Best Thermos Flask

best thermos flask

Nothing beats a hot brew when you’re tramping around the hills on a winter hike. It’s like we’re hard-wired to appreciate that soothing warmth spreading around our bodies like a “Ready Brek” glow.

You might think that the easiest way to get your hot drink fix is to lug a gas stove and camping kettle up to the summit in your rucksack.

Well help is at hand because there’s a rather nifty alternative. If you pack a thermos flask in your backpack then your hiking pit-stops are gonna immediately get a whole lot better!

A thermos flask can be a better alternative to the stove and kettle because not only do they keep hot drinks hot in winter, but they can also be used in summer to keep your cold drinks cold.

A Flask For All Seasons! Someone should make a film of that….

Thermos flasks use an ingenious mechanism which consists of dual layers separated by a vacuum cavity between them. The vacuum is a region of space with no air or anything at all in it – this creates a natural barrier to heat leaving or entering the innermost chamber holding your drink. That’s why liquids in the flask remain hot or cold for longer than they would in a standard plastic bottle.

You can see that a thermos flask will be a great companion for any outdoor activity!

Boost Your Green Credentials

Using a flask is actually also a very responsible thing to do in terms of recycling – apparently 99.75% of disposable coffee cups are not recycled!

Instead, if you take your thermos flask to the local coffee shop and ask them to fill it up instead of them using yet another disposable coffee cup and plastic lid then you will be reducing the amount of landfill and waste material which gets produced from these establishments. Win win in our books! 

Why ‘Thermos’?

Just a quick note about the word ‘Thermos’ . This is one of those brand names that have become descriptive of the product itself – a bit like Hoover being synonymous with ‘vacuum cleaner’. Thermos the brand certainly make some great flasks (see below), but we’re simply using the term as a general descriptive term for the whole category of flasks and insulated bottles!

Choosing a Coffee Flask

You may think choosing a vacuum flask is a simple matter of grabbing the shiniest example in the camping shop (if it’s open!) and hoping for the best. Well, unfortunately we’ve tried that approach several times and have frequently been disappointed.

There are literally hundreds of products available, and a high proportion of these seem to be pretty poor at accomplishing even the basics! From leaking lids to cold tea, there are some shockers out there which we’d like to help you avoid.

In this article we have reviewed a range of flasks that we have been impressed by and are also quite popular in the market (for good reason). We have personally used these items and have tried to provide honest and authentic reviews based on our experience.

The Tech to look out for

  • The best thermos flasks can keep liquids hot or cold for a long time – over 12 hours in some cases. Look for a two-layer construction with an intermediate vacuum layer. This prevents heat loss by convection and conduction.
  • The layers are typically made from stainless steel, plastic or glass. These materials are either poor conductors (plastic/glass) or able to reflect heat back into the hot liquid or away from cold liquid (like stainless steel). Take care with any flask which uses a glass inner chamber though as they can be quite fragile.
  • Some thermos flasks are designed to be handily-sized for solo use on hikes and outdoor adventures. Others are larger for bigger groups or car and campervan travel. Choose wisely, or get a couple for different activities.
  • Most decent thermos flasks employ a cup as a screw-on attachment to the flask. We rate any flask which shows useful design features like this to help save space.
arty thermos flask

Flask Size – How are you going to use it?

You should carefully consider the required capacity of your flask before buying one. Have a think about how many people will require a drink from it, and who will be the person carrying it around. Is it for home use (some people take a flask to their home office in the garden), or carrying on a walk for rest and recuperation?

For a group hike, you need to keep lots of liquid on board for everybody to get a cup. If this is your plan then you should buy a thermos which can hold lots of liquid for the whole squad. It is definitely more space efficient to use one big flask for the group rather than several small, individual flasks. If you think about it, the majority of a flask is wasted space and not actually used for carrying liquid – it is there for the heat insulation. Better to just carry one big flask.

The downside of this approach is that a high capacity flask will be big and bulky. There’s no getting away from it. However, if you’re able to split your gear equally between all members of the team (e.g. one walker carries lunch, another carries the picnic blanket etc) then overall you will use less space than if you each carried smaller flasks.

For solo trips you a smaller flask of a litre or less should suffice. These can be sleek and slimline and will fit in your pack nicely.

Check Your Lid!

Thermos Ultimate expandedHaving an effective lid for your flask is crucial. You should check the lid carefully on any product you’re considering. It should not leak and should be easily removable for use and for cleaning.

Make sure the lid screws in securely and tight enough not to come loose on a hike, but equally not be difficult to open when you need it – a tricky set of requirements!

Some thermos flasks use a cork or plug to seal the liquid in, but we really don’t feel this is an effective design. Our suggestion is to go with a screw cap and a clip-on outer cap which will double as a cup.

Warranty: We always suggest buying a vacuum flask from a manufacturer who will offer a warranty of at least one year, and preferably more. A warranty acts as a safeguard for your flask against unexpected defects, and ensures that you won’t have to buy again if something goes wrong. If you’re planning on spending a wodge of cash on a flask, then go for the one with the warranty.

Typical Heat Retention Times

Heat Retention is a major factor determining the usefulness of a thermos flask. It directly affects how long your drinks or food will stay warm or cold.

Normally, we’ve found that the heat retention varies between 4/5 hours to 12/15 hours. The flasks under about £20 are typically in the former category unfortunately and so we recommend paying a little extra to safeguard your precious heat as much as possible!

A word about BPA

You may not have heard of ‘BPA’, but in a similar way to aluminium in camping kettles, it should be avoided when you’re looking for a thermos flask. BPA stands for Bisphenol A, and it’s a chemical which has been used to make certain plastics and resins since the 1960s, and was commonly found in thermos flasks.

A range of scientific research has shown that BPA can seep into food or beverages and potentially cause a range of issues for the brains of fetuses, infants and children. Additional research suggests a possible link between BPA and increased blood pressure.

We recommend avoiding BPA where possible, and we have striven to let you know whether BPA is used in our choices of best thermos flask below.

So let's take a look at the top vacuum flasks on the market right now....

Stanley 18-8 Vacuum Insulation Water Bottle - 1 litre

Stanley 18-8 Flask_mainThe Stanley 18-8 flask is the best option for a solo user in our opinion. It is a great performer which looks superbly rugged for your next arctic exploration and goes about its job efficiently, discreetly and expertly.

The 18-8 flask is from the renowned brand Stanley. Stanley has been serving people since 1913 and this model is truly legendary. No other brand apart from Thermos themselves can boast such heritage.


The Hammertone Green flask is capable of holding 1 litre of any kind of drink. You can transport tea, coffee, hot water, iced water, milk, etc. It even works extremely well for soup we found. Measuring a reasonably discreet 36 x 13 x 10 cm and only 0.8kg we feel this is an excellent backpacking flask with a real sense of history.

Stanley 18-8 Flask unscrewedAll materials on the Stanley are very robust and have a quality utilitarian feel to them. You can be assured that the food-grade stainless steel used on the Stanley won’t give a metallic taste to your drinks at all, but offers excellent heat retention.

You can leave your house with a full bottle of coffee on Friday evening and can have a nice walk with a cup of hot coffee on your weekend destination even on Saturday evening! You will definitely get a duration of 24 hours, and this model performs equally well for hot and cold temperature contents.

In Use

The Stanley 18-8 is one of our favourite thermos flasks as it can be used effectively at your office site, during travel or even in your kitchen overnight so your drink is piping hot for the next day morning.

The flask has a BPA-free cup with enough space for several decent-sized cups of teas. The cup also helps ensure the flask remains leak proof.

Overall, the Stanley flask is a great option to explore – it will give you a cup of tremendous hot coffee without losing flavour, and at the end of the day, that’s all you can ask for. Would we award it the label of ‘best thermos flask’? Yes…. we probably would!

  • Strong stainless steel housing
  • Impressive heat retention
  • Renowned brand
  • Smell-free, BPA-free, non-toxic and leakproof performance
  • We’re struggling!

Hydro Flask Wide Mouth - 1.8 litres

hydro flask main

Hydro Flask are a fashionable US company who have made reusable flasks and water bottles immensely cool over on the West Coast of America. The Hydro Flask Wide Mouth is a quality vacuum flask employing some great tech, a lovely powder-coated exterior surface for great grip as well as excellent heat retention properties. The downside is that they are a touch on the expensive side – you don’t get this sort of style for free girl!

As we said before, vacuum insulation is important for keeping the heat energy inside your flask, and the Hydro Flask does this well. It is made of stainless steel that is extremely good for heat retention.

hydro flask lidThe temperature performance is excellent and compares favourably to the other top performers in the world of elite flasks. Your coffee will remain hot after 12 hours and your ice cubes will remain unmelted after a very decent 24 hours. So, you can still pour a chilled drink even after a long walk on a hot day. Part of this ability is due to the clever ‘honeycomb’ insulation used inside the Hydro Flask. It worked well for us!

Let’s talk about the lid. The Hydro Flask has a sturdy lid that engages well into the bottle neck and is thoroughly leak proof. Your rucksack contents will stay dry whether you put the flask in upside down or not. In addition, the lid comprises a useful carry handle on top.

hydro flask in use 2This particular Hydro Flask has a whopping 1.8 litre capacity although there are significantly smaller ones in the range if that’s what you need. Our test unit was the ‘wide mouth’ variety which enables you to take a decent glug of the contents and offers quick pouring. It’s also much easier to clean inside the flask. We were impressed.

If you are the type of person who likes a bit of style and glamour in your life then you will definitely like the Hydro Flask. Its bright blue colour is very attractive and is definitely a plus point. We recommend it, but the price could be off-putting for some, although there are smaller bottles available for better prices.

  • Looks amazing
  • Lifetime warranty
  • Impressive heat retention
  • Leak proof lid
  • Range of sizes available
  • BPA Free
  • Expensive
  • No cup

GSI Outdoors Microlite 1000 Twist

GSI Outdoors flask brown

The GSI MicroLite 1000 Twist Water Bottle is a vacuum insulated stainless steel flask that has a one litre capacity but impressively small size and density. It is extremely lightweight at 0.37kg which is less than half the weight of the Stanley 18-8 for the same capacity.

The GSI employs a thin but reliable stainless steel wall that reduces the weight and also increases the internal space of the bottle. According to GSI, you can store 25% more liquid than any other flask for the same size.

This thermos flask is made of a combination of Stainless Steel, Polypropylene and Silicone, and the dimensions are an excellent 21.1 x 12 x 9.1 cm. It is sized extraordinarily well for solo hikers who are concerned about size and weight but still wish to carry a hot brew around to warm up with. In our opinion this is one of the best thermos flasks on the market today.

The lid requires just a single twist to unlock (hence the name!) and is connected to the neck of the bottle and so cannot be lost accidentally. A great touch! This loop can also be used as a convenient carry handle.

The main aim of any vacuum flask is to prevent heat loss by either conduction, convection or radiation. If all these routes out are blocked then the heat will stay in – I remember that much from my physics degree! The GSI is successful on all fronts due to its innovative design. It will keep liquid hot for 18 hours and cold for 32 hours and is available with a very attractive and affordable price tag. If you search carefully, you can pick it up for around £35, which is a fantastic price for a piece of equipment which will serve as an investment over a lifetime adventures.

  • Useful flex handle
  • Extremely lightweight
  • Good capacity for 2 people
  • Strong carry loop
  • No attached cup

Be sure to read our in-depth roundup of the best camping kettles for your trips away.

Okadi Large Flask

Okadi Large Flask_main

This flask from OKADI gets full marks for its capacity and versatility. OKADI is a brand that has been serving people with quality products for the last 20 years and this particular model is their flagship model.

It’s a big flask for big groups

The Okadi is a 2.5L flask which offers extremely good capacity for both family and group adventures. It is fabricated from stainless steel which gives excellent core strength and durability. This thermos will last for a long period of time indeed.

Dimensions are reasonable for a flask of this capacity: 39.79 x 18.1 x 12.9 cm and a weight of 1.37 kg. It will certainly slide down the side of a largeish rucksack if required, but is also perfect for taking on car trips or picnics.

The Okadi incorporates two cups as part of its sleek design – these come in very handy when you are sharing drinks with your family. However, you’ll have to timeshare the cups if there’s more than two of you, or bring along a couple of extra lightweight mugs.

The heat retention capacity of the flask is exceptional. You will certainly be impressed with both the hot and cold performance. We were able to keep some tomato soup warm for 24 hours and on another occasion, some chilled orange squash remained cold after 36 hours. This is definitely enough for an overnight wild camping trip if you can handle the weight.

The inner layer is unusually coated with copper (not in contact with your drinks though) and this feature actually reflects the heat energy back into the liquid effectively. Equally impressive is the food-grade odourless silicone seal on the screw top lid which ensures that the cap won’t leak.

With a great carry handle and rugged aesthetic appeal, overall, this green thermos flask is a definite best pick if you’re looking for a larger-sized flask.

  • Stainless steel insulated bottle with inner copper coating
  • BPA-free dual cups
  • Best for travelling by car or campervan
  • Large capacity
  • No warranty
  • Too bulky for a solo user

Milu Thermo Food Flask

Milu Thermo Food Flask main

This time we picked a slightly different kind of product. This is not a standard slimline drinks flask, but we think it’s the best soup flask on the market.

The Milu Thermo Food Flask is a bowl-shape flask (16 x 10 cm) that is not only useful for keeping your drinks hot/cold, but also certain types of liquid-based food. You can keep soup, rice, noodles, fruits and even baby food in this great flask. All parents know that it’s often a problem carrying baby food at the right temperature. The 450ml flask Milu is a great solution to the problem.

The Milu offers decent quality materials and the strong housing mechanism makes it durable and comfortable to use in the outdoors without fear of breakages. The dual-wall of the flask is made of lightweight stainless steel which is heat resistant and won’t allow heat to drain away too quickly. Weight of the Milu is a respectable 0.4kg.

That being said, we found that performance was not quite as good as a dedicated drinks flask – the Milu will keep your hot drinks hot for six hours and the cold ones for 10 hours.

Milu Thermo Food FlaskAs we said before, the Milu is more bowl-shaped and can easily be eaten from directly. The 6.6cm wide opening is extremely convenient for easy filling and eating as well.

Slurping soup is great fun from the Milu – in fact, we’ve started using it at home because it keeps your minestrone piping hot, unlike a cold china bowl from the cupboard.

The Milu comes complete with a carry case and a folding stainless steel spoon to eat your food with which we thought was a useful inclusion. You also get a cleaning brush in the package. We love the Milu!

  • Great design and material quality
  • Convenient for both eating and drinking
  • Easy filling and chomping/slurping
  • Stainless steel spoon included
  • Not dishwasher proof
  • Can be used just for drinks but not optimum

Smardy Vacuum Water Bottle

Smardy flask_main

If you are searching for a great value thermos flask at the lower end of the budget range then we don’t think you can do much better than the SMARDY Vacuum Insulated Water Bottle. This is a compact size flask with a smallish capacity of 430ml – this is only enough for two cups of tea.

This is definitely one of the best thermos flasks for solo users due to the size of 21.8cm x 6.5cm and weight of only 0.33kg. This is light!

You can use it for multiple activities from sunrise to night. Picture this: you can take your morning coffee while going out for a walk. You can take a bottle of chilled drink to your yoga class. You can take it travelling, hiking, cycling, camping, even to your office. And all the while saving waste which you would have generated if you had grabbed a quick Starbucks. What’s not to like for less than £20.

Smardy flask sideThe Smardy flask is manufactured with high-quality stainless steel that makes it extremely durable and we are confident it will perform well over the long-term. It utilizes a silver coat in the vacuum portion of the flask which enhances heat retention.

The temperature performance of the Smardy is quite praiseworthy. It gives around 12 hours for hot drinks and 24 hours for cold drinks. Certainly good enough for day trips in any conditions.

The Smardy is BPA-free which certainly gives peace of mind, and we typically just drank directly out of the flask, although you have to watch out for scalding heat. The lid of the Smardy is 100% leakproof and non-toxic and can act as a small cup, but isn’t the biggest on offer in this roundup. The flask also has a practical lanyard that will allow you to carry it comfortably. For the price, we think this is one of the best coffee flasks you can buy.

  • Practical lanyard for convenient carry
  • Multi-purpose thermos flask
  • BPA-free cup
  • 100% leakproof and non-toxic lid
  • Excellent price
  • It’s not dishwasher proof
  • Limited capacity (430ml)

Oneisall 2L Insulated Thermal Flask

oneisall vacuum flask main

If you’re after a large flask, the Oneisall 2L is a great stainless steel option with a lovely ergonomic design. This is a high-quality two litre flask which is capable of serving drinks for the whole family – great to bring along for day trips in the car or campervan.

The Oneisall measures 34.5 x 11.5 x 11.5 cm and weighs 1.2kg. For a large capacity flask, those are not actually bad specifications. And it really does look the business! Beautiful lines as they say!

In terms of tech, the Oneisall employs a copper-coated layer outside of the vacuum space which ensures that the flask keeps your drinks hot without any heat reaching the outer surface.

oneisall car useAvailable in a range of colours (we like the electric blue), the flask is highly suitable for camping trips and general travelling refreshments. Pouring is very easy due to the superb flexible handle fabricated from silicone.

The flask also has a great additional feature which we haven’t seen before on flasks. You don’t need to open the lid to pour the drink but can instead click down a red button on top which opens up a little aperture. The drink can then be poured without taking the screwcap lid off and letting the heat escape. Brilliant!

In terms of heat retention, it is excellent. Starting with hot water at 95 ℃, you can expect it to drop to around 60 ℃ after 12 hours, 51 ℃ after 24 hours and 40 ℃ after 36 hours. Pretty incredible.

We have seen a few negative reviews of the Oneisall 2L flask, but they are in the minority, and could be due to shipping damage. If you buy from a reputable supplier then you should always be able to get a refund under these circumstances. The hordes of 5-star reviews more than outweigh the small number of unhappy owners – we like the Oneisall and can recommend it based on our experience.

  • Budget-friendly flask
  • Perfect for family trips
  • Great retractable side handle
  • Quite heavy (1.2kg)

Thermos 'Light and Compact' vacuum flask

Thermos flask main

Thermos should just rename their 1 litre ‘Light and Compact’ vacuum flask as “The Daddy” because it effortlessly delivers everything you would expect from the company who developed the vacuum flask as we know it today from Sir James Dewar’s original prototype in 1904

The Thermos has dimensions of 31 x 8 x 8 cm and weighs only 515g – a great size for a day hike. A 1 litre capacity is about the optimum in our view – enough to comfortably serve two people with hot or cold drinks throughout the day without being too much of an impediment to carry.

Thermos flask one clickWe really like the ‘one-click’ pour feature. When your hands are cold and you need a warm brew, you simply press down the blue switch to the ‘open’ position and you can immediately pour. So simple and yet so effective.

Heat and cold retention is exemplary, with the Thermos lasting well over 24 hours for both. We found that it is particularly effective for cold drinks, with ice cubes still being solid after a 30 hour test we performed. Thermos have obviously squeezed out all routes for heat to get into the inner chamber of the flask.

The Thermos Light and Compact flask (model 185515) is almost perfect and takes joint top honours as ‘best vacuum flask’ with the Stanley. The only negative feature we can think of is that there is no carry handle. However, the 8cm diameter size is easy to pick up in any case. The choice is so close between these two great brands that we can’t decide. Over to you!

  • Great dimensions for a day of hiking
  • Only 500g in weight
  • Excellent one-click pouring mechanism
  • No issues with heat retention – 24 hours+ is no problem
  • Superb Thermos brand and legacy
  • Great value for money – they could charge more!
  • Not alot, but possibly too large for a multi-day expedition
  • No carry handle

That concludes our roundup on the best thermos flasks on the market. Thanks to Matt Hoffman on Unsplash for some of the photographs.

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