Daily water intake calculator
How much water you need depends mainly on your body size, how much you exercise and the weather. FitCalcs sets a baseline of about 33 ml per kilogram of body weight, which lands near the EFSA adequate-intake figures of roughly 2 litres a day for women and 2.5 for men, then adds for exercise and hot days. About a fifth of your fluid comes from food, and pale-yellow urine is still the best day-to-day guide. This is general information, not medical advice.
How this is worked out
FitCalcs sets a baseline from body weight, then adds for exercise and hot weather:
baseline ml = weight kg x 33
exercise top-up = 350 ml per 30 min of activity
total x 1.15 on a hot or humid day
The 33 ml per kg baseline lands near the EFSA adequate-intake figures (about 2.0 litres a day for women and 2.5 for men, from all drinks and food). Food provides roughly a fifth of your fluid, and thirst plus pale-yellow urine are still the best day-to-day guides.
Informational only, not medical advice. Some conditions need fluid limits set by a doctor. Editorially reviewed by FitCalcs, with each figure citing its source.
Common questions
How much water should I drink a day?
It depends mainly on your body size, activity and the weather, but a common baseline is about 33 ml per kilogram of body weight. That lands near the EFSA adequate-intake figures of roughly 2 litres a day for women and 2.5 litres for men from all drinks. FitCalcs starts there and adds for exercise and hot days. This is general information, not medical advice.
Does the eight-glasses rule hold up?
It is a rough rule of thumb, not a scientific target. Your real need varies with your size, how much you move, the weather and what you eat, so a fixed glass count suits some people and not others. Using your body weight and activity, as this calculator does, gives a more personal starting figure than a flat eight glasses.
Do other drinks and food count towards my intake?
Yes. All drinks count, including tea and coffee, and about a fifth of your fluid typically comes from food such as fruit, vegetables and soups. The adequate-intake figures refer to fluid from all sources, so you do not need to drink that entire amount as plain water.
How can I tell if I am drinking enough?
Pale-yellow urine is the simplest day-to-day guide: dark urine suggests you need more, and very clear urine can mean you are drinking more than you need. Thirst is a reasonable prompt for most healthy adults. If you feel persistently thirsty or unwell, speak to your GP, as it can have medical causes.
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Editor, FitCalcs
Oliver Mackman is the named editor of FitCalcs and a director of Best Business Loans Ltd (Companies House 16833937), the company that operates the site. He is accountable for its editorial standards, sourcing and corrections, and requires every result to cite the underlying equation and the named UK source behind it, with health tools carrying the appropriate not-medical-advice caveats.
Last reviewed: 12 June 2026