lived and that these drinks still add to your water intake.
Do I need more water when I exercise or it is hot?
Yes. The Adequate Intakes are for typical daily life. People who are physically active or exposed to heat need more total water, and very active people in hot weather can need 6 litres or more a day. The American College of Sports Medicine suggests weighing yourself before and after exercise to see how much sweat you lose.
How much water do I need when pregnant or breastfeeding?
The Adequate Intake rises to 3.0 litres of total water a day in pregnancy (about 2.3 litres, or 10 cups, from drinks) and to 3.8 litres when breastfeeding (about 3.1 litres, or 13 cups, from drinks), for ages 14 to 50.
Is the 8 glasses a day rule right?
It is close for many women but low for many men. Eight 8-ounce glasses are 64 fluid ounces, or about 1.9 litres. The National Academies' figure for drinks is about 2.2 litres for women and 3.0 litres for men, and thirst and normal drinking at meals keep most healthy people hydrated.
Can I drink too much water?
Yes, but it is rare. The National Academies set no upper limit, but note that healthy kidneys can excrete at most about 0.7 litres an hour. Drinking far more than that can cause water intoxication and low blood sodium (hyponatremia), which the National Academies note can be life threatening.