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Best vacuum flasks and Thermos flasks
The best vacuum flasks keep drinks hot from morning through to night.
We've reviewed the best Thermos flasks, along with vacuum flasks from Chilly's, Stanley and Hydro Flask, and cheaper own-brand products from Wilko, Asda and Argos
We tested for heat retention, durability when dropped and how easily you can clean them by hand.
Three vacuum flasks get our Which? Best Buy recommendation. We also found one flask, among the cheapest we tested, that did an excellent job for the price, so we've named it our Great Value pick.
The best vacuum flasks
Only logged-in Which? members can view the Thermos flask and vacuum flask test results below.
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| Vacuum flask | Sizes available | Heat retention | Durability | Where to buy |
|---|---|---|---|---|
| Excellent | Excellent | |||
| Good | Excellent | |||
| Excellent | Poor | |||
| Good | Average | |||
| Good | Average | |||
| Good | Average | |||
| Good | Average |
Dates tested: January 2021 and September 2022. Page last checked February 2024.
The vacuum flasks we tested
All the vacuum flasks we tested are listed in alphabetical order below.
Only logged-in Which? members can view the best vacuum flasks from our tests.
Join Which? to get instant access to our test results and Best Buy recommendation below.
Chilly's Stainless Steel Vacuum Insulated Drinks Bottle
Date tested January 2021
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Size tested 750ml
Need to know Not dishwasher safe, twist-off lid, no cup, available in a range of colours and designs, available in 260ml, 500ml, 750ml and 1.8-litre sizes
Eurohike Rubberised Flask
Date tested September 2022
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Size tested One litre
Need to know Not dishwasher safe, button-operated lid, available in a range of colours, available in 500ml, 750ml and one-litre sizes
Flintronic Stainless Steel Vacuum Flask
Date tested September 2022
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Size tested 500ml
Need to know Not dishwasher safe, twist-off lid, LED temperature display on lid
George Home Stainless Steel Flask
Available from Asda (£4)
Date tested January 2021
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Size tested One litre
Need to know Not dishwasher safe, button-operated lid, comes with cup, available in a range of colours, available in 500ml and one-litre sizes
Habitat Stainless Steel Vacuum Flask
Available from Argos (£7)
Date tested September 2022
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Size tested 500ml
Need to know Not dishwasher safe, button-operated lid, comes with cup, one-year guarantee
Hydro Flask Wide Mouth Bottle
Date tested January 2021
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Size tested 1.2 litres
Need to know Dishwasher safe, twist-off lid, available in a range of colours, no cup, also available in 532ml, 596ml and 946ml sizes, lifetime warranty
Jack Wolfskin Kolima 1.0 Vacuum Flask
Date tested January 2021
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Size tested One litre
Need to know Not dishwasher safe, twist-and-pour stopper, rubber 'sure grip' coating, comes with two cups, available in 500ml and one-litre sizes
Lifeventure TiV Wide Mouth Flask
Date tested January 2021
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Size tested One litre
Need to know Not dishwasher safe, twist-and-pour stopper, comes with two cups, also available in 800ml size
Pioneer Stainless Steel Vacuum Flask
Date tested: January 2021
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Size tested: One litre
Need to know: Not dishwasher safe, button-operated lid, comes with one cup, also available in 350ml and 500ml sizes
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Stanley Classic Vacuum Flask
Date tested January 2021
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Size tested One litre
Need to know Only cup and stopper are dishwasher safe, twist-and-pour stopper, comes with one cup, available in one-litre and 1.9-litre sizes, lifetime warranty
Thermos King Flask
Date tested September 2022
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Size tested 1.2 litres
Need to know Not dishwasher safe, twist-and-pour stopper, comes with one cup, extendable handle, also available in 470ml size, five-year guarantee
Thermos Light and Compact Flask
Date tested September 2022
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Size tested One litre
Need to know Not dishwasher safe, button-operated lid, comes with one cup, range of colours, available in 350ml, 500ml and one-litre sizes, five-year guarantee
Thermos Mondial Flask
Available from Argos (1.8L) (£18)
Date tested January 2021
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Size tested One litre
Need to know Not dishwasher safe, twist-and-pour stopper, glass and plastic materials, comes with two cups, available in a range of colours, also available in 500ml and 1.8-litre sizes
Thermos The Revival Flask
Available from Flasks Online (£29.95) and Thermos (£30)
Date tested September 2022
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Size tested 500ml
Need to know Not dishwasher safe, button-operated lid, comes with cup, available in a range of colours and designs, five-year guarantee
Thermos Ultimate Range Flask
Date tested September 2022
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Size tested 900ml
Need to know Not dishwasher safe, twist-and-pour lid, comes with cup, available in a range of colours, available in 500ml and 900ml sizes, five-year guarantee
Thermocafe by Thermos Stainless Steel Flask
Date tested January 2021
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Size tested One litre
Need to know Not dishwasher safe, twist-and-pour stopper, comes with one cup, one-year guarantee, also available in 500ml and 350ml sizes
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How we test vacuum flasks
We select the UK's most searched vacuum flasks and popular Thermos flasks to test.
We buy everything we test – no freebies – so you know our reviews are unbiased.
Heat retention
- We pre-warm each flask before adding coffee to each flask.
- We take temperature readings at regular intervals over a 24 hour period to measure the temperature drop.
- The best vacuum flasks we've tested only dropped 30°C or less, leaving us with a nice warm coffee.
- The worst dropped over 50°C, giving us with room temperature coffee.
Leakage
- We open and close the lids and stoppers of each flask hundreds of times and check if water leaks from the flask.
- No Thermos flask or vacuum flask we've tested so far has leaked.
Durability
- We drop each flask multiple times, and check if flasks have large dents that inhibit function.
- The best vacuum flasks didn't even have a single dent on them, the worst were battered and bruised.
Cleaning
- We hand-wash each flask and let them air dry overnight before rating them on smell.
- The best flasks smell fresh, the others have the lingering smell of yesterday's contents.
Build quality
- We examine each flask and its pouring mechanism for faults. We penalise flasks that had faults that affect their usability.
Find out more about how Which? is not influenced by product manufacturers or retailers and how your support helps us to stay editorially independent.
Five things we learnt testing flasks
- Button-operated lids are more efficient to use than twist-and-pour stoppers We found the button-operated lids to be more secure than the twist-and-pour stoppers.
- Flasks are more 'smell prone' than 'stain prone' None of the flasks we tested had any staining after cleaning, but some retained a faint coffee smell.
- Textured coatings are useful It’s a nice touch to have a handle on a drinks flask but generally, for a one-litre flask, it’s more useful to have a textured coating that provides grip.
- None of the flasks leaked from the main body However, some flasks can't be filled to their stated capacity without liquid coming out when you screw on the lid.
- Paying more doesn't guarantee quality We found a Great Value flask for under £20 with good heat retention, and several over this price with bad durability and lukewarm coffee after 24 hours.
What's 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.
Although the terms are largely interchangeable, the original phrasing is 'vacuum flask'.
Not all flasks are vacuum insulated, though. Regular flasks will be cheaper and won't claim to keep contents hot or cold for long periods.
How does a vacuum flask work?
A vacuum flask keeps heat from coming in and out using double-walled insulation.
Essentially, there is a flask within a flask, and the two join together at the neck.
The space between the two flasks creates a vacuum preventing heat transfer.
How to clean a flask
- The rubber attachments in the stoppers hold the most smell Let them sit in hot soapy water for five minutes, then air dry them before reattaching.
- 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 It will help you get to hard-to-reach areas inside the flask but could damage 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.
- 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.
Are vacuum flasks safe?
There have been reports of vacuum flasks exploding during use.
Although rare, if you're not following proper guidelines, it is possible.
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.
- Don't leave food and drink in a flask for a long period of time Bacteria can breed and this produces gas. Pressure then builds up which can force the lid off the flask.
- Glass-lined flasks are more dangerous because glass walls can also shatter To be safe, it's best to empty the contents after 24 hours.
- Angle tough-to-open flasks away from your face Also, do it in an open space as this will protect you and others.
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