Daily Water Intake Calculator
A realistic daily target based on your weight, how much you move and the climate you're in — not a one-size-fits-all number.
Runs entirely in your browser. Nothing you type here is uploaded, stored or logged.
How the target is worked out
The base figure is roughly 33 ml per kilogram of body weight (about 0.5 fl oz per pound), which is the range most dietetic guidance converges on for adults. On top of that:
- Exercise adds about 350–500 ml per 30 minutes of activity, to replace sweat losses.
- Climate adjusts by 5–20%. Heat and humidity raise sweat rate; cold air is dry and raises respiratory water loss slightly, but overall need is a little lower.
- Occupation adjusts for whether you spend the day at a desk or on a building site.
Treat the result as a sensible starting point, not a prescription. Your kidneys are very good at their job, and thirst plus urine colour will fine-tune it better than any formula.
When you need noticeably more
- Illness — fever, vomiting or diarrhoea increase losses substantially.
- Pregnancy and breastfeeding — typically an extra 300 ml and 700 ml a day respectively.
- Altitude — above about 2,500 m, breathing rate and urine output both rise.
- Air travel — cabin humidity is often below 20%, drier than most deserts.
- High-protein or high-fibre diets — both increase water demand.
When you need less — and should check first
Some conditions require fluid restriction, including certain heart failure, kidney and liver conditions. If a clinician has given you a fluid limit, follow that and ignore this calculator entirely.
Practical ways to actually hit the number
- Drink a glass when you wake up — you've just gone seven or eight hours without.
- Keep a bottle you can see. Visibility does more than willpower.
- Attach drinking to existing habits: every time you make coffee, refill the bottle.
- Front-load the day. Drinking most of your intake after 8pm mainly costs you sleep.
- If plain water bores you, add citrus, mint or cucumber — flavour is not cheating.
General information only, not medical advice. Talk to a doctor about hydration if you have a kidney, heart or liver condition, or if you're taking diuretics.
Frequently asked questions
Is eight glasses a day actually right?
It is a memorable slogan with no strong evidence behind it. The figure appears to trace back to a 1945 US recommendation of about 2.5 litres a day — which also said most of that comes from food. Needs vary enormously with body size, activity, climate and diet, which is why this calculator asks about all four.
Does tea, coffee or juice count?
Yes. The mild diuretic effect of caffeine does not offset the fluid in the drink, so a cup of coffee still leaves you net hydrated. Food contributes too — fruit, vegetables, soup and yoghurt typically supply 20–30% of daily fluid intake. Alcohol is the exception; it is genuinely dehydrating.
How do I tell if I am drinking enough?
Urine colour is the practical everyday test: pale straw is well hydrated, dark amber means drink more. Thirst is a reasonable but lagging signal — by the time you notice it you are already mildly down. Note that B vitamins turn urine bright yellow regardless of hydration.
Can you drink too much water?
Yes, though it is rare. Drinking several litres in a short period can dilute blood sodium — hyponatraemia — which is dangerous and has killed endurance athletes and participants in drinking contests. Spread intake through the day rather than forcing large volumes at once.
Do I need electrolytes or sports drinks?
Only for prolonged heavy sweating, roughly beyond 60–90 minutes of hard exercise or in extreme heat. For ordinary daily hydration and shorter workouts, water and a normal diet supply everything you need, without the sugar.