Water intake calculator
A rough daily target from your weight, exercise and climate.
- Runs in your browser — nothing you enter is uploaded
- No sign-up, no quotas, free to use as often as you like
How it works
The familiar eight glasses a day has no particular evidence behind it and takes no account of your size or what you are doing. Scaling with body weight, exercise and climate is cruder than it sounds but still better than a fixed number.
How it is worked out
Roughly 33 millilitres per kilogram of body weight as a baseline, plus about 350 millilitres for each half hour of exercise, adjusted for a hot or cold climate. These are common rules of thumb rather than a clinical guideline.
Everything you drink counts, and so does food
Tea, coffee, juice and milk all contribute. The idea that caffeine dehydrates you does not hold at ordinary intakes — the fluid in a cup of coffee more than covers its mild diuretic effect. Food provides roughly a fifth of most people's total, more if you eat a lot of fruit, vegetables and soup.
The better indicator than any number
Urine colour. Pale straw suggests you are well hydrated; consistently dark suggests not. It responds to your actual needs, which no calculator can see.
When to be careful
Drinking far more than you need is not harmless — in extreme cases it dilutes blood sodium dangerously. And kidney disease, heart failure and some medications change fluid needs substantially, in which case follow medical advice rather than a calculator.
Water intake calculator — questions
Do tea and coffee count?
Yes. At normal intakes their diuretic effect is smaller than the fluid they provide.
Is eight glasses a day right?
It is a round number with little evidence behind it, and it ignores your size and activity entirely.
How can I tell if I am drinking enough?
Urine colour is the practical guide — pale straw is about right, consistently dark is not.
Can I drink too much?
Yes. Greatly excessive intake can dilute blood sodium, which is dangerous. More is not automatically better.
Is this medical advice?
No. Anyone with kidney, heart or medication-related restrictions should follow their doctor rather than this.