We tested thermos tasks to discover the best of the best - these are the top performing insulated flasks for 2025, all rated and reviewed by our experienced gear testing team.
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 here, 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 using our own temperature test.
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 back on. 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 results of our temperature tests.
In addition to this initial temperature test, all of these flasks have also been thoroughly tested across UK landscapes at various times of the year. This ensured that the flasks could perform well with both hot and cold liquids, and could hold up to the demands of life on the trail.
With a primary focus on hiking, walking, and backpacking; we also tested these flasks on kayaking trips, on travels abroad, on bikepacking adventures, and on everyday dog walks. These were key tests in determining factors such as durability and ease of use, as well as the almighty test of time.
The Best Vacuum Flasks Of 2025 – Our Results
Including top options from leading brands, as well as lesser-known brands with equally impressive offerings, here’s our favourite thermos flasks for 2025.
Thermos Ultimate Series Flask 500ml – Best Flask Overall in Our Tests
Quechua 0.8L Insulated Steel Flask 900 – Best Value Insulated Flask
Klean Kanteen TKPro 1L – Best Large Insulated Flask
Mizu D7 23oz Bottle – Best Plastic-Free Thermos Flask
Stanley Classic Legendary Bottle 0.75L – Most Durable Thermos Flask
Primus Trailbreak EX 1 Litre
Camelbak MultiBev Bottle 650ml with 480ml Cup
Sho Original 2.0 Bottle
Yeti Rambler 16oz
Outwell Wilbur Vacuum Flask
GSI Glacier Stainless Microlight 720
Hydro Flask Lightweight Trail Series 24oz Wide Mouth
Lifeventure TiV 700 Vacuum Flask
S’well Traveler 16oz Bottle
Editor’s Note: This article was updated in October 2025 to ensure that all of the products are still available for purchase. Further recent updates include the addition of the Yeti Rambler and the Quechua 900 thermos flasks. Both flasks were thoroughly tested in Eryri and the Lake District and impressed editor Will Renwick and the wider test team. Stay tuned for updates as we continue our testing throughout the winter season.
1. Thermos Ultimate Series 500ml – BEST BUY
The best thermo flask overall, according to our tests.
The Thermos Ultimate Series 500ml thermos flask is the best thermos flask overall, according to our tests.
Price: £25 Weight: 295g Best for: Hiking, hillwalking and general outdoor use Pros: Class-leading performance, durable, relatively lightweight, integrated cup
Cons: None so far!
The Thermos Ultimate flask is our original winner and, judging from the results of our most recent temperature tests, it’s 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.
This was our choice as best value insulated flask overall.
Pictured during our tests in the Lake District. Photo: Dave Macfarlane
Price: £15 Weight: 366g Best for: General outdoor and everyday use Pros: Easy to open/close, good temperature retention, very durable, useful handing loop Cons: Narrow opening, a little wide at the base, harder to clean than most
This was a solid performer in the temperature tests and also happens to be one of the lowest cost flasks in our tests. As such we’ve deemed this to be worthy of our pick as best value insulated flask overall.
It’s smart looking, very durable, and the flip top makes it very easy and convenient. We found we were able to drink from it one-handed, and the textured base made it usable while wearing gloves too.
We did find that there were some drawbacks, however. First of all, it’s narrow opening and multi-part lid did make it harder to clean than some of the other flasks in our tests. It’s also quite wide at the base so it’s not cup holder friendly or the easiest to slide into the side of a backpack – though it does have that useful hanging loop.
Full Specifications
Capacity: 800ml (27oz) / Temperature test: 0 hrs 90°c, 90 mins 88.9°c, 4 hrs 78.6°c, 6 hrs 71.8°c, 8 hrs °66.1c, 24 hrs 41.3°c / Stainless teel double wall vacuum with a layer of copper / Dishwasher safe.
Price: £48 Weight: 723g Best for: Camping and general outdoor use
Pros: Excellent performance, durable, lightweight, pours from any angle Cons: Narrow mouth, hard to clean, pricey
This is 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, in our opinion, this is the flask to take on 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.
Price: £47 Weight: 515g Best for: Hiking, hillwalking and general outdoor use
Pros: Durable, stylish, integrated cup, plastic-free construction, leakproof Cons: Didn’t perform as good as other flasks in our temperature tests
This 22oz (680ml) vacuum insulated bottle from Japanese brand Mizu combines classic looks with some more utilitarian, outdoorsy touches – namely that paracord carry handle and stylish powder-coat finish. It features a copper lining and a vacuum sealed cap, and the design is completely plastic-free.
The slimmer size slips easily into a pack and will also fit in most car cup-holders. On test, it proved completely leak-proof, and we particularly liked the screw-on double wall cup, which is a practical size for sipping coffee or soup. It doesn’t get hot enough to burn your fingers or lips either. It was a mid-range performer in our temperature test, placing 7th out of 14 flasks overall. The contents were still hot (well over 60°c) after 8 hours.
This gets our pick as the most durable thermos flask. It’s hard to beat a Stanley.
The Classic Legendary Bottle 0.75L by Stanley.
Price: £49 Weight: 595g Best for: General outdoor use
Pros: Excellent performance, robust and durable, integrated cup Cons: Heavy and bulky
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.
Pictured during our field tests in the Lake District. Photo: Dave MacFarlane.
The only possible drawback is that the chunky design adds weight and bulk to your pack. Still, it’s the kind of product that feels like it’ll last for years of use, and it’s versatile enough for everything from day hikes to winter camping. With its classic design and reliable performance, it’s easy to see why Stanley bottles have built such a strong reputation.
Price: £36.51 Weight: 740g Best for: Expedition use, winter adventures
Pros: Large capacity, very durable, comes with two stopper designs, easy to fill and clean Cons: Heaviest on the list, could have performed better in our tests
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.
Will using the Primus Trailbreak on a hike in Pembrokeshire.
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.
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.
7. Camelbak MultiBev Vacuum Insulated Stainless Steel Bottle 650ml with 480ml Cup
The Camelbak MultiBev thermos flask.
Price: £22 Weight: 645g Best for: Hiking, hillwalking and everyday use
Pros: Durable, BPA-free, easy to clean Cons: Limited insulating abilities
Though perhaps best known for its hydration systems and water filters, Camelbak also makes a range of double-wall vacuum flasks. This includes the versatile MultiBev, which is a 2-in-1 vessel water bottle and tallboy-style coffee mug. This makes it a great daily companion, as you can use it for morning coffee and for water throughout the day. The mug unscrews from the bottom of the flask and there’s a handy roll-and-fold silicon lid hidden in the top cap.
Our tester Ian using the Camelbak Multibev in Eryri. Photo: Chris Johnson.
It also has a non-slip silicone base and a practical carry handle. The flask is also easy to clean, dishwasher safe and BPA-free. It works better as a bottle for chilled water than a coffee flask though, as overall heat-holding performance was disappointing in comparison to rivals. In fairness the contents of the flask was still hot after six hours, measuring over 60°c. But beyond that, the insulation seems to reach its limits.
Price: £15 Weight: 483g Best for: General outdoor and everyday use Pros: Excellent performance, good value, versatile, hard-wearing, useful carabiner accessory Cons: Narrow neck and mouth, a little tricky to clean
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.
Pictured during our field tests. Photo: Dave Macfarlane.
Price: £32 Weight: 460g Best for: General outdoor and everyday use Pros: Cup holder-friendly, easy to open/close and pour, easy to clean, durable Cons: Didn’t perform well in our temperature tests, top of flask expands in heat
We were left a little disappointed by this. Our expectations were high: a well-regarded brand, a smart-looking design and good quality feel.
We had to do two rounds of tests on this. The first time round we weren’t able to take any temperature records as the heat had caused the top to expand, making the lid impossible to unscrew. In the second round of tests, while we were able to open it, the results weren’t too impressive. It finished with the coldest reading after 24 hours.
There are aspects we liked though. The twist-and-pour cap was easy to operate and gave a smooth pour and it was very easy to clean. We also liked that it’s cup holder friendly and that we could slide it easily into the stretchy mesh pocket on the side of a backpack. It’s certainly very durable too.
Price: £20 Weight: 489g Best for: General outdoor and everyday use
Pros: Great value, integrated cup, well-made, reliable Cons: Fairly bulky, slippery finish, prone to scratching
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.
Price: £46.30 Weight: 306g Best for: Lightweight hiking and backpacking
Pros: Very light, design enables one-handed drinking, wide mouth for easy filling and cleaning Cons: Thin outer wall, dents and scratches easily
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.
12. Hydro Flask Lightweight Trail Series 24oz Wide Mouth
The Hydro Flask Lightweight Trail Series 24oz Wide Mouth Thermos.
Price: £40 Weight: 289g Best for: Lightweight hiking and backpacking
Pros: Very light, stylish finish, useful grab handle, easy to fill and clean Cons: Thin outer wall, dents and scratches easily, didn’t perform as well as others
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.
Price: £22.99 Weight: 464g Best for: Hiking and general outdoor use
Pros: Versatile, good value, scratch resistant, condensation-free build, lightweight and slim Cons: Slightly slow and splashy pouring, narrow mouth, small cup
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.
Price: £22.99 Weight: 360g Best for: Everyday use. Pros: Stylish looks, leak-proof, portable, lightweight, wide mouth for easy drinking and cleaning Cons: Small capacity, expensive for size
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.
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.
What Makes a Good 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 doesn’t 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.
The original article was written by Matt Jones with further edits from editor Will Renwick and the wider OM team.
