How Much Water Do You Need in a Day?

Most healthy adults get enough water by drinking when they’re thirsty and with meals. As a reference, the US National Academies found that adequately hydrated women take in about 2.7 litres of total water a day and men about 3.7 litres, counting drinks and the water in food; the European Food Safety Authority (EFSA) considers 2.0 litres adequate for women and 2.5 litres for men. These are averages of what hydrated people consume, not a target you must hit.

This article is general information, not medical advice. Some conditions, such as kidney or heart disease, call for a fluid limit or a fluid target set by your doctor.

Total Water, Not Just Drinks

Both figures are total water: plain water, every other drink, and the water in food. The National Academies’ 2004 report estimated that about 80% of total water comes from drinks and the other 20% from food; fruit, vegetables, soups, and cooked grains are mostly water. Coffee, tea, and milk count too.

Source	Women	Men	What it is
US National Academies (2004)	2.7 L a day	3.7 L a day	Adequate Intake: the average total water of adequately hydrated people
EFSA (2010)	2.0 L a day	2.5 L a day	Adequate Intake of total water, for moderate temperatures and moderate activity

The two differ because they’re set from different populations’ intakes. Neither is a minimum. The National Academies’ panel offered no rule of thumb for how many glasses to drink, since needs can be met from many sources, and found that most healthy people meet them by letting thirst be their guide. It also found that caffeinated drinks count towards it as much as others, with no convincing evidence that caffeine leaves the body short of water.

Where “Eight Glasses” Comes From

In a 2002 review, Heinz Valtin looked for the scientific basis of “drink at least eight 8-ounce glasses of water a day” and found no studies supporting it. Surveys of healthy adults suggested such large amounts aren’t needed, caffeinated drinks can count towards the total, and the body’s water balance is tightly regulated. He stressed that the conclusion is limited to healthy adults in a temperate climate leading a largely sedentary life.

When You Need More

• Heat and hard exercise, which raise sweat losses. The National Academies note that very active people continually exposed to hot weather often need six litres or more of total water a day.
• Illness with fever, vomiting, or diarrhoea.
• Pregnancy and breastfeeding, which have reference values of their own.
• Some medical conditions, such as kidney stones, where a doctor may advise drinking more.

Following It in Kaizora

Dietary Water is on the Food Log, the water in what you ate and drank. Foods logged from the Food Database carry their water from the USDA’s FoodData Central, and Cronometer and Apple Health imports bring each food’s and drink’s water. It’s shown in millilitres, or fluid ounces in the US, and a gram of a food’s water counts as a millilitre. A glass of water is logged as a food: add water as a custom food (100 g of it holds 100 mL of water) and log it by its amount in grams, so a 250 mL glass is 250 g. See Add a Custom Food.

Kaizora has no default reference range for water, since the reference values describe typical intakes rather than a need. A habit such as “at least” an amount of Dietary Water a day works if you want a reminder; count only what you actually log, since a day with nothing logged reads as zero.

The Short Version

• Healthy adults usually get enough by drinking to thirst and with meals.
• Reference total intakes: 2.7 L (women) and 3.7 L (men) in the US; 2.0 L and 2.5 L in Europe, including food’s water.
• “Eight glasses a day” has no evidence behind it.
• Heat, exercise, illness, pregnancy, and breastfeeding raise your needs.

Sources

• National Academies of Sciences, Engineering, and Medicine, Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (2005). doi:10.17226/10925
• National Academies, Report sets dietary intake levels for water, salt, and potassium to maintain health and reduce chronic disease risk (2004).
• EFSA Panel on Dietetic Products, Nutrition, and Allergies, “Scientific opinion on dietary reference values for water,” EFSA Journal 8, no. 3 (2010): 1459. doi:10.2903/j.efsa.2010.1459; EFSA, EFSA sets European dietary reference values for nutrient intakes (2010).
• Valtin H, “‘Drink at least eight glasses of water a day.’ Really? Is there scientific evidence for ‘8 × 8’?,” American Journal of Physiology: Regulatory, Integrative and Comparative Physiology 283, no. 5 (2002): R993–R1004. doi:10.1152/ajpregu.00365.2002