“Drink eight glasses of water a day” is one of the most repeated pieces of health advice there is. It’s printed on water bottles, repeated by trainers, and quietly accepted by most of us as fact. The problem is that it was never based on solid evidence, and a large international study has now shown why a single number doesn’t work for everyone.
Where the “8 glasses” rule came from
The eight-by-eight rule (eight 8-ounce glasses, or about 2 liters) has been questioned by researchers for decades. A well-known 2002 review in the American Journal of Physiology searched for the scientific basis of the advice and found none. Most of the studies that tried to pin down a better number had a weak spot of their own: they relied on people remembering and reporting how much they drank, which is easy to get wrong.
How the new study measured water
Published in the journal Science in 2022, the study took a more objective approach. Researchers followed more than 5,600 people of all ages across 26 countries, from newborns to adults in their 90s.
Each participant drank a small amount of “doubly labeled water,” which is safe water containing harmless, traceable forms of hydrogen and oxygen. By measuring how quickly those markers left the body over about a week, scientists could calculate each person’s water turnover, which is the amount of water the body uses and replaces every day.
That turnover is a better guide to real needs than any rule of thumb. It’s also worth knowing that turnover counts water from all sources, including food and the water your body produces as it burns energy. So the amount you need to drink is lower than your total turnover.
What the researchers found
The headline finding is simple: water needs vary a lot from person to person. The biggest factors were:
- Age. Turnover was highest for men in their 20s and for women from their 20s through their mid-50s. It began to fall after about age 40 in men and after 65 in women. Newborns had the highest turnover relative to the total water in their bodies.
- Sex. With other factors equal, men turned over roughly half a liter more water per day than women, largely because of differences in body size and composition.
- Body size. Every additional 110 pounds (50 kg) of body weight added about 0.7 liters a day.
- Climate. A 50% rise in humidity added about 0.3 liters a day. Heat and altitude also pushed needs up.
- Activity and lifestyle. People in wealthier countries with mostly sedentary jobs used less water than manual laborers and people in traditional, physically demanding ways of life.
Because of that spread, the authors concluded that one average figure says very little about what any one person needs. Some people need less than eight glasses a day, and some need considerably more.
What this means for you
You don’t need to count glasses to stay well hydrated. A few practical habits work for most healthy adults:
- Let thirst guide you. For most healthy people, thirst is a reliable signal. Drink when you’re thirsty and with meals.
- Check your urine color. Pale yellow usually means you’re well hydrated. Dark yellow is a sign to drink more.
- Drink more when conditions call for it. Hot or humid weather, exercise, physical work, illness with fever, and pregnancy or breastfeeding all raise your needs.
- Count your food. Fruits, vegetables, soups and other water-rich foods all contribute to your daily intake.
- Older adults should be more deliberate. The sense of thirst can weaken with age, so a regular routine of drinking water through the day helps.
Some people do need specific fluid targets, including people with kidney disease, heart failure, or certain other medical conditions. If that applies to you, follow the guidance of your doctor rather than general advice.
The bottom line
The eight-glass rule isn’t harmful, but it isn’t a scientific standard either. Your water needs depend on your age, size, sex, activity level and where you live. For most people, the best approach is to drink regularly, respond to thirst, and drink more when it’s hot or you’re active.
This article is for general information and isn’t medical advice. Talk with your healthcare provider about your individual needs.
