How much water to drink: do you really need eight glasses?

“Drink eight glasses of water a day” is one of the most repeated health tips. It’s said so confidently that few people ask what it rests on. Let’s ask.

Where “eight glasses” comes from

Physiologist Heinz Valtin tried to track down the origin of this advice and any studies that support it. His review’s conclusion: no scientific studies in support of “8 × 8” were found (Valtin, 2002). On the contrary, surveys of food and fluid intake in thousands of adults suggest such large amounts aren’t needed — the people surveyed were healthy. Caffeinated drinks, and to a lesser extent mild alcoholic ones like beer in moderation, can likely be counted toward the daily total.

What official targets say

The target from EFSA (the European Food Safety Authority) is an Adequate Intake of total water: 2.0 L a day for women and 2.5 L for men. “Total” means all water, including from food: on average about 20% comes from meals and the rest from drinks. The values are set for a temperate climate and moderate activity. So from drinks that’s roughly 1.6 L for women (six to seven 250 ml glasses) and 2.0 L for men (eight glasses) — not “eight glasses of plain water” for everyone.

Why there’s no single number for everyone

A large study of water turnover — how much water the body uses each day — included 5,604 people from infants to 96-year-olds in 23 countries. Turnover depended on age, body size and composition, physical activity, athletic status, pregnancy, socioeconomic status and climate: latitude, altitude, air temperature and humidity (Yamada et al., 2022). People in countries with a low human development index (a combined measure of health, education and income) had higher water turnover than those in high-index countries.

That’s why there is no widely accepted water requirement for different groups: it depends on a complex regulatory system and big differences between people (Armstrong, Johnson, 2018). The authors of that review found that a mild hormonal defence of body water begins when total intake falls below 1.8 L a day, and that between 19% and 71% of adults in various countries drink less than this. Worth knowing: the authors have ties to water companies (Danone Nutricia Research, the Drinking Water Research Foundation) — the paper says so explicitly.

And there are no longer-term randomized trials of how water volume affects health, with big gaps remaining in how fluid intake and hydration status are measured (Popkin et al., 2010). So any precise figure is a guide, not a law of nature.

What this means in practice

• Count all your water, not just glasses: soup, fruit, vegetables and porridge supply it too — about a fifth of the daily target.
• Tea and coffee can count toward the daily total rather than “taking water away”, as is often said (Valtin, 2002).
• You need more water during hard work and exercise, especially in the heat (Valtin, 2002); water turnover is also linked to pregnancy and climate (Yamada et al., 2022).
• The target doesn’t depend on body weight: formulas like “30 ml per kilogram” have no footing in these sources.

In the calculator the water target is shown separately — split into drinks and water from food, in 250 ml glasses. It’s a starting point, not a prescription.