How much water do you actually need?

Not eight glasses. The official numbers are higher than that, they include food, and they were never meant as a target for you personally.

"Drink eight glasses a day" is the most repeated hydration rule there is. It's easy to remember, and it isn't what the main reference values say.

The official numbers

In 2004 the US National Academies (then the Institute of Medicine) set reference values for water. For healthy adults living in temperate climates, the "adequate intake" for total water is about 3.7 litres a day for men and 2.7 litres for women.¹

Three details change how those numbers read:

• They include food. About 20% of the water people take in comes from food, the rest from drinks. So the drinks portion is closer to 3.0 litres for men and 2.2 litres for women. Those are the "13 cups" and "9 cups" figures you'll sometimes see.
• Coffee and tea count. The panel counted caffeinated drinks toward total water.¹
• They aren't requirements. An adequate intake is based on what healthy people typically consume. The panel said directly that it shouldn't be read as a specific requirement, and that active people and people in hot climates may need more.¹

What people actually turn over

A 2022 study in Science measured how much water 5,604 people, aged from 8 days to 96 years, across 26 countries, actually move through their bodies each day. It used a tracer method: participants drank a small dose of water labelled with deuterium, and researchers tracked how fast it left the body.²

• Average daily turnover was about 4.3 litres for men and 3.4 litres for women.
• Individual averages ranged from roughly 1 to 6 litres a day, with some outliers near 10.
• Turnover rose with body size, physical activity, heat and humidity, and was higher in people living in countries with lower human development scores, where more physical work is done and more people live without climate control.

Turnover isn't the same as how much you need to drink. It includes water from food and water your body makes when it burns fuel. The authors' point was that a single number for everyone, eight glasses included, doesn't fit how differently people use water.

So how much do you need?

For a healthy adult on an ordinary day, the honest answer is: probably somewhere in the range of the reference values, and thirst will usually get you there. Food covers a real share. Coffee, tea and milk count.

Your needs move up a lot with:

• Exercise, especially in heat. Sweat losses can add one or two litres per hour of hard training. Here's how to measure yours.
• Hot or humid weather and physical work.
• Body size.
• Pregnancy and breastfeeding.

And some groups shouldn't rely on thirst alone, including older adults and people on medications that change thirst. That's covered in Drink to thirst or drink to a plan?