Best Thermos Flasks Reviewed 2020 - Outdoors Magic

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Best Thermos Flasks Reviewed 2020

Top performing insulated thermos flasks rated and reviewed, plus an in-depth buyer's guide

Nothing beats a hot brew on a chilly day in the hills, and the best way to keep your tea, coffee or soup steaming hot is by packing a good thermos flask in your rucksack.

All insulated flasks work on the same basic principle. They use a double-wall construction, separated by an airless cavity, which creates a thermal insulation layer that keeps liquids hot for longer.

The same vacuum principle applies to keeping drinks cold, so double-walled flasks are also a great way to keep drinks ice-cold on a hot hiking day.

We’ve reviewed a selection of the best thermos flasks, ranging in size from 470ml to a whopping 64oz (1.8 litres). Bigger flasks usually retain heat better than smaller flasks, but are typically bulkier and heavier. We’ve compared their performance with our own temperature test.

What Is The Best Thermos Flask?

The ideal flask will have good hot and cold performance, with the ability to keep a brew hot after hours out in the cold, or your drink nicely chilled even when the mercury rises. The other critical feature is a secure cap, so that even if your flask gets knocked around or accidentally up-ended in a rucksack it won’t leak.

Other useful design features to look for include a wide mouth, which makes for easier cleaning and filling (especially if you want to add ice cubes to your flask). In addition, many larger flasks incorporate ‘twist and pour’ spouts, so you can pour from the flask without having to completely remove the stopper. An integrated cup is also useful. Smaller flasks often feature lid designs that allow on-the-go swigging or one-handed operation.

Like almost any decent bit of outdoor kit, a good vacuum flask should also be robust and durable. All the flasks we tested are made of high-quality 18/8 food-grade stainless steel that resists dents, dings and scratches. The trade-off with steel flasks is that they can be heavy, although modern designs are shaving weight all the time, and in some cases, they now weigh little more than a standard plastic drinks bottle.

Another advantage of stainless steel it that does not leave a metallic taste in the mouth or residual odours. However, it is always worth cleaning your flask thoroughly with hot soapy water between uses.

To get maximum performance from your flask, you should always pre-warm it before use. Fill it with boiling water and leave to stand for five minutes or so before emptying and refilling with your chosen brew – such as tea, coffee, soup or hot chocolate. Filling your flask right to the top will also help minimise heat loss.

Temperature Test: Method

To test the heat-holding power of the flasks, we firstly primed each one for five minutes by filling them with boiling water. We then refilled them with water at 90°c to simulate the temperature of hot coffee and fitted the screw tops and cups. We placed the flasks outside on a level surface and reopened them after 90 minutes, 4 hours, 6 hours and 8 hours, taking the water temperature at each interval. The ambient air temperature was 13°c. Wind speed was zero.


The Best Vacuum Flasks Of 2020

  • Thermos Ultimate Series Flask 500ml – Best Insulated Flask
  • Klean Kanteen TKPro 1L – Best Large Insulated Flask
  • S’well Roamer 64oz – Best Beer Cooler
  • Sigg Hot & Cold 1.0L Flask
  • Primus Trailbreak EX 1 Litre
  • Hydro Flask 32oz Wide Mouth Flask
  • Stanley Classic 25oz Bottle
  • Sho Original 2.0 Bottle – Most Versatile Insulated Flask
  • Outwell Wilbur Vacuum Flask
  • GSI Glacier Stainless Microlight 720 – Best Lightweight Insulated Flask
  • Lifeventure TiV 700 Vacuum Flask – Best Value Insulated Flask
  • MontiiCo Original Bottle – Best Bottle For The Gym
  • Camelbak Hot Cap 20oz Flask – Best Insulated Coffee Mug
  • S’well Traveler 16oz Bottle – Best Bottle For The Office

Thermos Ultimate Series 500ml (Best In Test)

Price: £31.95
Weight: 295g
Capacity: 500ml

The original and, judging from the results of our temperature test, still the best. This stainless steel flask is very durable, with a silicone base to protect against dents. The two-part stopper closes securely yet pours easily and has an integrated cup. In terms of outright heat-holding performance for up to 8 hours, nothing can touch the Ultimate Series.

Pros: Durable, portable and relatively lightweight too. Retains heat superbly for 8 hours and beyond, making it the best-performing flask we tested overall.

Cons: Nothing to fault. And if you’re concerned that the 500ml capacity will be too small, it’s worth noting that Thermos also makes a larger 900ml version.

Temperature test: 0 hrs 90°c, 90 mins 89.3°c, 4 hrs 82.8°c, 6 hrs 79.2°c, 8 hrs 75.3°c, 24 hrs 50.9°c

Klean Kanteen TKPro 1L

Price: £44.95
Weight: 723g
Capacity: 1 litre/33.8oz

An exceptionally durable and robust steel flask with no plastic components. The integrated cup is double-walled and has a generous capacity, while the stopper features a unique thread design that aims to minimise heat loss while also facilitating pouring from any angle.

Pros: Despite its chunky looks, this was the lightest of the 1-litre flasks we tested. It also retains heat impressively well and was the best performer after 24 hours. The flask to take for extended trips when you need to keep a brew hot for hours and hours – and hours…

Cons: The mouth of the flask is relatively narrow, which probably helps to keep heat in but makes filling and cleaning a little tricky. This was also the priciest flask we tested.

Temperature test: 0 hrs 90°c, 90 mins 86.5°c, 4 hrs 81.6°c, 6 hrs 78.1°c, 8 hrs 74.7°c, 24 hrs 53.7°c

S’well Roamer 64oz Flask

Price: £75
Weight: 816g
Capacity: 1.8 litres/64oz

This is a big flask. In fact, the 64oz version is frickin’ huge, though a smaller 40oz  (1.1 litre) version is also available. Both designs are essentially what the Yanks call a ‘growler’, which is basically a large jug or bottle with a handle designed to carry cold beer. For this purpose, it’s ideal, though it’s not so good for keeping liquids warm. Its sheer size also means this is a flask best reserved for group camps and family picnics.

Pros: Well-made and stylish. We loved the teakwood print finish, which has a cool wilderness vibe. It doesn’t retain heat very well but comes into its own when used to keep chilled drinks cold. This makes the Roamer a great option for camps and picnics, whether you’re pouring ice-cold beer or your favourite cocktail. The mouth of the flask is wide enough for ice cubes and makes cleaning easy too.

Cons: Pretty chunky and heavy. Didn’t hold heat as well as any of the other flasks. It was also the priciest flask we tested.

Temperature test: 0 hrs 90°c, 90 mins 72.9°c, 4 hrs 50.8°c, 6 hrs 38.2°c, 8 hrs 29.5°c, 24 hrs 22.2°c

Sigg Hot & Cold 1.0L Flask

Price: £39.95
Weight: 728g
Capacity: 1 litre/33oz

An extremely robust and durable flask with a large, integrated cup. Both the flask and the cup have rubberised silicone bases to prevent them from sliding around on a flat surface.

Pros: Very effective heat retention in a sturdy and durable flask – this came joint second in our temperature test. The cup has a generous capacity, although it’s very hot to hold.

Cons: Tall and fairly heavy. The turn and pour spout works well enough, but it’s not as easy to use as some other designs.

Temperature test: 0 hrs 90°c, 90 mins 87.2°c, 4 hrs 82.2°c, 6 hrs 78.2°c, 8 hrs 74.7°c, 24 hrs 52.4°c

Primus Trailbreak EX 1 Litre

Price: £39.99
Weight: 740g
Capacity: 1 litre/33oz

This is a robust and durable bit of kit for even the most demanding conditions. It’s supplied with a non-slip silicone sleeve that improves insulating performance and makes the flask very resistant to scratches and dents. You also get two stoppers as standard – one regular stopper for maximum temperature retention and one ClickClose-stopper that enables you to drink directly from the flask.

Pros: Extremely rugged construction. The wide mouth makes filling and cleaning the flask easy, while the two different stoppers included with the flask make it very versatile too. We also liked the cup design.

Cons: The heaviest flask here. We also expected it to perform better in our temperature test – it only placed 7th overall. We found it a bit difficult to screw the regular stopper in properly.

Temperature test: 0 hrs 90°c, 90 mins 85.5°c, 4 hrs 79.1°c, 6 hrs 74.3°c, 8 hrs 69.9°c, 24 hrs 44.5°c

Hydro Flask 32oz Wide Mouth

Price: £37.95
Weight: 444g
Capacity: 946ml/32oz

This attractive flask comes in a kaleidoscope of colours, with a powder-coated finish that is easy to grip. It has an extremely wide mouth that makes it very easy to fill and clean. The supplied cap is simple in design, though it is interchangeable with other Hydro Flask lids and stoppers (sold separately). The company also offers a lifetime warranty against manufacturing defects.

Pros: Robust and durable yet lightweight for its size, making it a versatile option for hot and cold use.

Cons: Long-term heat retention performance doesn’t match up to other flasks of similar capacity. Relatively expensive for what you get, while the chunky dimensions take up a fair bit of space in your pack.

Temperature test: 0 hrs 90°c, 90 mins 83.8°c, 4 hrs 74.1°c, 6 hrs 67.5°c, 8 hrs 61.8°c, 24 hrs 33.2°c

Stanley Classic 25oz Bottle

Price: £36.99
Weight: 595g
Capacity: 750ml/25oz

This heavy-duty flask has an effective easy-pour stopper that reduces the chance of spillages. The integrated cup is a good size and has an insulated rim to prevent burnt lips. You can actually get more in the flask than the manufacturer’s stated capacity too – we filled it up to 800ml.

Pros: A well-made and durable flask with very good heat retention. This was one of the best performers in our temperature test. It features a simple but effective twist and pour stopper, and has a useful integrated cup.

Cons: The chunky design adds weight and bulk to your pack.

Temperature test: 0 hrs 90°c, 90 mins 85.2°c, 4 hrs 79.5°c, 6 hrs 75.3°c, 8 hrs 71.4°c, 24 hrs 48.4°c

Sho Original 2.0 Bottle

Price: £24.99
Weight: 483g
Capacity: 750ml/25.4oz

Another good-looking flask with a hard-wearing, textured, non-slip finish. The design prevents condensation from forming on the outside of the flask and ensures it stays cool to the touch. We particularly liked the useful carabiner accessory (sold separately, £3.99), for attaching the flask to a pack.

Pros: Punched above its weight in our temperature test, being beaten only by the Thermos Ultimate Series 500ml and the two larger capacity flasks from Sigg and Klean Kanteen. A versatile option for a multitude of everyday and outdoor uses.

Cons: The only design drawback is the flask’s narrow neck and mouth, which minimises heat loss but makes cleaning a little tricky.

Temperature test: 0 hrs 90°c, 90 mins 87.2°c, 4 hrs 81.8°c, 6 hrs 78°c, 8 hrs 74.1°c, 24 hrs 50.6°c

Outwell Wilbur Vacuum Flask M

Price: £17.99
Weight: 489g
Capacity: 750ml/25oz

This flask has a familiar design with a classic ‘pop’ stopper that makes for easy pouring. With cheap, inferior flasks these are often prone to leaking but with this Outwell version the component seems well-made and reliable. It certainly didn’t leak on test.

Pros: A simple, inexpensive flask that makes use of a classic tried and tested design. The wide mouth makes the flask easy to fill and clean, while the cup’s plastic liner ensures you won’t burn your mouth.

Cons: Fairly bulky for a 750ml flask, with a small cup compared to some other designs. The external finish is a bit slippery and prone to scratching.

Temperature test: 0 hrs 90°c, 90 mins 83.6°c, 4 hrs 74.4°c, 6 hrs 68.2°c, 8 hrs 62.5°c, 24 hrs 33.6°c

GSI Glacier Stainless Microlight 720

Price: £39.95
Weight: 306g
Capacity: 720ml/24oz

Designed for gram-counting backpackers, this is an extremely lightweight flask with double walls that are 60% thinner than standard vacuum flasks. The design is neat and compact, but also incorporates a locking, flip-top cap for one-handed drinking. A non-slip silicone pad helps to prevent accidental damage to the base of the flask.

Pros: An impressively light and versatile design. The wide mouth makes for easy filling and cleaning, while the ergonomic spout is easy to drink from.

Cons: Treat the flask with care, as the stainless steel outer wall is very thin and will dent more easily than others. The smooth finish is also easily marked and scratched – though perhaps that just adds character?

Temperature test: 0 hrs 90°c, 90 mins 83.7°c, 4 hrs 75°c, 6 hrs 69.1°c, 8 hrs 63.4°c, 24 hrs 35.4°c

Lifeventure TiV 700 Vacuum Flask

Price: £18.99
Weight: 464g
Capacity: 700ml/24oz

This vacuum flask incorporates an internal copper coating in its construction for greater heat retention. We liked the solid screw-in stopper and integrated cup, as well as the external powder finish that stays condensation-free and cool to the touch, while being resistant to scratches.

Pros: A good value flask with solid all-round performance, finishing a respectable sixth overall in our temperature test. It’s relatively lightweight and the slim design slips easily into a pack.

Cons: Despite the ‘pouring spout’, pouring can be a bit slow and splashy if you don’t completely remove the stopper. The mouth of the flask is quite narrow, while the integrated cup is quite small.

Temperature test: 0 hrs 90°c, 90 mins 85.7°c, 4 hrs 79.5°c, 6 hrs 74.8°c, 8 hrs 70.5°c, 24 hrs 44.6°c

MontiiCo Original Bottle

Price: £19.95
Weight: 296g
Capacity: 600ml/20.3oz

This attractive flask looks great and incorporates a number of practical design features, including a grippy, scratch-resistant powder-coated finish and a plastic-free stopper with an eco-friendly bamboo insert. We also liked the integrated carry handle.

Pros: A lightweight and versatile flask that makes a good general-purpose drinks bottle, while also keeping a brew warm in a pinch. The slim design slides easily into a pack. You can also buy a sports cap to fit this flask, which makes it a great option for active use or taking to the gym.

Cons: Tests showed that heat retention was not as good as other flasks, and the stainless steel outer wall of the flask is very thin and thus likely to dent more easily than others.

Temperature test: 0 hrs 90°c, 90 mins 79.6°c, 4 hrs 66.5°c, 6 hrs 58.3°c, 8 hrs 51.6°c, 24 hrs 22.2°c

Camelbak Hot Cap 20oz

Price: £29.99
Weight: 305g
Capacity: 591ml/20oz

This innovative flask has a leak-proof cap with an unusual twist stopper that allows 360° drinking from any angle. It’s ideal for swigging coffee on the go. The wide mouth also makes for easy filling.

Pros: A lightweight and portable design with a nice powder-coated finish, which makes the flask easy to hold in one hand. Keep it in your commuting bag or in the car for coffee stops.

Cons: We found the cap design a bit awkward to clean. The flask’s heat-holding power reaches its limits after about 4 hours.

Temperature test: 0 hrs 90°c, 90 mins 79.8°c, 4 hrs 67.2°c, 6 hrs 59.3°c, 8 hrs 53°c, 24 hrs 25.9°c

S’well Traveler 16oz

Price: £35
Weight: 360g
Capacity: 470ml/16oz

When single-use plastics started getting a bad rap, S’well became the drinks bottle of choice for a host of celebrities and ‘influencers’. But they’re not all style over substance – the design consists of a triple-walled construction incorporating a copper layer for improved insulation.

Pros: Lightweight and practical – the ergonomic shape makes it easy to hold, the really wide mouth makes for easy drinking, filling and cleaning, and the grooved lid makes it easy to open, even if screwed on tightly. It’s sure to draw admiring glances if you leave it on your desk at the office.

Cons: Small capacity and relatively expensive for a little flask.

Temperature test: 0 hrs 90°c, 90 mins 83.9°c, 4 hrs 74.1°c, 6 hrs 67.5°c, 8 hrs 61.8°c, 24 hrs 31.6°c



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