We’ve heard the advice before. Feeling tired? Drink more water. Bad skin? Drink more water. Fighting off a cold? Drink more water. But to be the best, most hydrated version of yourself you could possibly be, how much water does science say you should actually drink per day?

According to current Australian dietary guidelines, you should drink “plenty” of water. But that’s not exactly scientific.

While there are varying guidelines and opinions on the topic, the most common advice dished out is what’s known as the 8×8 rule. This advice goes back to 1974 when nutritionist Dr Fredrick J Stare suggested drinking between six and eight 240ml glasses of water per day, on top of any other drinks, for optimal hydration. This guideline has evolved over the years to more specific amounts: most commonly 2.1 litres for women (more if pregnant or breastfeeding) and 2.6 litres for men.

This advice doesn’t, however, take into consideration individual circumstances such as how active you are, your age, weight, diet and general health, not to mention the climate you live in. Someone who exercises often or who lives in a very humid location is likely to lose more water to sweating, so should replace their fluids more regularly to stay hydrated.

According to a Harvard Medical School Special Health Report, “water keeps every system in the body functioning properly” and plays a huge role in most chemical reactions happening inside us. It helps to carry nutrients and oxygen to your cells, aids digestion, cushions joints, regulates body temperature and flushes bacteria from your bladder, for starters. Another study by Barry M. Popkin at the University of North Carolina, details water’s influence on cognition. “Mild levels of dehydration can produce disruptions in mood and cognitive functioning,” he says.

But that doesn’t mean Australians should be reaching for bottled water. We have world-standard drinking water, and organisations including Clean Up Australia actively encourage people to invest in reusable bottles. “Although plastic bottles are recyclable, many end up in landfill and take up to 1000 years to break down,” says a recent Clean Up Australia report.

The bottom line? You lose water every day through your sweat, breath, urine and bowel movements. And while tea, coffee, and even alcoholic drinks all contribute to hydration, tap water is by far the healthiest, cheapest and most sustainable option for quenching thirst. With no calories and no sugar, it’s the drink we should be reaching for most often. Two litres a day is a good start.

For more non-alcoholic drinks news, head here.