Water Intake Calculator
A daily fluid estimate, given as a range.
Runs entirely in your browser — nothing you enter is uploaded or sent to a server.
There is no scientifically established daily water target for a healthy adult, and any tool that gives you one confident number is overstating what's known. This shows the range a widely used rule of thumb produces. Fluid needs move with heat, illness, pregnancy, kidney and heart conditions and several common medications — if any of those apply, ask a doctor rather than a calculator. Read our full disclaimer.
About this tool
Almost every water calculator on the web returns one confident number, and none of them can justify it: there is no established daily fluid requirement for healthy adults, and the widely repeated 'eight glasses a day' has no clear source. What does exist is a rule of thumb used in practice — somewhere around 30 to 40 millilitres of total fluid per kilogram of body weight per day — plus an allowance for what you lose training, commonly quoted at 350 to 500 ml per half hour of activity. So this tool gives you the band that rule of thumb actually describes, in litres, millilitres and cups, and tells you where the numbers came from. A range is less satisfying than a target. It's also the truthful answer.
Example
At 70 kg with no exercise, the rule of thumb gives about 2.1–2.8 litres of total fluid. Add an hour of training and it becomes roughly 2.8–3.8 litres.
How to use
- 1Choose metric or imperial and enter your weight.
- 2Add roughly how many minutes of exercise you're doing today.
- 3Read the range in whichever unit you think in.
Features
- A range rather than a single false-precision target.
- Shown in litres, millilitres and 240 ml cups.
- Adds an allowance for exercise, per 30 minutes.
- States the rule of thumb it uses, right next to the inputs.
Frequently asked questions
Does this mean plain water only?
No — it's total fluid, which includes tea, coffee, milk and juice. Food contributes a further 20% or so on top, more if you eat a lot of fruit, vegetables or soup. So the amount of water you need to actively drink is usually less than the figure shown.
Where does the eight-glasses-a-day figure come from?
Nowhere identifiable. It's been traced to a misreading of a 1945 recommendation that mentioned around 2.5 litres a day and noted in the next sentence that most of it comes from food. The number survived; the caveat didn't.
Can I drink too much water?
Yes, though it's uncommon. Drinking a very large volume quickly can dilute blood sodium — hyponatraemia — which is serious. It's mostly seen in endurance events and specific medical conditions. This is one reason the tool gives a range rather than encouraging you to hit a number.
Should I just drink when I'm thirsty?
For most healthy adults, thirst is a reasonable guide, and that's the position a lot of clinicians take. A calculator like this is useful mainly as a rough sense of scale, or for people who genuinely forget to drink during a busy day.
Why does the estimate change with exercise?
Because sweat is the main variable. Losses depend heavily on intensity, heat and humidity, and can range from a few hundred millilitres an hour to well over a litre — so the per-30-minute allowance here is a broad average, not a measurement of your sweat rate.
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