How Much Water Should You Really Drink?

"Eight glasses a day" is the most repeated hydration rule in the world. It is also far shakier than it sounds. Here is a more useful way to think about water.

Plantrino Guides · Updated 2026 · About a 9-minute read

Few pieces of health advice are as universal — or as rarely questioned — as "drink eight glasses of water a day." It is simple, memorable, and repeated everywhere. It is also not based on strong evidence. This guide explains where the rule came from and offers a more practical way to gauge your own needs.

Where "8 Glasses" Came From

The famous figure does not trace back to a clinical study. It is often linked to a mid-twentieth-century recommendation that suggested a certain volume of water for adults — while also noting that much of that water is already present in the food we eat. Over the decades, the first half of that advice was remembered and the second half quietly dropped. The result is a tidy rule that overstates how much you need to actively drink.

Food Counts Too

This is the key correction. Your hydration does not come only from a glass. Fruits, vegetables, soups, and many other foods are largely water, and they contribute meaningfully to your daily total. Other drinks — including tea and coffee — also count toward fluid intake. So the amount of plain water you need to drink is less than the amount of total water your body uses.

What Actually Determines Your Needs

Rather than a fixed glass count, your water needs depend on several real factors:

• Body size — a larger body simply needs more.
• Activity level — exercise and sweating raise needs, sometimes substantially.
• Climate — hot or humid conditions, and dry indoor heating, increase fluid loss.
• Diet — a diet rich in fruit and vegetables provides more water from food.
• Health factors — illness, pregnancy, and breastfeeding all change requirements.

Because of this, no single number fits everyone. General guidance from health bodies tends to land around two to three litres of total water a day from all sources, but that is a rough average, not a personal prescription.

A Rough Way to Personalise It

If you like having a ballpark figure, one common rule of thumb scales water with body weight — very roughly in the region of 30 to 35 millilitres per kilogram of body weight per day, counting all sources.

For a 70 kg adult that works out to about 2.1 to 2.5 litres of total water a day. Remember two things before treating that as a drinking target. First, food and other drinks typically supply a meaningful share of it, so the plain water you need to pour is less. Second, it is a starting estimate, not a medical figure — a hot day, a workout, or your own body can move it considerably. Use it to sense-check your habits, then let the everyday signals below do the fine-tuning.

The Two Best Everyday Signals

Instead of counting glasses, your body offers two simple, reliable indicators.

Thirst

For most healthy people, thirst is a perfectly good guide. It is the body's built-in signal, and responding to it generally keeps you well hydrated. The main exceptions are older adults, whose thirst signal can weaken, and intense exercise, where it helps to drink proactively.

Urine colour

A pale straw colour usually indicates good hydration. A darker yellow suggests it would be wise to drink more. It is a quick, honest check that needs no calculation. Note that the target is pale, not completely clear — consistently colourless urine usually just means you are drinking more than you need. Some vitamin supplements can also turn urine bright yellow for a few hours regardless of hydration, so read the colour with that in mind.

When Needs Genuinely Rise

The fixed-number rules fail most visibly on the days that are not average. An hour of hard exercise can cost the better part of a litre in sweat — more in an Australian summer — and that fluid needs replacing over the following hours, not in one uncomfortable gulp. Long stretches in air-conditioned offices or heated rooms dry you out more quietly. Illness matters too: fever, vomiting, and diarrhoea all drain fluids quickly, which is why "keep the fluids up" is standard advice for a sick day — and why persistent vomiting or diarrhoea, especially in children and older adults, deserves a doctor's attention rather than a bigger water bottle.

Pregnancy and breastfeeding raise fluid needs as well, and some medical conditions and medications change how much you should drink in either direction. If any of those apply to you, your GP's guidance beats any general rule, including everything on this page.

Does It Have to Be Plain Water?

No. Plain water is the cheapest and simplest option, and it comes with no sugar or kilojoules — but hydration arithmetic counts nearly everything you drink. Tea, coffee, milk, and juice all contribute. Sparkling and still water hydrate equally well. Even soft drinks hydrate, though their sugar makes them a poor way to do it routinely. The main exception is alcohol, which increases fluid loss and does not count toward your total. If plain water bores you, a slice of lemon, chilled sparkling water, or tea gets you to the same place.

Practical Habits

• Drink to thirst through a normal day, and a little ahead of thirst when exercising or in heat.
• Glance at urine colour as an easy check — aim for pale, not dark.
• Eat water-rich foods. Fruit and vegetables hydrate while doing other good things.
• Adjust for the day. A hot day or a hard workout simply needs more; a mild, quiet day needs less.

Common Mistakes to Avoid

Treating a target as a rule. Whether it is eight glasses or a calculator's estimate, a fixed number is a starting point. Forcing down water you do not want on a quiet day — or stopping at the target on a scorching one — gets the logic backwards.

Counting only plain water. If you drink tea with breakfast, coffee mid-morning, and soup at lunch, you are further along than your water bottle suggests. Ignoring those sources makes hydration feel like a chore it is not.

Aiming for completely clear urine. Clear is not the goal — pale straw is. Chasing clarity usually means drinking well past your needs.

Expecting water to work miracles. Water is regularly credited with transforming skin, melting fat, and "flushing toxins." The evidence for those claims is thin. Staying hydrated genuinely helps energy and concentration, and replacing sugary drinks with water is an easy win — but your liver and kidneys handle the detoxifying, and no quantity of water changes that.

Forgetting the exceptions to thirst. Thirst serves most healthy adults well, but it weakens with age and lags behind during long or intense exercise. In those situations, drinking on a loose schedule rather than waiting for thirst is the safer habit.

Frequently Asked Questions

The honest answer to "how much water?" is: enough that you are rarely thirsty and your urine stays pale. Your body, your diet, and your day all shape the amount — far more reliably than any fixed count of glasses. This guide is general information, not medical advice; if a health condition, medication, pregnancy, or anything else complicates the picture, your GP is the right person to ask.