How Much Water Should You Drink a Day?
Published June 6, 2026
The "8 glasses a day" rule is a myth. Your real water needs depend on your body size, activity, climate, and even what you eat.
- No published study supports drinking exactly 8 glasses of plain water. Thirst is a reliable guide for most healthy adults.
- The U.S. National Academies recommend roughly 3.7 L/day for men and 2.7 L/day for women as total water, including food.
- About 20% of your intake comes from food. Coffee and tea count too. Only alcohol works against you.
- Pale-straw urine is the easiest real-time hydration check, no math needed.
Where the “8x8” myth came from
The rule traces back to a 1945 U.S. Food and Nutrition Board recommendation that adults need about 2.5 litres of water daily. That report included a sentence almost everyone forgot: most of that quantity is already in prepared foods. The food part got dropped, the number stuck, and a tidy myth was born.
There is no published study showing that healthy adults need precisely eight glasses of plain water, and no evidence that hitting that number produces any specific health benefit beyond what thirst-guided drinking already does.
What the science actually recommends
The most authoritative figures come from the U.S. National Academies of Sciences, Engineering, and Medicine, which set Dietary Reference Intakes for total water from all sources combined.
These are population averages for healthy adults in a temperate climate, not personal targets you must hit to the millilitre. The National Academies were clear: the vast majority of healthy people meet their needs simply by drinking to thirst and eating normally.
The UK’s NHS guidance suggests 6 to 8 cups of fluid a day and explicitly counts low-fat milk, tea, and coffee, not just plain water.
Not just the glass in your hand
Your body does not care whether water arrives as a glass of water, a cup of tea, a bowl of soup, or a slice of watermelon. It all counts.
Water, tea, coffee, milk, soup, juice, and water-rich foods like cucumber (90%+ water), tomatoes, and yoghurt.
Net fluid gain even from coffee and tea.
Alcohol has a strong diuretic effect. It makes you lose more fluid than it provides, which is a key reason hangovers feel like dehydration.
Net fluid loss.
The CDC confirms that high-water foods like fruits and vegetables contribute meaningfully to daily fluid intake. If you eat a reasonable amount of produce and drink throughout the day, you are already closer to your target than the “plain water only” framing suggests.
What raises your needs
The standard figures assume a comfortable climate and a sedentary-to-lightly-active day. These factors push requirements up:
Other factors that raise needs: larger body size, high altitude (which increases breathing rate and urine output), fever, vomiting, or diarrhoea. Pregnancy raises the adequate intake to roughly 3.0 L/day total; breastfeeding raises it further to about 3.8 L/day, per the National Academies.
The easiest check: your urine colour
You do not need to count glasses. The colour of your urine is a free, instant hydration signal.
This is the target. You are well hydrated.
Drink more. Your body is concentrating urine to conserve fluid.
Consistently clear, colourless urine may mean you are drinking more than you need. Some vitamins and medications tint urine regardless of hydration, so the colour check works best as a daily pattern, not a one-off reading.
Can you drink too much?
Yes, though it is rare. Drinking far more than your kidneys can clear dilutes sodium in your blood, a condition called hyponatremia. Severe cases cause nausea, confusion, seizures, and can be life-threatening.
This almost never happens from normal thirst-guided drinking. It tends to occur in extreme endurance events, certain medical conditions, or water-drinking challenges. Your kidneys can process a large volume in a day, but there are limits, and pushing well beyond thirst carries real risk with no benefit.
As the USGS explains in The Water in You, water makes up roughly 60% of the adult body. For the overwhelming majority of people, the real risk is not overhydration but simply not drinking enough on hot, active, or busy days.
Your daily hydration checklist
This guide is for general education and isn’t medical advice. If you have a heart, kidney, or other condition that affects fluid balance, or you take medication that does, talk to a healthcare professional about the right amount of water for you.