How Much Water Should You Drink a Day?
Almost everyone has heard that you should drink eight glasses of water a day. It is a tidy, memorable rule — and like many tidy rules, it is more of a rough guide than a scientific truth. Your real water needs depend on your body, your activity and even the weather. Here is how to work out a sensible target for yourself rather than a one-size-fits-all number.
Where the '8 glasses' rule came from
The famous '8x8' rule — eight 8-ounce glasses, roughly two litres — has no single strong study behind it. It became popular because it is easy to remember, and two litres is a reasonable ballpark for many adults. But a small, sedentary person in a cool climate and a large, active person in summer heat have very different needs, so the same number cannot fit both. It is a starting point, not a prescription.
A better estimate based on your body
A more personalised guideline is to aim for roughly 30 to 35 millilitres of water per kilogram of body weight per day. For someone weighing 70 kg, that works out to about 2.1 to 2.45 litres. This already accounts for the fact that bigger bodies need more, and it is a far better baseline than a fixed glass count. Remember that this is total fluid, not only plain water.
What raises your needs
- Exercise: you lose water through sweat, so add roughly 350 to 700 ml for every hour of activity.
- Hot or humid weather: heat increases sweating and therefore fluid loss.
- Illness: fever, vomiting or diarrhoea drain fluids quickly and need replacing.
- Pregnancy and breastfeeding: both raise daily fluid requirements.
- High-protein or high-salt diets: these make your body use more water to process them.
Food and other drinks count too
You do not have to get every drop from a glass of plain water. Tea, coffee, milk and even food contribute meaningfully — fruits and vegetables like watermelon, cucumber and oranges are mostly water. Roughly a fifth of most people's fluid intake comes from food. The mild diuretic effect of moderate coffee or tea is small enough that these drinks still count towards your total for most people.
How to tell if you are drinking enough
Rather than obsessing over an exact number, watch your body's own signals. The simplest is urine colour: a pale straw yellow usually means you are well hydrated, while dark yellow suggests you should drink more. Persistent thirst, dry lips, tiredness and headaches can all be signs of mild dehydration. On the other hand, you can overdo it — drinking far more than you need offers no extra benefit and, in rare extreme cases, can dilute the body's sodium dangerously. Steady sips through the day beat forcing down litres at once.
- Start the day with a glass of water to rehydrate after sleep.
- Keep a bottle visible at your desk — sight is a strong reminder.
- Drink a little before, during and after exercise.
- Check your urine colour as a quick, honest hydration gauge.
The bottom line
Forget the rigid eight glasses. A better target is around 30 to 35 ml per kilogram of body weight, adjusted up for exercise, heat and illness — and topped up by the water in your food and other drinks. Let pale urine and the absence of thirst tell you when you have got it right. Use our water intake calculator to turn your own weight and activity into a clear daily goal.