How this calculator works
It combines three things, and shows each one so you can see where the number comes from:
- A weight-based range. A widely used rule of thumb is 30 to 35 ml of fluid per kilogram of body weight. For 70 kg, that's about 2.1 to 2.5 litres.
- Australia's official guideline. The NHMRC's "adequate intake" for fluids is about 2.6 litres a day for adult men and 2.1 litres for adult women, rising to 2.3 litres in pregnancy and 2.6 litres while breastfeeding (ages 19 to 50). It counts plain water, milk and other drinks; food supplies more on top.
- An extra for activity. If you enter today's steps, active calories or exercise minutes, the calculator adds water using the same rates as Hydration Hero's optional Health Power Up: about 30 ml per 1,000 steps, 1.5 ml per active calorie and 8 ml per minute of exercise.
"Today's suggested total" is the middle of your weight-based range plus the activity extra. It's a sensible target, not a rule.
Why your number isn't eight glasses
"Eight glasses a day" was never based on evidence, and your needs depend on your size, how active you are, the weather, and whether you're pregnant or breastfeeding. Adults living or working in very hot climates, especially if they're active, may need more than the guideline. The full story is in how much water should you drink a day?
Better than any calculator: check your urine
Pale straw-coloured urine usually means you're drinking enough. Dark yellow urine, thirst, headaches and tiredness are early signs of dehydration. On training days, use how much to drink when you exercise or it's hot.
Let your goal update itself
Hydration Hero sets a personal daily goal from your details and adjusts it as your day goes. It weights drinks by their real hydration effect, so coffee and alcohol don't count like water, and an avatar reacts to every sip. With the optional Health Power Up and weather features switched on, your goal rises on active and hot days by itself. On iPhone you can try it instantly as an App Clip, with nothing to install.
Frequently asked questions
How much water should I drink a day based on my weight?
A common rule of thumb is 30 to 35 ml of fluid per kilogram of body weight a day, so about 2.1 to 2.5 litres for someone who weighs 70 kg. Australia's official guideline is about 2.6 litres a day of fluids for men and 2.1 litres for women, from all drinks, not just water.
Do coffee, tea and milk count towards my water intake?
Yes. Australia's guideline counts plain water, milk and other drinks. Caffeine and especially alcohol change how much of a drink your body keeps, which is why Hydration Hero weights drinks by their real hydration effect.
How much more water do I need when I exercise?
It depends on how much you sweat, which varies from about half a litre to two litres an hour. Weighing yourself before and after exercise is the most reliable guide: each kilogram lost is roughly a litre of sweat to replace.
Is this calculator medical advice?
No. It gives general estimates for healthy adults. If you're pregnant, breastfeeding, unwell, or have been told to limit fluids for a heart, kidney or liver condition, follow your doctor's advice.
Sources
- Water: Nutrient Reference Values for Australia and New Zealand, NHMRC / Eat For Health
- Drinking water and your health, healthdirect
- Hot weather risks and staying cool, healthdirect
- Exercise and fluid replacement: position stand (2007), American College of Sports Medicine
- Hydration Hero privacy policy, Digital Sauce
Last updated 7 October 2026. General information for healthy adults, not medical advice. If you have a heart, kidney or liver condition, or have been told to limit fluids, follow your doctor's advice.