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10 February 2021

The best vacuum flasks: Thermos flask, Hydro Flask and more tested

Wondering which vacuum flask keeps the contents hottest for longer and is more durable? Read our test results
Someone pouring a hot drink from a vacuum flask outdoors.
Olivia Woodhouse

In January 2021 we put 11 vacuum flasks (including Thermos flasks and the Instagrammers' favourite Hydro Flask) to the test. Cheaper own brand vacuum flasks from Wilko and Asda joined these popular brands in our line-up. We tested models costing from £4 to £43 to see whether paying more for a vacuum flask pays off. 

Each flask was put through a number of tough tests to discover the most durable and which kept our coffee hottest for longest. We also investigated how easy each flask was to clean and which had the best build. 

The best vacuum flasks

Only logged-in Which? members can view the full vacuum flask test results below. If you're not yet a member, you'll see an alphabetically ordered list of the vacuum flasks we tested.  

Join Which? now to get instant access to our test scores and Best Buy recommendation below.

Chilly's Stainless Steel Vacuum Insulated Drinks Bottle 

£25 Available from Amazon, Chilly's, John Lewis, Karavan Eco, Outdoor Gear

Material: Stainless steel Size: 750ml 

Key features: Not dishwasher safe, twist-off lid, wide range of colours and designs, available in 260ml, 500ml, 750ml and 1.8 litres, no cup.

Chilly's bottles' appeal is the masses of colours and designs you can pick from to suit your mood or personality.  But is a Chilly's bottle style over substance? 

Log in now or join Which? to unlock our test results and find out.

George Home Stainless Steel Flask 

£4 Available from Asda

Material: Stainless steel Size: 1 litre

Key features: Not dishwasher safe, button operated lid, comes with one cup.

This stainless steel vacuum flask is the cheapest in our line-up but looks almost identical to some of the other flasks we tested. 

If you want to find out whether it's worth saving the pennies for a hot drinks flask you'll have to log in or join Which? to find out.

Hydro Flask Wide Mouth Bottle 

£42.95 Available from Amazon, Cotswold Outdoor, Hydro Flask, Trekitt

Material: Stainless steel Size: 1.2 litres

Key features: Dishwasher safe, twist-off lid, also available in black, blue and pink, also available in 532ml, 596ml and 946ml, lifetime warranty, no cup.

Hydro Flask gets the thumbs of from social media where you'll see pictures of it popping up in beautiful spots around the world. But how did the most expensive of the bunch we tested cope in the real world?

Log in or join Which? to find out. 

Jack Wolfskin Kolima 1.0 Vacuum Flask 

 £26 Available from Amazon, Jack Wolfskin, Wow Camping

Material: Stainless steel and plastic Size: 1 litre

Key features: Not dishwasher safe, twist and pour stopper, comes with two cups, also available in 500ml, 'sure grip' coating

Jack Wolfskin claims this vacuum flask keep drinks warm for up to 12 hours but we tested all our flasks for 24 hours to see which would keep our coffee hottest for longest. 

Log in now or join Which? to unlock our test results and find out how it fared.

Lifeventure TiV Wide Mouth Flask 

£20.99 Available from Cotswold Outdoor, Go Outdoors, Lifeventure

Material: Stainless steel and plastic Size: 1 litre

Key features: Not dishwasher safe, twist and pour stopper, comes with two cups, available in 800ml.

If you prefer your flask chunky, this portly wide-mouth model from Lifeventure (with a retractable handle) might be right up your street. But does chunky mean more durable? 

Log in now to find out or join Which? to unlock all our test results and find out how this flask fared against its stainless steel competitors. 

Pioneer Stainless Steel Vacuum Flask 

£13.96 Available from Amazon, Hearts of Stur, Pioneer

Material: Stainless steel Size: 1 litre

Key features: Not dishwasher safe, button operated lid, comes with one cup, available in 350ml and 500ml.

Pioneer describes its design as unbreakable both inside and out. Is it? Log in now to find out or join Which? to unlock all our test results. 

Stanley Classic Vacuum Flask

£32.99 Available from Angling Active, Leisure Outlet, TFM

Material: Stainless steel Size: 1 litre

Key features: Only cup and stopper are dishwasher safe, twist and pour stopper, comes with one cup, available in 1.9 litres, lifetime warranty. 

The Stanley Classic has long been Stanley's flagship vacuum flask. It's even got a lifetime warranty, but how long do we think it will last? Our tests told us enough to know whether we'd want to invest in this flask.

If you want to find out what we thought log in or join Which? for all of our results.

Thermos Stainless King Flask

£26.99 Available from Argos, Amazon, John Lewis, Robert Dyas, Sainsbury's, Tesco, Thermos

Material: Stainless steel Size: 1.2 litre

Key features: Not dishwasher safe, twist and pour stopper, comes with one cup, also available in 470ml, five-year guarantee, extendable handle. 

Thermos flasks have been round since 1904. It's the ubiquitous vacuum flask brand and you can pick up a Thermos pretty much everywhere, even if your local supermarket. We tested three of them. 

Log in or join Which? to find out whether the results reflect this brand's heritage.

Thermos Mondial Flask 

£13.22 Available from Amazon, Flasks Online, Ocado, The Range 

Material: Glass and plastic Size: 1 litre

Key features: Not dishwasher safe, twist and pour stopper, comes with two cups, available in black, blue and green, available in 500ml and 1.8 litre.

This thermos flask is glass lined to help keep the heat in. We tested two glass lined vacuum flasks to see how they compared to those without glass. We were particularly interested to see whether they survived our drop test to check durability

Log in now to find out or join Which? to find out how this Thermos flask did. 

Thermocafe by Thermos Stainless Steel Flask 

£12.99 Available from Amazon, Argos, Flasks Online

Material: Stainless steel Size: 1 litre

Key features: Not dishwasher safe, button operated lid, comes with one cup, available in 500ml and 350ml, one-year guarantee.

Many stainless steel vacuum flasks look almost identical and this Thermocafe is almost a twin (on looks) of the George Home flask we also tested.

Log in now or join Which? to see whether they matched up on how well they kept drinks warm and how durable they were in our testing.   

Wilko Thermal Flask With Liner 

£4.50 Available from Wilko

Material: Glass and plastic Size: 1 litre

Key features: Not dishwasher safe, twist-off lid, comes with two cups, available in 1.8 litre.

This is the second glass lined flask in our line-up. It's also one of our cheapest. Surprisingly it outdid one of the big name brands in our durability test. 

Log in now or join Which? to find out whether it keeps drinks warm better than others we tested.

Five things we learned about flasks

  1. Button operated lids are more efficient to use than twist and pour stoppers. 
  2. Flasks are more smell prone than stain prone.
  3. Textured coating can be more useful than a handle for holding your flask.
  4. None of the flasks leaked out of the main body.
  5. Paying more doesn't guarantee quality. Our most expensive flask came last in our hot coffee test.

Hot coffee test results

To see a more detailed run down of our findings log in now or join Which? .

How we tested the vacuum flasks 

We tested 11 popular flasks for the following: 

Build quality

We examined each flask, its pouring mechanism, and attachments for faults. We penalised flasks which had faults affecting their usability.  


We opened and closed the lids and stoppers of each flask 100 times. When we reached 50 times and then again at 100 we filled the flasks with water and held them upside down to see if anything escaped. 

Which flask keeps contents hottest for longest?

To test for heat retention we put coffee granules (measured out to match the cups produced per flask) in each flask and then, using a water boiler to control the water temperature, we filled each flask with hot water. We took temperature readings at regular intervals across 24 hours. 

To ensure each flask was prepared we had pre-warmed them with hot water for 11 minutes prior to carrying out the test. Once we'd filled the flasks we took thermal images (see below) to illustrate where heat was being lost. 


After the hot coffee had been poured out, we hand washed each flask with hot soapy water and let them air dry over night. The next day we ranked them on odour and even made tea in all of them to see if any residual coffee flavour came through in the taste of the tea.


We dropped each flask five times from 90cm to mimic what might happen in a real life scenario - a dropped flask while hiking for instance. 

How we chose the vacuum flasks

We chose from the most popular brands were most popular.

At Which? we order and pay for all of the products we test, just like you, the consumer. This means we can give you an honest, unbiased review.

How to clean a flask

All but one of the flasks we tested included full cleaning instructions. 

It's important to rinse your flask out with hot soapy water before use.

When it came to cleaning after our coffee test we used hot soapy water, as suggested by each flask guide. Certain brands suggest leaving a mixture of hot water and bicarbonate of soda or vinegar inside the flasks overnight for hard to shift stains and while this might help heavy staining, we didn't feel it was necessary for our test as none of the flasks stained.

Below are some tips and tricks were learnt from our testing:

We found that the rubber attachments in the stoppers held the most smell. We recommend letting them sit in hot soapy water for five minutes and then air drying them before reattaching. If a smell still remains it's worth letting the attachment sit in either a bicarbonate of soda or vinegar mix. 

Don't fully submerge the flask in water. Doing this might allow water to get inside the insulation layers affecting the quality of insulation. 

Invest in a sturdy bottle brush for stainless steel flasks. A bottle brush will help you get hard to reach areas inside the flask but could damage the glass lining in glass lined flasks.

Some flasks are dishwasher safe but others should only be hand washed. Every flask is different and it's best to check before you clean. The only flask we tested that is dishwasher safe is the Hydro Flask and the attachments of the Stanley Classic, but not the flask itself. 

Avoid abrasive detergents and materials when cleaning your flask. Bleach will stain and damage stainless steel and scouring pads will scratch the outside of your flask.

What is the difference between a vacuum flask and a thermos flask?

Since the American manufacturer, Thermos, came to fame the term has become slang for vacuum insulated flasks. Though the terms are largely interchangeable the original scientific phrasing is 'vacuum flask'. 

It should be noted that not all flasks are vacuum insulated either. Regular flasks will be cheaper and won't claim to keep contents hot or cold for long periods because they won't have the technology to do so. 

How does a vacuum flask work?

A vacuum flask manages to keep heat from coming in and out using double walled insulation. Essentially, there is a flask within a flask and both join together at the neck. 

The space in-between the two flasks creates a vacuum, as labelled above, preventing heat transfer. The wall of the inner flask is silvered to prevent heat radiation deflecting heat away from the flask. 

Surrounding these two flasks is an outer case which protects the flask and acts as another layer of insulation.   

Are flasks safe?

There have been reports of vacuum flasks exploding during use. Though rare, if you're not following proper guidelines, it is possible. 

You should never leave food and drink in a flask for long periods of time because it allows bacteria to breed and this produces gas. Pressure then builds up which can force the lid off the flask. 

With glass lined flasks the risk becomes more dangerous because the glass walls can also shatter. To be safe it's best to empty contents after 24 hours. 

If the flask feels particularly tough to open, angle it away from yourself and ensure you're in an open space. 

The two glass-lined flasks we tested were the only flasks to fail our drop test: one shattered on the first drop, the other fell apart on the second.