How Much Water Should You Drink Per Day?
There is no single correct number of glasses. Hydration needs scale with your body, your activity, and your environment.
Where the Common Advice Comes From
The familiar advice to drink eight glasses a day has no strong evidential basis; it appears to be a simplification of older dietary reference values that included water obtained from food. Those reference values suggested total water intakes of roughly 2.7 litres per day for women and 3.7 litres for men, but they explicitly noted that most of this is met through food and all beverages, not plain water alone. Food typically contributes around 20 percent of total water intake.
Estimating Your Own Needs
A widely used practical estimate is 30 to 35 millilitres of fluid per kilogram of body weight per day for healthy adults in a temperate climate. That puts a 70 kg adult at roughly 2.1 to 2.5 litres of total fluid. Activity adds to this: you can lose roughly 0.5 to 1 litre per hour of moderate exercise through sweat, more in heat. Hot and humid environments increase losses further. Our Water Intake Calculator applies these adjustments based on your weight, activity level, and climate.
Signs of Good Hydration
Thirst is a useful signal for most healthy adults, though it becomes less reliable with age. Practical indicators include urine that is pale straw-coloured, urinating four to seven times per day, and rarely feeling thirsty. Dark yellow urine, headaches, fatigue, and difficulty concentrating can all indicate insufficient fluid. Overhydration is possible but rare in everyday life; it occurs mainly with extreme endurance events or when large volumes are consumed very quickly.
What Counts Toward Hydration
Plain water is ideal, but tea, coffee, milk, and juices all contribute to fluid intake. The mild diuretic effect of caffeine is smaller than once believed and does not offset the fluid consumed. Alcohol is different: it genuinely increases fluid loss and is a poor hydration choice. Foods with high water content -- cucumber, watermelon, soups, and yogurt -- also contribute meaningfully.
Practical Guidance
Keep a bottle within reach, drink with meals, and increase intake on hot days and around exercise. If you have kidney, heart, or liver disease, or take diuretics, ask your doctor about appropriate fluid intake rather than following a general guideline. Our related guide on healthy eating covers how food contributes to hydration.