Water Intake Calculator
How much water should you drink each day?
A general guide for healthy adults - not medical advice. Needs vary with health, medication, pregnancy and weather; when in doubt, ask a clinician.
Forget the one-size-fits-all "eight glasses a day" slogan - how much water you need depends mostly on your size. This free Water Intake Calculator starts with the widely used guide of 35 ml of water per kilogram of body weight, then adjusts it for how active you are (sedentary, moderate or active) and adds 500 ml on hot days when you lose more through sweat. You get your daily target three ways: in millilitres, in litres, and in familiar 250 ml glasses, so you can actually plan your day around it.
A worked example: a 70 kg person with a moderate activity level needs 35 x 70 x 1.15 = about 2,818 ml a day - roughly 2.82 litres or 11 glasses. Treat the result as a sensible target, not a medical prescription. Food provides around a fifth of daily water for many people, thirst is a reliable guide for healthy adults, and needs change with illness, medication, pregnancy, breastfeeding and very long exercise. People with kidney or heart conditions may be told to drink less, not more - if a clinician has given you a fluid limit, that always wins over any calculator. Your weight and answers stay in your browser; nothing is uploaded.
How to use
- Enter your body weight and choose kg or lb.
- Pick your activity level - be honest; it changes the result by up to 30%.
- Choose "Hot" if today is a hot day or you will be sweating a lot.
- Click "Calculate my water target" (the result also updates as you change the inputs).
- Read your daily target in ml, litres and 250 ml glasses.
Frequently asked questions
Where does the 35 ml per kg rule come from?
It is a long-standing rule of thumb in nutrition and sports guidance: roughly 30-35 ml of water per kilogram of body weight per day for a healthy adult in a temperate climate. This calculator uses 35 ml and then adjusts for activity, which is why a larger or more active person gets a higher target than the flat "2 litres for everyone" advice.
Do tea, coffee and food count towards the target?
Mostly, yes. Tea, coffee, milk and juice are almost all water, and normal food - especially fruit, vegetables, soup and yoghurt - typically provides around 20% of daily fluid. This calculator gives you a drinking target for water itself; if much of your fluid comes from other drinks and watery food, you may comfortably drink a little less plain water than the headline number.
Can you drink too much water?
Yes, though it is uncommon. Forcing down very large amounts in a short time can dilute blood sodium (hyponatraemia), which is dangerous. Spread your intake through the day, drink to thirst around your target, and never force yourself to finish a quota quickly. For all-day endurance events, follow event medical guidance, not a general calculator.
Is this calculator right for children, pregnancy or illness?
No - it is calibrated for healthy adults. Children need weight-based amounts set by paediatric guidance, pregnancy and breastfeeding raise fluid needs, and conditions such as kidney or heart disease can come with strict fluid limits. In any of those cases, follow a clinician's advice rather than this page.
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