Best Thermos Flasks 2024 - Outdoors Magic

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Best Thermos Flasks 2024

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 a 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 2024

  • Thermos Ultimate Series Flask 500ml – Best Buy
  • Klean Kanteen TKPro 1L
  • S’well Roamer 64oz
  • Earthwell 20oz Roaster Loop Bottle
  • Sigg Hot & Cold 1.0L Flask
  • Primus Trailbreak EX 1 Litre
  • Hydro Flask 32oz Wide Mouth
  • Stanley Classic 25oz Bottle
  • Sho Original 2.0 Bottle
  • Outwell Wilbur Vacuum Flask
  • GSI Glacier Stainless Microlight 720
  • Hydro Flask Lightweight Trail Series 24oz Wide Mouth
  • Lifeventure TiV 700 Vacuum Flask
  • MontiiCo Original Bottle
  • Camelbak Hot Cap 20oz Flask
  • S’well Traveler 16oz Bottle

BEST BUY: Thermos Ultimate Series 500ml

Price: £29.99
Weight: 295g
Best for: Hiking, hillwalking and general outdoor use
Key attributes:
Class-leading performance, durable, relatively lightweight, integrated cup

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. It was the best-performing flask we tested overall. It is relatively lightweight too. 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.

Full Specifications

Capacity: 500ml / 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 / Stainless steel interior and exterior / Cool-touch, no-slip finish / Protective silicone rings for ultimate durability / Two part stopper for smooth pouring.



Klean Kanteen TKPro 1L

Price: £44.95
Weight: 723g
Best for: Camping and general outdoor use
Key attributes:
Excellent performance, durable, integrated cup

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. 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 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. Still this is the flask to take for extended trips when you need to keep a brew hot for hours and hours – and hours…

Full Specifications

Capacity: 1 litre (33.8oz) / 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 / Plastic-free stainless steel and silicone construction / 18/8 food-grade stainless steel interior doesn’t retain or impart flavours / Integrated double-wall construction cup / Twist and pour cap with 360° pour-through design / Internal Thread Design for increased thermal performance / Durable Klean Coat finish.




S’well Roamer 64oz Flask

Price: £75
Weight: 816g
Best for: Car camping, picnics and parties
Key attributes:
Huge capacity, stylish finish, wide mouth

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 and weight also means this is a flask best reserved for group camps and family picnics. It is well-made and stylish. We loved the teakwood print finish, which has a cool wilderness vibe. The Roamer would be 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.

Full Specifications

Capacity: 1.8 litres (64oz) / 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 / Grab handle / Made from 18/8, food-grade stainless steel / Condensation-free exterior / BPA and BPS-Free.



Earthwell 20oz Roaster Loop Bottle

Price: £32.95
Weight: 394g
Best for: Hiking, travel and everyday use
Key attributes: 
Leakproof, practical design enables one-handed drinking, wide mouth for easy filling and cleaning

Earthwell is a US brand from Portland, Oregon that designs drinkware for adventurous types. The company is also part of 1% for the Planet, which gets a firm thumbs up from us. Their 20oz Roaster bottle – that’s a capacity of just under 600ml, for us Brits and Europeans – has a standard double-wall vacuum insulated build in 18/8 stainless steel. But unlike most other flasks featured here, the screw-in loop cap incorporates a locking, flip-top lid for one-handed drinking. It’s completely leakproof, and the loop makes it easy to attach it to a pack with a carabiner or similar. Meanwhile, the wide mouth makes for easy filling and cleaning, and the powder-coated finish is easy to grip, while staying perfectly cool and decidedly un-sweaty. Performance-wise, it placed 12th overall in our temperature test, which doesn’t sound too impressive, but it was up against some much larger flasks that you would expect to beat it. Compared to other bottles of similar size, it was a much more respectable third (though it was some way behind our outright champ, the Thermos Ultimate). Still, for daily carry, travelling, or just sipping coffee on the trail, this is a solid and attractive flask.

Full Specifications

Capacity: 592ml/20oz / Temperature test: 0 hrs 90°c, 90 mins 81.7°c, 4 hrs 72.4°c, 6 hrs 65.1°c, 8 hrs 59.9°c, 24 hrs 36.2°c / Double wall vacuum stainless steel construction / Screw in stopper with leakproof flip top lid / Powder coat finish / BPA-free and phthalate-free / Dishwasher safe (hand wash recommended).



Sigg Hot & Cold 1.0L Flask

Price: £39.95
Weight: 728g
Best for: General outdoor use
Key attributes:
Large capacity, durable, large integrated cup

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. There’s a grab handle on the lid, so it’s easy to pull out of a rucksack. It offers very effective heat retention too – this was a top performer in our temperature test. The cup also has a generous capacity, although it gets very hot to hold. Other minor negatives are that it is tall and fairly heavy. The turn and pour spout works well enough, but it’s not as easy to use as some other designs.

Full Specifications

Capacity: 1 litre (33oz) / 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 / Grab handle / Large capacity cup / Made from 18/8, food-grade stainless steel / BPA and BPS-Free.



Primus Trailbreak EX 1 Litre

Price: £39.99
Weight: 740g
Best for: Expedition use, winter adventures
Key attributes:
Large capacity, very durable, comes with two stopper designs

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. In addition to its 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. But it is one of the heaviest flasks 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.

Full Specifications

Capacity: 1 litre (33oz) / 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 / Made of food grade 18/8 stainless steel that doesn’t impart or retain flavour / Lid doubles as a mug ensuring you always have a cup handy / Silicon sleeve / Drinking stopper opens and closes with a slight turn / Additional standard stopper for optimum heat retention included / Tapered design for easier handling / Large mouth opening is easy to fill and clean.



Hydro Flask 32oz Wide Mouth

Price: £37.95
Weight: 444g
Best for: Everyday use
Key attributes:
Large capacity, stylish finish, robust

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. It is robust and durable yet lightweight for its size, making it a versatile option for hot and cold use. Having said that, our temperature test showed that long-term heat retention performance doesn’t match up to other flasks of similar capacity. It’s also relatively expensive for what you get, while the chunky dimensions take up a fair bit of space in your pack.

Full Specifications

Capacity: 946ml (32oz) / 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 / Durable 18/8 Pro-Grade Stainless Steel construction / Wide Mouth for easy drinking, filling and cleaning / BPA-Free and Phthalate-Free / Dishwasher safe / Lifetime warranty.


Stanley Classic 25oz Bottle

Price: £36.99
Weight: 595g
Best for: General outdoor use
Key attributes:
Excellent performance, robust and durable, integrated cup

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. It’s a well-made and durable flask with very good heat retention. In fact, this was one of the best performers in our temperature test. The only possible drawback is that the chunky design adds weight and bulk to your pack.

Full Specifications

Capacity: 750ml (25oz) / 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 / Made from 18/8 stainless steel / Leak-proof / Insulated lid doubles as a cup / Dishwasher safe / Lifetime warranty.

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Sho Original 2.0 Bottle

Price: £24.99
Weight: 483g
Best for: General outdoor and everyday use
Key attributes: Excellent performance, good value, versatile

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. It 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. All in all, it’s a versatile option for a multitude of everyday and outdoor uses. The only design drawback is the flask’s narrow neck and mouth, which minimises heat loss but makes cleaning a little tricky.

Full Specifications

Capacity: 750ml (25.4oz) / 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 / Made from 18/8 Stainless Steel / Lifetime guarantee / BPA Free and EU safety tested / Over 10% SHO profits donated to charity.



Outwell Wilbur Vacuum Flask M

Price: £17.99
Weight: 489g
Best for: General outdoor and everyday use
Key attributes:
Great value, integrated cup

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. It’s 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. On the other hand, it is fairly bulky for a 750ml flask, with a small cup compared to some other designs. The external finish is also a bit slippery and prone to scratching.

Full Specifications

Capacity: 750ml (25oz) / 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 / Made from stainless steel / Easy to open and close / Integrated lid functions as a cup / Odour and taste free.



GSI Glacier Stainless Microlight 720

Price: £39.95
Weight: 306g
Best for: Lightweight hiking and backpacking
Key attributes:
Very light, design enables one-handed drinking, wide mouth for easy filling and cleaning.

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. It is an impressively light and versatile design. The wide mouth makes for easy filling and cleaning, while the ergonomic spout is easy to drink from. You do need to treat this 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?

Full Specifications

Capacity: 720ml (24oz) / 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 / 2mm walls result in a compact footprint that fits anywhere / Made from 18/8 stainless steel / Push-button, flip top cap locks closed for transport and locks open for easy drinking.



Hydro Flask Lightweight Trail Series 24oz Wide Mouth

Price: £47.99
Weight: 289g
Best for: Lightweight hiking and backpacking
Key attributes:
Very light, stylish finish, grab handle.

This is another impressively lightweight flask with a neat and compact design. The wide mouth makes for easy filling and cleaning, and the silicon grab handle means it is easy to pull this flask from the side pocket of a rucksack. Like the GSI Microlight, the stainless steel outer wall is necessarily thin and will dent more easily than others. Our temperature test found that heat-holding performance wasn’t quite as good as the GSI rival, and nor does it offer one-handed drinking to make life easier on the trail (though it is compatible with the Hydro Flask straw lid, sold separately). On the other hand, it is a few grams lighter than the GSI Microlight 720.

Full Specifications

Capacity: 720ml (24oz) / Temperature test: 0 hrs 90°c, 90 mins 81.1°c, 4 hrs 71.5°c, 6 hrs 63.7°c, 8 hrs 57.7°c, 24 hrs 31.7°c / 25% lighter than standard Hydro Flask 24oz wide mouth bottle / Fits most backcountry hydration filters / Made from 18/8 pro-grade stainless steel / Insulated wide mouth cap / Flexible perforated strap and aluminum pivots for added weight loss / BPA-Free and Phthalate-Free / Dishwasher safe.



Lifeventure TiV 700 Vacuum Flask

Price: £18.99
Weight: 464g
Best for: Hiking and general outdoor use
Key attributes:
Versatile, good value.

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

Full Specifications

Capacity: 700ml (24oz) / 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 / Stainless steel construction with copper coated inner wall / Screw in stopper / Lid can be used as a cup.



MontiiCo Original Bottle

Price: £19.95
Weight: 296g
Best for: Everyday and active use – great for the gym!
Key attributes:
Versatile, good value.

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. It is 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. Admittedly, our tests showed that heat retention was not as good as other flasks. The stainless steel outer wall of the flask is very thin and thus likely to dent more easily than others.

Full Specifications

Capacity: 600ml (20.3oz) / 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 / Stainless steel construction / Wide mouth opening /
Stylish, eco-friendly bamboo lid / Grab handle / Bottle + packaging 100% recyclable / No plastic comes in contact with your drink (silicone seal only) / BPA-free / Hand wash only.



Camelbak Hot Cap 20oz

Price: £29.99
Weight: 305g
Best for: Your daily coffee hit.
Key attributes:
Unusual cap design, leak-proof, lightweight and portable.

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. It’s 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. In terms of heat-holding power, be aware that it reaches its limits after about 4 hours. The cap design is also a bit awkward to clean, though you can just stick it in the dishwasher.

Full Specifications

Capacity: 591ml (20oz) / 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 / 360° cap allows drinking from any side of the bottle / Leak-proof rotating valve twists open for easy sipping / Sweat-proof insulation ensures a no-slip grip / Dishwasher safe cap.



S’well Traveler 16oz

Price: £35
Weight: 360g
Best for: Everyday use.
Key attributes:
Stylish looks, leak-proof, portable.

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. This flask is 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. It’s just a shame that the capacity is a bit small, and it’s relatively expensive for a little flask.

Full Specifications

Capacity: 470ml (16oz) / 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 / Wide mouth for stirring coffee or adding ice cubes / Made from 18/8, food-grade stainless steel / Condensation-free exterior / BPA- and BPS-free / Hand-wash only.



Why Use a Insulated Flask?

Quite simply, it’s a good idea to use a reusable flask simply for the sake of preserving the world and the natural landscapes us hikers enjoy exploring. Every year, an estimated 8 million tons of plastic end up in our oceans, with a significant portion originating from single-use plastic bottles. Embracing reusable flasks presents a sustainable solution that significantly reduces plastic production and waste, benefiting the environment in several ways.

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