How much water do you need?
A common starting estimate is about 35 millilitres of water per kilogram of body weight, adjusted up for how active you are. For a 70 kg person that's roughly 2.5 litres a day at rest, rising with exercise as you lose more through sweat. This tool applies that rule and an activity factor to give a personalised daily target.
What affects your needs
Hot or humid weather, high altitude, fever, pregnancy and breastfeeding all raise how much fluid you need, as does intense or prolonged exercise. Food also contributes — fruits and vegetables can supply a meaningful share of your daily water — so the figure here is total intake, not only what you drink from a glass.
A practical target, not a strict rule
Use the result as a guide rather than a hard limit. A simple sign of good hydration is pale-yellow urine and rarely feeling thirsty. Spread intake across the day rather than drinking a lot at once.
Frequently asked questions
Does coffee or tea count toward my water?
Yes. Despite their mild diuretic effect, tea, coffee and most drinks contribute to daily fluid intake. Water is still the best default because it has no sugar or calories.
Can you drink too much water?
Very rarely, but drinking extreme amounts in a short time can dilute blood sodium (hyponatraemia). For almost everyone, normal thirst-guided drinking around this target is safe.
How much water should I drink a day?
A common guide is about 30–35 mL per kilo of body weight, so roughly 2–2.5 litres for most adults, and more with heat or exercise.
Does the 'eight glasses a day' rule hold up?
It is a rough guide, not a strict rule. Needs vary by size, activity and climate, and food and other drinks count toward the total.
How do I know if I am drinking enough?
Pale-yellow urine and rarely feeling thirsty are good signs. Dark urine or frequent thirst suggests you need more.