Does Water Go Off? Here's How Long a Glass of Water Really Lasts

We're all guilty of filling a glass with water before bed, drinking a few mouthfuls, and then forgetting about it until morning. As you wake up and look at it, one question usually comes to mind, does water go off?

The answer is both yes and no. The good news is that water doesn’t expire like milk or fruit juice will, but that doesn’t mean your glass of water will stay perfectly fresh forever.

In this short guide, we’re going to look at how long water can safely be left out, why water tastes different after leaving it for a while, and what you can do to keep your drinking water fresher for longer.

Does Water Go Off? 

Whilst the water itself won’t expire, everything around it will. The second you pour yourself a drink, that water makes contact with the air. Dust, tiny floating particles and even bacteria from your mouth (if you’ve already taken a sip) can all end up in the glass.

That won’t make your drink unsafe, but it will have an impact on how fresh your water tastes.

How Long Can You Leave a Glass of Water Out?

If you’ve forgotten a glass of tap water on your bedside table overnight, you’re almost certainly not going to have any issues if you drink it. Leaving water indoors for 12-24 hours isn’t usually a problem, however it may taste a little odd, but that’s to be expected.

After that, you’re probably better off pouring yourself a fresh glass instead.

The longer water is left out, the more dust and particles it will gather, and if you’ve already started drinking from it, bacteria from your mouth will become a factor too.

It won’t make you unwell if you drink water that’s been left out for a couple of days, but it certainly won’t taste very nice. 

Why Does Water Taste Different After Sitting Out?

If you’ve drank water that’s been left sitting in a glass overnight, you’ll probably have noticed it doesn’t quite taste the same.

This is down to a few factors.

Your Water Absorbs the Air 

Water naturally absorbs gases from the air around it, and as this process takes place, it can change the flavour of your drink ever so slightly, causing it to taste flatter than when it was freshly poured.

The Chlorine Starts to Fade 

UK tap water contains small amounts of chlorine to keep the water clean and free from germs, but it can also affect the taste when left for a while.

Dust Finds Its Way In 

Even in a clean home or office, tiny particles of dust are always floating around. An uncovered glass will gradually collect some of these, even if you can't see them.

Does It Matter If You’ve Already Drunk From It?

Yes, a little. 

While you’ll be fine drinking water that you’ve taken a sip from overnight, every mouthful you take will introduce new bacteria to the glass. Tiny amounts that won’t harm you, but will multiply over a period of days.

That's why a glass you've been drinking from is best finished within the day rather than left for several days.

Does Temperature Make a Difference? 

Absolutely. 

If you leave a glass of water on your desk in a cool office, it will taste fresher for longer than if you leave it in direct sunlight by a window. Heat speeds up the growth of bacteria, so water should always be stored cool if possible.

Can Bottled Water Go Off? 

Many people seem to think bottled water is immune to going ‘off’, but that isn’t entirely true.

Unopened bottles are fine for long periods when stored properly, though they’ll usually have a best before date on the packaging. But this is due to the bottle degrading, not the water inside.

Once opened, treat bottled water like you would a glass from the tap. If you take sips directly from it, try to finish within a few days and keep it refrigerated where possible.

How Can You Keep Water Fresh? 

The easiest way to keep water tasting its best is to keep it protected from the environment.

Here are a few things you can do:

• Pour fresh water each day, rather than topping up yesterdays glass
• Keep bottles and reusable water flasks clean
• Store water in a cool place, out of direct sunlight
• Cover water if you know it will be left out for long periods
• Keep reusable bottles clean by washing them regularly

None of these take much effort, but they help ensure every glass tastes as fresh as it should.

Fresh Water Is Easier to Drink

One thing many workplaces notice is that people naturally drink more water when it's fresh, chilled and easy to access.

Instead of waiting for bottles to be refilled or filling up kettles and jugs all day long, employees have a constant supply of fresh, chilled water right when they need it. Not only will it improve the taste, but it makes hydrating throughout the day easy for both employees and visitors.

At Thirsty Work, we help businesses across the UK create healthier workplaces with reliable hydration solutions, from bottled water coolers to modern mains fed water dispensers and filtered drinking water systems.

Final Thoughts 

So, does water go off? 

Not in the traditional sense. Water doesn't suddenly expire, but it does become less fresh the longer you leave it exposed to the air.

A glass left overnight is generally nothing to worry about, although it may not taste quite as good as a freshly poured one. If it's been sitting there for a day or two, or it's been left somewhere warm, you're usually better off emptying it and pouring a fresh glass.