The "8 glasses a day" rule is everywhere. It's also not based on any specific scientific evidence. Your actual water needs depend on your body size, activity level, diet and where you live. Here's how to figure out the right amount for you.
The origin is a 1945 U.S. Food and Nutrition Board recommendation that people consume about 2.5 litres of water daily — but the same document noted that most of this comes from food. The food part got dropped, and "8 glasses" stuck as a cultural shorthand with no particular scientific basis.
Body weight: A larger body has more cells requiring hydration. The standard estimate used by most health guidelines is 35ml per kilogram of body weight per day. So a 70kg person needs around 2.45 litres (8.3 cups) from all sources.
Activity level: Exercise increases water loss through sweat significantly. Add roughly 400–600ml for moderate exercise, more for intense activity or hot weather.
Climate: Heat increases sweating. People in hot or humid climates need considerably more water than those in cool, dry environments.
Diet: About 20% of water intake comes from food — fruits, vegetables and soups contribute substantially. People who eat a lot of whole foods need to drink less than those eating mostly dry processed food.
Enter your weight and activity level to get a personalised daily water target in litres and cups.
Open Water Calculator →Yes, but it's rare in normal circumstances. Hyponatraemia (water intoxication) occurs when someone drinks so much water so quickly that sodium levels in the blood drop dangerously. This typically only happens during endurance events where athletes drink excessive amounts without replacing electrolytes. For everyday life, drinking when thirsty and aiming for pale yellow urine is sufficient.