How much water should you drink every day — two liters, eight cups? Guidelines generally revolve around that ballpark, but according to a new study that used isotopes, the actual need could vary wildly, with averages ranging from 1 to 6 liters.

We’re all different in more than one way, but we all need water to survive and function. How much water though, is not an easy question to answer. Some estimates rely on self-reports from people who estimate their consumption of food and water; but humans are notoriously unreliable when it comes to recalling things accurately. Other studies focused on observation studies, but only tracked a specific group of people.

To get a more accurate estimate, Yosuke Yamada and colleagues from several universities used “labeled water” to see how water passed through the bodies of over 5,600 people.

This water contained trackable hydrogen and oxygen isotopes, atoms with slightly different atomic weights that make them distinguishable from other atoms.

“If you measure the rate a person is eliminating those stable isotopes through their urine over the course of a week, the hydrogen isotope can tell you how much water they’re replacing and the elimination of the oxygen isotope can tell us how many calories they are burning,” says Dale Schoeller, a University of Wisconsin–Madison emeritus professor of nutritional sciences. His lab, in the 1980s, was the first to apply the labeled-water method to study people.

It was a mammoth study that involved over 90 researchers and tracked participants from 26 countries, ages ranging from 8 days to 96 years old, to get a more comprehensive picture of how water usage and requirements vary based on different parameters.

The averages varied greatly. For instance, all things equal, men and women differ by close to half a liter. The mean temperature and physical activity were also found to be important sources of variation. For instance, a male non-athlete aged 20 years old and weighing 70 kg (154 pounds), in a developed country with a mean temperature of 10 degrees C (50 F) and a relative humidity of 50% needs 3.2 liters of water every day — way more than the 8 cups or 2 liters usually touted. A similar woman living in the same place and weighing 60 kg (132 pounds) would still need 2.7 liters.

“There are outliers, too, that are turning over as much as 10 liters a day,” says Schoeller, a co-author of the study. “The variation means pointing to one average doesn’t tell you much. The database we’ve put together shows us the big things that correlate with differences in water turnover.”

Water intake varies wildly

The database established by the researchers also yielded some important correlations. For instance, 10 kg more in body weight adds a requirement of about 0.14 extra liters per day. A 50% increase in humidity asks for 0.3 more liters; athletes need about a liter more than non-athletes. All these are ballpark figures, but they paint a good picture of how our need for water can vary.

“That’s representing the combination of several factors,” Schoeller says. “Those people in low HDI countries [less developed] are more likely to live in areas with higher average temperatures, more likely to be performing physical labor, and less likely to be inside in a climate-controlled building during the day. That, plus being less likely to have access to a sip of clean water whenever they need it, makes their water turnover higher.”

No matter how you look at it, though, current guidelines don’t really make much sense and there’s just too much variation to sustain a single value.

“The science has never supported the old eight glasses thing as an appropriate guideline, if only because it confused total water turnover with water from beverages and a lot of your water comes from the food you eat,” says Schoeller. “But this work is the best we’ve done so far to measure how much water people actually consume on a daily basis — the turnover of water into and out of the body — and the major factors that drive water turnover.”

The measurements are all the more important since our water needs will be increasingly challenged by climate change and unsustainable water usage. In areas where water is insufficient, or where a calamity is restricting water access and water must be rationed, the measurements could also help develop more equitable rationing.

“Look at what’s going on in Florida right now, or in Mississippi — where entire regions have been exposed by a calamity to water shortages,” Schoeller says. “The better we understand how much they need, the better prepared we are to respond in an emergency.”

“Determining how much water humans consume is of increasing importance because of population growth and growing climate change,” Yamada also adds. “Because water turnover is related to other important indicators of health, like physical activity and body fat percent, it has potential as a biomarker for metabolic health.”

The study was published in Science.