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Water Intake Calculator

A daily fluid target from your body weight, your training and the weather.

What this calculates

The "eight glasses a day" rule has no particular evidence behind it and takes no account of how big you are or what you did today. Scaling to body weight and adding for training and heat gets you somewhere more useful — though thirst remains a decent guide for most healthy adults.

Common questions

Does tea, coffee or food count?

Yes. Tea and coffee hydrate you despite the caffeine, and food supplies roughly a fifth of most people's fluid — more if you eat a lot of fruit, vegetables and soup. This target is for what you drink.

Can you drink too much water?

Yes, and it is not a trivial risk. Drinking far past thirst over a short period dilutes blood sodium, which can be dangerous. Endurance athletes are the usual cases. Spread intake through the day rather than forcing large volumes at once.

How do I know if I am drinking enough?

Urine colour is the practical test: pale straw is about right, dark yellow means drink more, and completely clear all day usually means you are overdoing it.

Do I need more when training?

Yes — the calculator adds for your activity level, and on a hard or hot session you may want another 400–800 ml per hour on top. Weighing yourself before and after a long session tells you what you actually lost.

Estimates for healthy adults — not medical advice, diagnosis or treatment. If you are pregnant or breastfeeding, under 18, managing a condition such as diabetes or thyroid disease, or have a history of disordered eating, talk to a doctor or a registered dietitian before acting on any of this.

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Content last reviewed 29 August 2026. Estimates for healthy adults, not medical advice.