A Thermos isn’t just for Christmas. Our top seven vacuum flasks keep their contents freezing cold in summer as well as piping in winter

We'll stop using "Thermos" as a generic term now. After all, Thermos is just one of many brands to master the art and science of the vacuum flask, that ingenious insulated container that does the job of a portable kettle, oven or fridge without the need of a plug socket.

Whether it's a chunky "food flask", sleek "vacuum bottle" or something in between, the job of a flask is to keep food and drink hot or cold without leaking, all day long. Some of the winners on our list can insulate for more than 24 hours, which is long enough to survive the toughest hike or most gruelling commute.

For this review we've picked out seven flasks that excel at insulation, leak-proof seals, durability (no shattering in a single drop, thank you) and, of course, value for money. Before we reveal our list, here's a quick shopping guide.

Flasks have come a long way since your great-grandmother sent her man off to work with a Thermos of oxtail soup. Today's best vacuum flasks are mini engineering marvels, made from multiple layers of BPA-free rust-proof stainless steel that will survive years of bumps and knocks and keep you in hot cuppas and ice-cold drinks season after season.

They also win big on green points, doing away with the need to buy drinks in single-use cups. If your flask is big enough for lunch you can save money and plastic there, too.

You can buy flasks for next to nothing, with even Ikea selling one for just £4. But given how often you're likely to use your flask, it's worth spending a bit more for quality and durability.

You tend to get what you pay for when buying vacuum flasks. Of course you can get flashy designer flasks (Paul Smith's stripy flasks cost hundreds of pounds), but if you're looking for quality alone you won't have to spend more than about £60. That may seem extravagant for a drinks container, but you'll soon save much more on drinks and lunches.

18/8-grade steel: Today's best flasks are made from stainless steel that doesn't secrete chemicals (they're BPA-free, in other words) or weird flavours. The best of the best are made from 18/8-grade steel (aka 304-grade), whose high nickel content makes it rust-proof and extremely durable. It's also non-magnetic, which may disappoint if you'd hoped to adorn your Thermos with fridge magnets.

Multi-wall insulation: Double-wall insulation creates a vacuumed layer, which is vital for keeping hot drinks hot and cold drinks cold. Some flasks include more layers for even better insulation. The 100% stainless steel construction of today's flasks means there's no risk of inner walls shattering when dropped.

Insulated cup: Good flasks also pay attention to the insulation of the detachable cup, if they have one. This is so your drink stays warm while you're drinking it. Meanwhile the outside stays cool, so you don't risk burnt hands.

Capacity: Bigger capacity means more contents to enjoy. It also means more weight to carry, but only if you fill it to the brim. For best insulation, buy a flask that's slightly bigger (but not heavier) than you need, and leave a couple of centimetres of air at the top. We've included capacity and weight info for all our flasks below.

New from the creators of the 106-year-old Stanley Classic, this sleek, rugged flask keeps hot drinks hot for longer than the promised 27 hours and iced drinks cold for a staggering 100+ hours (good luck if you're caught out in a heatwave for that long, frankly). So if you're packing the car for a camping trip, this is the flask to take.

The Master achieves its insulating magic with a 'QuadVac' quadruple-walled insulation system, made entirely from BPA-free 18/8-grade rust-proof steel. The insulated cup is made from the same stuff, and does a brilliant job of retaining a drink's temperature while you take your time over lunch. The stopper is plastic but lined with steel, so your drink never comes into contact with plastic, and we found it to be reliably leak-proof.

Our only grumble is that the flask weighs nearly a kilo even before there's anything in it, and isn't available in a smaller size.

Key specs – Material: 18/8 stainless steel with steel-lined plastic inner cap (dishwasher-safe); Capacity: 750ml; Size: 30 x 8.8 x 8.8cm; Weight: 889g; Colours available: 1

At the other end of the weight spectrum comes this gorgeous newcomer from Thermos, weighing in at barely more than 200g (before there's anything in it, of course) and topped with a 'direct drink' spout that's perfect for the train to work.

Thermos designed the flask to be as light and ergonomically comfortable as possible, with a cool sweat-proof exterior. It can't match the Stanley Master for sheer insulating power (no flask can, to be honest) but it still does a great job of keeping cold drinks cold for a couple of days, and hot drinks piping hot all day.

Key specs – Material: stainless steel (not officially dishwasher-safe); Capacity: 470ml; Size: 22 x 6.5 x 6.5cm; Weight: 210g; Colours available: 1

Drop it, dent it, dishwash it, drink straight from it, even brew tea in it. This fab lightweight flask is designed to survive anything, without messing with the temperature of its contents.

The two-part lid doubles as a spout and doesn't leak, working more like a drinking bottle than a flask, so it’s ideal for taking a sip when you're walking. The classic Sigg handle doubles as a lever, so you can open it using a stick if your hands are cold or dirty. There's even a built-in tea filter, so you can make tea using loose tea leaves or whatever you find on your adventures.

This 500ml flask is still half price from Cotswold Outdoor, but even at £20 it'd be a good buy. Cotswold also has a 750ml version (£26), and Wiggle has a 1-litre version with detachable cup (£22.49). Neither larger version has a tea filter.

Key specs – Material: 18/8 stainless steel (dishwasher-safe); Capacity: 500ml; Size: 25.5 x 6.9 x 6.9cm; Weight: 381g; Colours available: 1

Keep drinks cold in the blazing sunshine with this stylish, lightweight, double-wall insulated bottle, made from sturdy stainless steel and a powder-coating finish to stop it slipping out of sweaty hands. It does keep hot drinks hot for 12 hours, but it's on summer adventures that this flask really comes into its own, keeping iced drinks cold for more than 24 hours.

Hydro Flask also comes in a smaller 12oz version and in a 'wide mouth' design, but only the standard mouth fits the insulated silicone Sport Cap (£9.50), designed for taking generous sips on the go.

Key specs – Material: 18/8 stainless steel (dishwasher-safe); Capacity: 600ml; Size: 26.5 x 7.3 x 7.3cm; Weight: 295g; Colours available: 3 (more colours available from other retailers)

The Thermos King is surprisingly light given its size (it's bigger than the Stanley Master but lighter and cheaper) and looks spectacular. Thoughtful details include a twist-and-pour stopper that lets you pour drinks without removing it, and a space-saving handle. However, it doesn't insulate for as long as the Stanley Master, and it's a bit drippy when pouring from the spout, at least when it's full.

A Thermos King will last years but it's not officially dishwasher-safe, and you're not even meant to submerge it in water (if you do, you kill the manufacturer's 50-year guarantee). Inquisitive users report that it does survive the dishwasher intact, albeit with peeling colour.

The pictured flask is in Cranberry Red; Argos also has it in Midnight Blue (£27.99). More colours and sizes are available from Thermos.

Key specs – Material: 18/8 stainless steel (not dishwasher-safe); Capacity: 1.2l; Size: 31 x 10.5 x 9.4cm; Weight: 670g; Colours available: 2 (more colours available from other retailers)

This large, robust flask comes with a removable shoulder strap, so you could throw it over your shoulder while you're heading up a mountain for the weekend. And if you're heading out there with a friend, even better, because there's a second plastic 'buddy cup' hidden inside the larger metal one. The flask is wide enough to be used as an insulated lunchbox, too, but you'd need to bring your own spoon. Insulation is excellent, keeping contents piping hot for over 12 hours.

Key specs – Material: 18/8 stainless steel with copper coating (dishwasher-safe); Capacity: 1l; Size: 23 x 12 x 11.5cm; Weight: 708g; Colours available: 1

The King is back, and this time he's full of lunch. Here, the Thermos's insulated stainless steel lid doubles as a serving bowl, and comes with a full-size folding stainless steel spoon. Stews and soups stay very hot all day, while fruit and tubs of yoghurt stay cool all day – or fridge-cold if you include an ice pack in the flask. Ice cream won't quite go the distance, but if you pack the flask with a handful of wrapped lollies straight from the fridge they'll stay frozen for a few hours.

The pictured flask is Midnight Blue but Argos also has it in Red (pictured at the top of the page) for £18.99. Find more colours on the Thermos website.

Key specs – Material: 18/8 stainless steel (not officially dishwasher-safe); Capacity: 470ml; Size: 14.2 x 9.4 x 9.4cm; Weight: 390g; Colours available: 2 (more colours available from other retailers)

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