How to clean a thermos bottle

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We all love a good insulated flask – right? They’re the perfect way to keep our hot drinks hot and our cold drinks cold – all while avoiding plastic waste to save and look out for the planet.

They’re great for traveling, sports, and even just for sitting at your desk.

And yes, these are just a few of the reasons why we recommend that everyone tries our Kool8 water bottles.

Yet there’s one benefit that one-use plastics have that thermos flasks don’t… you don’t have to clean them!

We all know the struggle every now and again when your hot water and soap method isn’t quite doing the trick after having one too many black coffees.

Tea and coffee do stain over time, which can be irritating when you’ve bought a beautiful new bottle that you’d pretty much like to stay perfect forever.

Luckily, we’ve picked up some great tips and tricks over time on how to clean a thermos bottle, and we’re going to share them with you in this article.

You’re bound to find at least one method here that works for you and removes even the toughest of stains.

Let’s get started…

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Baking soda and vinegar

clean a thermos bottle

This classic combo works out to be a pretty easy way to clean a thermos bottle as you probably have the ingredients waiting in your home already.

Make sure to pop your bottle into the sink before trying this one, as it could get a little messy!

Pour around half a cup of distilled white wine vinegar into your bottle and add around a tablespoon of baking soda.

Using distilled white wine vinegar is best as it is much more acidic, and won’t leave your bottle tasting a little strange.

The mixture will froth up as the two ingredients react, which is perfectly normal.

Leave the ingredients to work their magic until the frothing has come to a halt, then fill the bottle up with hot water to free the stains and distribute the cleaning mixture throughout the bottle.

Make sure not to put the top back on the bottle at any point as these ingredients are extremely reactive, and bottling them up (literally) could cause a large build up of pressure.

Leave the flask to soak for around ten minutes, or a little longer if it has particularly deep stains.

Use a cloth, bottle brush or even an old toothbrush to scrub away at the stains, then wash as normal in hot, soapy water.

Denture cleaning tablets

clean a thermos bottle

Yes, you read correctly! In the same way that denture tablets clean our fake teeth, they can clean a thermos bottle, too.

Make sure the tablets you own or buy contain sodium bicarbonate, which most do.

First, you’ll need to fill your bottle with warm to hot water, not boiling. Let the bottle soak for a few minutes to allow the water to loosen up the stains, before adding one or two tablets.

In the same way as the vinegar and baking soda, this will make a mess. It’s probably best with this one to take your bottle outside and once again, don’t put the top on the bottle.

With this method, you should let the bottle sit for around half an hour. Not only will the tablets remove stains, but they will also sterilize the bottle as they clean. In fact, there are many different ways to use denture tablets out there!

Once the reaction has slowed down, you can clean the bottle with a cloth or brush in the same way as the first method.

Rinse the bottle with cool water until all remnants of the tablets have gone and it looks and smells as good as new.

Dishwasher powder

clean a thermos bottle

Now, you claim your dishwasher doesn’t work, but have you ever tried using dishwasher powder directly? We’re now talking about your usual soap or dishwasher liquid here – we’re talking about actual powder.

It’s likely that you don’t have it laying around as it is pricey, but it can clean a thermos bottle really effectively.

You’ll need to use one tablespoon per quart that the thermos holds and add warm water.

Leave the cap off and let the bottle soak overnight.

All you need to do then is follow the same instructions as the previous methods – give the bottle a good scrub and rinse.


clean a thermos bottle

Egg shells are a great, environmentally friendly way of cleaning your bottle. You’ll need to dry them out in the sun before use, so it’s recommended that you save your eggshells every time, then you’ll always have some handy.

Crush the dried shells up, and pop them into the flask with boiling water and shake (with the lid shut this time!) before taking the lid back off, leaving to soak for 15 minutes, and scrubbing and rinsing like before.

Lemon juice

clean a thermos bottle

You’re probably starting to see a trend here. When it comes to how to clean a thermos bottle, acidic products are best.

Squeeze a few tablespoons of lemon juice into the bottom of your flask and shake for around a minute before scrubbing and rinsing. Make sure to wash your bottle out really well after this one- unless you like the taste of lemon tea!

Ice and salt

clean a thermos bottle

Start by filling your bottle about a quarter of the way up with either crushed or smaller, uneven pieces of ice. If you don’t have crushed ice handy, simply take a bag of normal ice cubes outside and crush with a rolling pin, or, if you’re feeling more domestic, use a food processor.

Add two to three heaped spoons of salt. A large grain such as sea or rock salt will work best.

If not, don’t worry. Simply use an extra half-tablespoon of fine salt.

Make sure the lid is securely on your bottle, then shake vigorously for as long as you feel is necessary considering the state of your bottle.

This mixture will exfoliate the bottle to remove stains without causing unwanted scratches or scrapes.

Our Kool8 bottles are made with industrial strength steel, so they can definitely handle a bit of salt and ice.

When finished, empty the ingredients out and rinse the flask. You won’t need to scrub this time as the salt and ice will have done that for you.

How to clean a coffee thermos

clean a thermos bottle

Now that you have a good idea of the range of options available to you when it comes to general thermos cleaning, we’re going to look at how to specifically clean a coffee thermos.

If you’re reading this, you’re probably the type of thermos owner who loves their caffeine hit in the morning, but the bottle is then often left unwashed at your desk until the end of the working day.

Have no fear! We know this can make your other drinks taste a little strange, or could even start to grow a dangerous mould around the top of your flask.

That’s why we’ve sourced a few methods especially for you.

Hydrogen peroxide and baking soda

clean a thermos bottle

This sounds a little daunting, doesn’t it? However, these two ingredients are the perfect pair when it comes to cleaning a coffee thermos,

Mix half a cup of hydrogen peroxide in a bowl with two tablespoons of baking soda.

Then, simply stir and pour into your flask. You don’t need anything within the likes of goggles or safety equipment to do this, but we do recommend stepping outside just in case of any reactions that may cause a little mess to your kitchen.

Shake the thermos gently without putting the lid on, and leave it to soak for thirty minutes, and rinse thoroughly.

That’s all it should take to remove any harsh coffee stains and smells from your thermos!

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clean a thermos bottle

Effervescent antacids such as Alka-Zelter are great for cleaning coffee thermoses. Drop three to four tablets into the bottom of your flask, and add hot water. The hotter the water is, the more effectively this will work.

The great thing about using an antacid is they’re non-toxic, so you don’t have to worry about rinsing your flask over and over again before use!

Same as with the denture tablets listed about, leave the thermos overnight with the lid off and rinse in the morning with soapy water.

It seems these tablets don’t only have a range of health benefits but can clean a thermos bottle well, too!

How to get tea stains out of stainless steel thermos

stainless steel thermos

Yes, tea lovers, we hear you. Coffee drinkers aren’t the only ones who come across problems when it comes to thermos bottles! We know you struggle just as much with tea stains, which is why we’ve also created a little guide for you guys, too.

Tea contains tannins that naturally stain the surfaces of a thermos. However, as the stains don’t go any deeper, they’re pretty easy to clean off by using a mild acid or slightly abrasive cleaning tool.


stainless steel thermos

Fill a spray bottle with distilled, white wine vinegar and cover the stained area (in some people’s cases – the entire bottle) with the spray.

Wait ten minutes, then fill the bottle with boiling water. Set aside, and once completely cool, pour the mixture out.

Then, simply buff away the stain with a cloth and rinse with warm, soapy water.

Exfoliating paste

stainless steel thermos

When it comes to surface stains, often the best way to go is to use an exfoliator, similar to the ice cube and salt method. Once again, if you’re working with stainless steel, then you have absolutely nothing to worry about when it comes to scrapes and scratches.

Create a paste using corn starch and vinegar to add acidity.

Apply the paste onto the surface of the stains, and let it sit for two to three minutes.

Then, get scrubbing! Using a bottle brush or scourer will leave you with the best results as these are much more erosive options.

Simply rinse off with a cloth and warm, soapy water.


If you’re looking for a cleaning method that is a little more heavy duty, or something that applies quite a few of the ideas listed above into one time-saving method, then this soak is for you.

For this, we’ll use the juice from a quarter-wedge of lemon, a tablespoon of bicarbonate of soda, a quarter cup of Realemon juice, two denture tablets, and two tablespoons of dishwasher detergent. Pop all of these ingredients into a bowl (once again, it’s best to work outside) and stir.

Fill your thermos halfway with the soak, top up with boiling water, and leave overnight.

Rinse, scrub and wipe away the stains the following morning effortlessly.

Take care

All of these methods are fantastic for cleaning stains from your thermos, but remember that – they’re just stains! There’s no need to be using these methods daily or you could end up ruining your lovely thermos bottle.

The best way to normally clean a thermos bottle is to simply use soap and hot water as you would with your other dishes, and wipe gently with a cotton cloth.

Please remember that as convenient as it may be, the dishwasher is not the place for your thermos as it is way too abrasive for it to handle on a daily basis.

Avoid using bleach as this is not only hazardous for the thermos lining but your health, too.

And while stainless steel is extra tough, it’s best not to test the waters! Avoid using extra harsh abrasives as they won’t cause harm the first three or four times, but regular use could cause the lining to crack.

Also, as great as thermos bottles are for storing water, tea, coffee, juices, and soups, they should not be used for milk as this could cause a harmful bacterial build up – warm or cold.

You’re good to go

You’re now all set to live your best life with your thermos bottle! Which is your favorite method for keeping your bottle clean, safe and stain-free? There’s plenty of options to experiment with here!

Our Choice
Kool8 water bottle
Stainless steel water bottle
Your modern, drop-dead gorgeous, fully insulated water bottle. Kool8 moves with you.
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