How Much Water Do You Need Each Day? What the Numbers Mean

Where the daily water numbers come from

The figure most people have heard, eight glasses a day, is not a scientific reference value. The values that are used in nutrition policy come from the Dietary Reference Intakes for water published by the Institute of Medicine (now part of the National Academies) in 2005, and from a separate scientific opinion issued by the European Food Safety Authority in 2010. Both are worth understanding, because they are built differently from most nutrient recommendations.

Most vitamins and minerals have a Recommended Dietary Allowance, which is set from evidence about what prevents deficiency. Water has no such evidence base for a precise number: healthy people regulate their water balance closely and tightly, and no experiment has identified a single intake that all adults need. The Institute of Medicine therefore set an Adequate Intake, a value based on the median intake of healthy, apparently well-hydrated people in national surveys. An Adequate Intake is an observation of what healthy people actually drink and eat, not a calculation of what they require.

The two sets of values disagree by more than a liter for men. That is not because one body of experts made an error. The two panels worked from different populations, applied different assumptions about food moisture and climate, and reached different judgments about what counts as adequate. When two reputable authorities publish reference values that far apart, the honest reading is that the true individual figure sits within a wide band and varies from person to person.

Total water is not the same as water you drink

Both reference values describe total water. That includes plain water, but it also includes coffee, tea, milk, juice, soup and the water held in solid foods. The Institute of Medicine noted that roughly 80 percent of total water intake typically comes from beverages and about 20 percent from food. Fruits, vegetables, yogurt, cooked grains and soups all contribute materially.

In practice, that means the headline figures overstate how much a person needs to pour into a glass. Applied to the US value for women, 2.7 liters of total water implies something like 2.2 liters from drinks across a whole day, and for men about 3.0 liters from drinks. Someone who eats a lot of produce and soup needs less from beverages than someone who eats mostly dry, processed food.

The Centers for Disease Control and Prevention has published national survey data on what US adults actually drink and eat, which shows that many healthy adults live comfortably around or below the headline reference values. That is consistent with how an Adequate Intake is defined: it was derived from healthy people who were not experiencing the effects of deficiency.

What raises or lowers your needs

A single daily figure hides most of the things that actually move water requirements. The reference values are for adults in temperate conditions at modest activity levels, and several ordinary circumstances shift the picture.

• Heat and humidity increase sweat losses, sometimes by a liter or more over a day for someone working outdoors.
• Exercise adds sweat. The American College of Sports Medicine position stand on exercise and fluid replacement describes sweat rates that commonly fall in the range of roughly 0.4 to 0.8 liters per hour, with large differences between individuals and between environments.
• Altitude increases breathing rate and urine output over the first days at elevation.
• Fever, vomiting and diarrhea cause rapid fluid loss, and replacing it is a medical matter if it persists.
• Pregnancy and breastfeeding raise requirements, and the Institute of Medicine set higher adequate intakes for both.
• Some medications, including diuretics, change fluid balance, and some medical conditions require fluid to be restricted rather than increased.

Older adults deserve a note of their own. Thirst perception can be blunted with age, and some older people drink less than they need without noticing. For someone in that position, a deliberate habit of regular fluids is more reliable than waiting for thirst. This is one place where the generic advice to drink to thirst needs a qualifier.