Guides · do you need 8 glasses of water a day
Do you need 8 glasses of water a day?
No, there is no official "8 glasses a day" rule. The US guidance gives a total water target that includes the water in your food and in every drink.
What the guidance says
The US National Academies set an adequate intake of about 3.7 litres (≈125 oz) of total water a day for men and about 2.7 litres (≈91 oz) for women. That is from all foods and beverages combined, not just plain water. The panel did not translate it into a number of glasses.
Where the water comes from
Fluids supply about 81% of total water in the report's figures, so roughly a fifth comes from food. Fruit, vegetables, soup and many other foods are mostly water. Eight 8-oz glasses add up to about 1.9 litres (≈64 oz), which is only part of the totals above.
What people actually drink
CDC survey data (NCHS Data Brief 242) reports US adults' real daily intake from all foods and liquids at about 3.46 litres for men and 2.75 litres for women. Those are roughly in line with the targets.
Your own number varies
Mayo Clinic notes that needs depend on health, age, activity level and climate. A hot day, a long workout, pregnancy, breastfeeding or illness raises it.
Turning the targets into glasses
If about 81% of total water comes from drinks, as the report's figures say, that is roughly 2.2 litres of drinks for the women's target (about nine 8-oz glasses) and 3.0 litres for the men's target (about twelve and a half). Those are totals that include coffee, tea, milk, juice and the water in food. So "eight glasses of plain water" is neither a requirement nor a ceiling.
Why one number cannot fit everyone
Mayo Clinic notes that needs depend on health, age, activity and climate. A person training in a hot place needs more than someone at a desk in a cool office, and no single glass count can reflect that.
A practical way to use this
- Add up what you drink in a normal day, including coffee, tea and milk.
- Do not forget the water in food.
- Drink more on hot days and around long workouts.
- If you have a medical condition, ask your doctor what is right for you.
What counts toward your total
The National Academies targets cover water from all foods and beverages, so the water in soup, fruit and vegetables counts, and so do milk, juice, tea and coffee. Because fluids supply about 81% of the total, roughly 19% comes from food. For the women's target of about 2.7 litres that is about 0.5 litres from food and about 2.2 litres from drinks. For the men's target of about 3.7 litres it is about 0.7 litres from food and about 3.0 litres from drinks.
Why the targets are not a rule for your body
The figures are intake levels for healthy people, not a requirement you must hit each day. Mayo Clinic notes that how much you need depends on your health, age, activity and climate, and the CDC survey data shows people's real intakes spread widely around the averages. Treat the numbers as a rough guide to a normal day, and use the sections above to adjust for heat, workouts and your own situation.
What to do instead
- Treat the targets as a rough total, not a glass count.
- Drink more when you sweat or when it is hot.
- Count the water in food, and count other drinks too.
- Ask a doctor if you have a condition that affects how much you should drink.
Quick answers
Is the 8 glasses rule official?
The National Academies panel that set the water targets does not give a rule of thumb in glasses. The targets are for total water from all foods and drinks.
Does coffee or tea count?
The targets cover total water from all beverages and foods. So drinks count toward the total.
Is the 2.7 and 3.7 litres a requirement?
They are the National Academies' adequate-intake levels for total water from all foods and drinks. Your own need varies with health, age, activity and climate.
Sources
- National Academies: report sets dietary intake levels for water, salt and potassium
- National Academies: Dietary Reference Intakes for water, potassium, sodium, chloride and sulfate (chapter on water)
- CDC / NCHS Data Brief 242: water intake of US adults
- Mayo Clinic: how much water do you need to stay healthy?
General information, not medical advice. Calorie figures are estimates and vary by person.
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