Water Intake Calculator
How much water you need depends mainly on your size and how active you are. A common rule is 35 ml per kilogram of body weight, plus extra for exercise.
Enter your weight and daily exercise to see your target in litres and glasses.
How the calculation works
We believe numbers should be transparent. Here is the formula behind this calculator.
Worked example
Worked example: 70 kg, 30 minutes of exercise a day
Important assumptions
- Uses the common 35 ml per kg guideline.
- Exercise add-on is approximate — hotter weather and heavy sweat mean more.
- Tea, coffee and food all count towards your fluid intake.
Rates & official sources
Every calculation is based on the official rates and rules published by GOV.UK and HMRC.
Frequently asked questions
Is 8 glasses a day the right amount?
It is a decent shorthand for smaller people, but your real need scales with body size and activity. A 90 kg active person needs well over 8 glasses; a 55 kg sedentary person needs fewer.
Can I drink too much water?
Yes, though it is rare. Drinking far more than you need can disturb your salt balance. If you are not exercising hard, staying near your target is enough.
Does tea and coffee count?
Yes. Despite the old myth, the caffeine in tea and coffee is mildly dehydrating at most, and the fluid counts towards your total. Decaf and herbal drinks count fully.
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View calculatorRelated guides
New to the topic? Read a plain-English guide on health & lifestyle, with worked examples and tips.
These calculations are estimates for guidance only and are not professional financial, tax or legal advice. UK rates and rules change, so always verify important figures with official GOV.UK / HMRC information or speak to a qualified professional. See our full disclaimer.