When the temperature drops, a flask or thermal cup is a worthwhile investment, as you can keep a drink or soup hot while out and about.
You’ll also save money, as you’ll avoid the need to buy coffee or soup in shops and reduce your use of disposable cups and single-use plastics. In the summer, flasks can also help keep drinks cold.
We put popular products through their paces to find the ones you should consider. For more, visit our reviews section and find over 400 practical buyer’s guides offering unbiased advice on what equipment is worth investing in, including the best lunchboxes and best reusable coffee cups.
Best flasks and thermal cups to buy
Thermos stainless steel 1.2-litre flask
Best overall flask
- Feels light even at full capacity
- Could do with a second cup
Tea test score: 10/10
This stainless steel flask is large and proudly bears the Thermos name. Most importantly, it works.
Despite plenty of competition, Thermos is still the most recognisable name in the market, offering an extensive range of products to keep food and drink hot or cold.
We tested the 1.2-litre capacity flask in bold red, but there are plenty of colour options available. All aspects of its design have been carefully considered. For instance, the cup is insulated with plastic, so our tea retained its heat without being too hot to hold.
The flask features a simple screw-top cap that can be loosened for pouring, and ensures not too much heat escapes.
Although the flask is on the hefty side, a sturdy, loose-hinged metal handle makes it easy to use and carry. Its handsome metal cup easily screws on over the cap.
The flask is just over 30cm tall (including the cup) and weighs in at 1.8kg when full, despite being quite light when empty. We didn’t find any signs of leaks around the cap, and pouring was easy with no excessive dripping.
Best of all was its performance. Its label promises to keep drinks hot for 24 hours, and when we finally opened it a day after making our tea, it was as hot as when we’d brewed it the morning before.
This flask is reliable and immensely practical, and with a confident 50-year guarantee, it could remain a staple of family days out for years.
Klean Kanteen Insulated TKWide, 473ml
- Available from: Klean Kanteen (£27.95)
Best thermal flask hybrid
- Handy carry loop
- Easy-twist lid catch for sealing on-the-go
- Standard weight when empty
Tea test score: 9/10
This Klean Kanteen flask gets top marks for versatility. It’s a simple re-design of a classic thermal coffee cup; a function it performs with class thanks to climate lock insulation.
The leakproof café cap and the twist valve seal on its lid are so effective that you also have an insulated water bottle, soup carry-cup and a handy little ice caddy for filling with ice cubes, all wrapped up in one package.
The stainless steel loop is a classy design addition for hooking onto your finger or clipping into a bag. There’s a twist valve closure for sealing its mouthpiece and preventing spills when on the go.
The seal itself is tight but the actual valve can be loosened one-handed, making it a great insulated cup for using whilst driving – it fits a standard cup holder.
A little ridge in the twist valve to make room for your nose is just another small design feature that makes a big difference to the comfort of drinking out of this insulated flask. The brand uses a Klean Coat matte-finish for durability which improves its grip.
The insulation itself is ‘climate lock’ and passed our test with flying colours, keeping water close to boiling (too hot to drink!) even after two hours and a cup of tea hot for over 12 alongside the standard test.
Klean Kanteen has good sustainability credentials and if you’re looking to buy a thermal flask for the long haul, this is a great option.
Microlite insulated flask, 1000ml
Best big flask for one
- Large capacity
- Small footprint
- Surprisingly lightweight
- Sturdy carry loop
- Lacks cups
Tea test score: 9/10
The only thing this insulated flask lacks is a set of cups. Its stainless steel outer has a deceptively large capacity and is vacuum insulated with 2mm walls. Efficiency is a major selling point with this bottle.
GSI claims this bottle is a third lighter and holds 25 per cent more liquid than the same size traditional bottles. When empty, it was certainly the lightest of all we tested of similar capacity.
Having warmed it with hot water for the recommended five minutes, we produced a steaming cup of tea after 12 hours, a hot one after 18 and a passable cuppa at the 24 hour mark.
The broad mouth was ideal for slotting in a handful of ice cubes in one go – and chilled down quickly with water from the tap. The insulation claims to keep water cold for 32 hours, which turned out to be underselling its capability.
The lid is attached to the bottle by a durable strap which provides great grip and leverage for easily unscrewing, particularly with slippery or frozen hands. Once on, the bottle is completely leakproof thanks to an inner silicone lip.
The bottle has a wide-base design, so if you’re looking for a slimline flask to fit in a rucksack’s bottle carrier we’d suggest an alternative. However, the efficiency of this insulated flask and its brushed stainless steel styling makes it one of our favourites.
Klean Kanteen Insulated TKPro 25oz flask, 750ml
- Available from: Klean Kanteen (£39.95)
Most stylish flask
- Superlative insulating capabilities
- Good size cup
- BPA free
- Only one cup
- Capacity under 1 litre
Tea test score: 9/10
You could fill this insulated flask up with iced water on a Thursday evening, take it on a long summer weekend of wild camping and still have a refreshingly chilled drink for a Sunday afternoon walk.
But this bottle works just as well for hot drinks as cold. The lid design allows you to unscrew it half way and achieve a controlled pour, perfect when decanting its contents into its accompanying lid.
This doubles up as a cup and holds just over 250ml per fill; about the same as a large mug. You can expect to get three large cuppas out of this flask fully filled.
We tested the bottle’s impressive claims of keeping hot drinks hot for 28 hours and cold drinks iced for 92 hours and it lived up to both. Out of sheer curiosity, we also filled it with ice over a weekend and discovered minimal melt at the end of it.
It’s an industrial-slick bit of kit. The black styling of the Klean Coat matte finish is contrasted against the stainless steel of its inner screw lid. The result is a good looking flask that will cope well with the marks of wear and tear and regular use. The steel is food-grade 18/8 so didn’t retain any smell or flavour from a tremendously strong cup of tea or coffee.
Lifeventure 1000 widemouth flask with handle
- Available from: Lifeventure (£20.99)
Best value flask for families
- Two cups
- Retractable side handle
- Optional shoulder strap
- Robust but lightweight construction
- Wide base
Tea test score: 8/10
The Lifeventure has brought back the retractable handle with style. It’s an excellent feature on the 1000ml flask, which although feels a bit top heavy, is lightweight overall and also comes with the option of an adjustable shoulder strap.
Its matte black screw lid acts as a bowl-shaped cup and there’s another which sits beneath it, this time in white plastic.
Both are broad and work well filled with cereal and soup alike. Unscrew the stopper part way and you can control the pour of the flask’s liquid contents. Or for filling, the whole lid unscrews, leaving a broad opening.
The specifications state that hot drinks will stay steamy up to eight hours, whilst cold ones will be cool for 12. When tested, this insulated flask over-performed, keeping contents chilled for up to three days.
Within its stainless steel construction is a copper lining, although there’s no metallic smell on opening.
Considering its performance, this litre flask is of remarkable value and would be a great investment for anyone wanting an easily portable insulated thermal flask for sharing.
Rex flask and cup, 350ml
- Available from: Rex London (£12.95)
Best value flask for one person
- Small and convenient
- Suitable for any occasion
- Cup isn’t insulated
Tea test score: 7/10
There’s certainly no lack of choice in the Rex range, with plenty of designs to choose from that appeal to both adults and kids.
We tested the Astrid Flower flask and cup, which was stylish and cheerful, as well as robust. With a capacity of 350ml – about the size of an average mug – we found it ideal for one person to use.
The single cup attached to its top holds roughly as much as a small tea cup, and has a tiny handle for extra support. Although the cup is basic and lacks insulation, it was perfectly adequate.
The flask itself has a press-button top, meaning you press it in to pour your drink and then press it again to lock it. Not having to remove the full cap reduces the amount of heat that escapes when using it, so liquids stay hot for longer.
We found no signs of leaks, pouring was trouble-free and the flask was a doddle to wash-up by hand. The cup easily slotted back on top until needed again.
The leaflet that comes with the flask doesn’t make any specific claims about how long it keeps drinks hot. This could be frustrating if you’re looking for one to use all day long, and may put some people off. We gave it an initial six-hour test and found our tea was still very drinkable after that time.
Tefal travel mug, 360ml
Best thermal cup
- Easy to open and close
- Safe in the dishwasher
- Quite pricey
Tea test score: 7/10
This easy-to-grip travel mug with a slightly raised, soft-feel outer surface is a sound investment if you’re looking to keep drinks warm for yourself.
It has a sleek appearance and comes loaded with practical features. We liked the push button on the lid – push it in to drink from it, push it again to lock it and seal in heat. There’s no need to remove the lid to drink, which is great for use on the go.
As a plus, it’s safe for use in the dishwasher so it can be thoroughly cleaned.
At 360ml, it holds a decent-sized mugful and promises to keep drinks hot for four hours or cold for eight. After four hours, our drink was still a perfect temperature, and even after our six-hour test it was still fairly hot.
The mug isn’t the cheapest, but a five-year guarantee should mean it’s built to last, be it on the commute or otherwise on the go.
Primus Trailbreak vacuum bottle stainless steel, 750ml
Most reliable midsized flask
- Drinks came out as hot as they went in
- No handle for carrying it
Tea test score: 10/10
This Swedish-designed double-walled flask promotes itself as crucial kit for the rugged adventurer, so you whilst you might feel it’s a bit much for a Sunday afternoon stroll in the countryside, you can be assured it won’t let you down, no matter the activity.
It feels sturdy, and at 26cm is quite tall, but doesn’t feel bulky – its sleek shape means it can easily be slipped into a backpack without much fuss.
We found it easy to grip, and its slightly textured surface means it won’t slip if your hands are wet or you’re wearing gloves.
It comes with two stoppers, regular and click-close – the standard feels hefty and helps retain heat while the click-close stopper allows you to drink straight from the bottle. Both have grooves to help with grip. You can also use the stainless-steel cup that screws on over the stopper.
We were impressed by its ability to keep liquids hot: it promised eight hours, but even after 12 hours, our tea was still warm.
Easy to wash and with no fiddly accessories, this is a go-to flask for any occasion.
16 oz Hydro Flask coffee flex flask
- Available from: Hydro Flask (£26.95)
Best innovative cup for one person
- Flex handle for easy carrying
- A little complex to dismantle the first time
Tea test score: 7/10
The coffee flex cup features all the performance specs of Hydro Flask’s larger insulated water bottles, within a compact footprint that happily fits in an average-size cup holder in your car.
Available in a range of bright colours, you’re unlikely to miss one of these if you’re rummaging in the bottom of a packed bag.
Its double wall insulation locks in temperature and produces a boiling cup of tea after over 12 hours. An ice cold drink remained freezing even after a day spent in a hot car.
Hydro Flask’s added its iconic flex handle for easy carrying. It also gives you great leverage for tightening the Flex Sip lid. Once screwed on firmly, its top section rotates to open and close the spout. Colour coding helps you know when it’s open and when it’s not to prevent spillages.
The cup arrives with a rather complex looking set of instructions of how to dismantle its lid to clean it. Considering the simplicity of other insulated mugs, this could be quite off putting for some.
It’s a wide-mouth cup, so doubles up as a handy little ice carrier for having chilled drinks on the go.
Switch up the lid to get an insulated water bottle sports cap. Thankfully within its price you’re given a lifetime warranty against manufacturer defects.
Which flask should I buy?
Flasks come in a huge range of shapes, sizes and openings, and with varying claims as to how long they’ll keep drinks hot or cold. Here are some factors to consider when choosing a flask.
We tested flasks that could hold enough for several people (or someone wanting several cups during the day), as well as some that were designed to serve one. The size you pick should be based on your individual needs, as well as the activities you might be doing while using it.
A large-capacity flask that’s completely full can be a hefty addition to your bag, so you may want to look for a streamlined, lighter flask.
For how long will it keep drinks hot?
If you’re buying a flask for the commute, a short ‘keep hot’ time will be fine, but if you’re a hiker planning day-long treks, you’ll want one with a longer promise. Our samples made pledges ranging from four to 24 hours, so there’s plenty to choose from.
Mug or flask?
Some flasks are effectively insulated travel mugs, perfect for drinking directly from them. Flasks usually require you to pour the drink into a cup.
How will you carry your flask? If you want to slip in into a pocket or take it in backpack, you’ll want to pay attention to the shape. Would you prefer one that’s tall and thin, or will that end up poking out of the top of your pocket? Will a shorter, wider one take up too much space in your bag? Consider your options carefully before investing.
On some flasks, you need to unscrew the top before pouring, while others open by pushing a button (or similar). This keeps the contents hotter for longer, as you’re not exposing them to cold air. If you’re after a single cup of tea, that won’t be an issue, but it’s worth considering if you’re planning to enjoy your drink over a few hours. If you’ll be juggling a number of things, like children and shopping, you might prefer a flask with a simpler, one-button mechanism or similar.
As a general rule, flasks can’t go in the dishwasher, and some slender models might be tricky to wash without a bottle brush. Wider openings are generally easier to clean.
Our sample ranged from £10 to £80.
Is there a guarantee with your flask, and if so, how long does it last? A couple of our samples came with lifetime guarantees, which was very impressive.
What we looked for in a flask
We tested the flasks on two different ‘effectiveness’ criteria. First, did it stand up to its stated claim? For example, if it said it would keep water hot for 12 hours, did it do so? We also filled the flasks or cups with boiled water and then used that water to make a cup of tea for our ‘tea test’, and scored it out of ten based on how hot the resulting drink was.
Value for money
Was the flask worth its price tag?
A flask can be practical and stylish. We looked at the design from an aesthetic point of view.
Design and functionality
We looked for important design features that can make a real difference, such as insulated cups, easy-open tops and strong handles.
We awarded extra points for flasks with particularly clever features.
A flask that leaks is at best annoying and at worst dangerous. We tested all the flasks for leaks, and we’re pleased to report that none of them did.
This review was last updated in July 2020. If you have any questions or suggestions for future reviews, or spot anything that has changed in price or availability, please get in touch at firstname.lastname@example.org.