GSI Outdoors Microlite Flasks Review

© Pegs Bailey

American company GSI's Microlite flasks are a range of stainless steel-walled vacuum bottles that claim to be 1/3rd lighter than traditional designs. We've been using three different sizes, two with a flip-open easy-pouring top, and one with a screw top.

The flip-top models have a spout you can drink from  © Dan Bailey
The flip-top models have a spout you can drink from
© Dan Bailey

It's worth stating from the off that I don't know which traditional designs the Microlites are being compared with. But they certainly do seem light for the impressive insulating performance on offer

Microlite 500 Flip - £30

The smallest of the range, the Microlite 500 Flip has, unsurprisingly, a capacity of 500ml. A twist-top model is also available, but we were sent the flip-top version.

This has a robust sprung plastic lid, which can be locked closed to avoid accidental opening in transit (a good call). The lip is shaped for easy pouring, though you have to bring your own cup if you want one - something not factored into the overall weight. It's also designed for drinking directly from the bottle without getting hot coffee down your front.

Microlite 500  © Dan Bailey
Microlite 500
© Dan Bailey

Locking mechanism  © Dan Bailey
Locking mechanism
© Dan Bailey

Flip-top lid  © Dan Bailey
Flip-top lid
© Dan Bailey

It's very small - discounting the lid, not much larger than a single-walled 500ml plastic water bottle. How did GSI fit a vacuum space in there?! At only 230g (minus contents, obvs) it is surprisingly light for an insulated bottle, but its ultra-thin stainless steel walls are also pretty robust - yes it could get dented if sat on or dropped, but I think you'd be unlucky to really damage it.

At this weight I'd be happy carrying it on big hill days, or for winter climbing, when saving grams does count but it's still nice to have a hot drink.

GSI claim it will keep hot drinks hot for 10 hours, or cold ones cold for 20 hours. Out in the hills it certainly seems to do the trick, but to be a bit more scientific (and I use the term loosely) I conducted a home test for all three flasks on review. Standing them at room temperature (around 20C), I filled them straight from the kettle and then stuck in a thermometer periodically. Here are the 500's results:

  • 0hrs: 98C
  • 4hrs: 78C - hot on the lips if drinking from the spout
  • 8hrs: 63C - still a hot drink
  • 12hrs: 54C - lukewarm
  • 24hrs: 38C - body temperature, but still warmer than the air

I think that's pretty impressive for a flask this small and light.

Easy pouring...  © Dan Bailey
Easy pouring...
© Dan Bailey

Microlite 720 Flip - £35

This is simply a larger-capacity alternative to the 500, and again comes either with a twist-top or a pop-up lid. I'm not sure why this is called the 720 since its capacity is advertised at 24 fl oz, which is actually 709ml. Still it's nearly 3/4 of a litre either way, making it a useful size for a cold day out.

Microlite 720 on a chilly early spring Rannoch Moor  © Pegs Bailey
Microlite 720 on a chilly early spring Rannoch Moor
© Pegs Bailey

Weight-wise, you're looking at only 308g for this bottle, and again it is compact in size, and robust. Squeeze hard and you can get the outer wall to meet the inner, but it readily pops out again, so I don't think this is a concern in terms of longevity.

They don't come with a cup, so you may want to bring your own  © Pegs Bailey
They don't come with a cup, so you may want to bring your own
© Pegs Bailey

As for insulation, the Microlite 720 Flip promises 12 hours hot, and a full 24 hours cold. Here's what I found:

  • 0hrs: 98C
  • 4hrs: 80C
  • 8hrs: 65C - still at this stage what I'd call a hot drink
  • 12hrs: 55C - lukewarm
  • 24hrs: 39C

A house in summer weather is a less rigorous testing environment than a hill in winter (perhaps I should've put them in the fridge?). But from my experience of using these flasks, backed up by the home results, I think that on the average hill or crag day you could fill a Microlite in the morning and still expect to be having a hot drink on your way home - and that's really all I'd ask of a flask. GSI's 12-hour figure does look a little ambitious though.

GSI say:

Sleek, light and very efficient. The slim, incredibly lightweight design is 1/3rd lighter than traditional designs due to its impossibly thin 2mm walls; actually thinner than most single-wall plastic bottles. The simple push-button, flip-top cap locks open for easy drinking and closed for transport, and the 18/8 stainless steel boasts unrivaled purity and durability. Available in six colours.

  • Capacity: 500ml or 709ml
  • Weight: 500 - 230g; 720 - 308g
  • Material: Stainless steel
  • 2mm walls are actually thinner than most single wall plastic bottles for an amazingly compact footprint that fits anywhere
  • 18/8 stainless steel boasts unrivaled purity and durability
  • Push-button, flip top cap locks closed for transport and locks open for easy drinking

For more info see

Microlite 1000 Twist - £40

Simpler than the flip-top versions, the Twist is basically a glorified bottle, with a screw-on cap and no spout for easier pouring. You can still drink from the mouth of this bottle, but as you're likely to scald your lips in the first few hours I've tended to bring a folding cup along too - which of course adds (a little) weight and bulk.

Its rugged simplicity is the 1000's strong point - that and its lightness compared with traditional vacuum flasks. In rough size and shape it's reminiscent of a classic 1 litre Nalgene bottle, with a nice wide mouth, but the difference is the vacuum insulation. Its 2mm aluminium walls are claimed to be thinner than most single wall plastic bottles, but of course there are two layers with space for a vacuum in between, so in total this is a little fatter.

It's built tough...  © Dan Bailey
It's built tough...
© Dan Bailey

The chunky plastic lid is good and strong, and shaped to give you added leverage for easier opening. Its retaining loop doubles as a hanging point (though who hangs 1-litre bottles about their person?).

As I've said, I'm not sure what rivals it is 1/3 lighter than, but it does indeed feel surprisingly light for a metal 1-litre flask, at only 370g. In terms of weight-for-warmth the Microlite 1000 must be one of the better models currently available. By comparison, the Thermos Ultimate .9L flask, also sold on the basis of its lightness, is listed as weighing 380g. The Microlite 1000 Twist is even more robust than its smaller siblings - I can't bend the walls at all for instance. This thing is made to take some knocks.

With its larger capacity and the added insulation of a twist cap (which is better than a push-button design in terms of heat retention) you'd expect the Microlite 1000 Twist to keep things hotter for longer than its smaller siblings. GSI say it keeps liquid hot for 18 hours, and cold for 32 hours.

Having filled it from the kettle in the early morning, and still found the contents too hot to sip when halfway up a Munro at mid day, I'd say anecdotally that heat retention is very impressive. I've actually taken to leaving the lid open when sat down in order to cool it off.

Here's the hard data from the home test:

  • 0hrs: 98C
  • 4hrs: 85C
  • 8hrs: 76C - hot on the lips if you sip from the flask
  • 12hrs: 67C - still a steaming hot drink
  • 24hrs: 51C - tepid, but a lot warmer than the air outside

Though I didn't manage to test it at the claimed 18 hours (even reviewers have to sleep) I think that set of numbers is pretty impressive!

To illustrate the performance of all three of these bottles in the home test, I sketched a quick graph (take it with a pinch of salt - it's very rough):


I really like these GSI bottles. They're rugged, they're compact, they're lighter than most, and they keep drinks hot for ages. Considering all that, the prices seem fair. Being genuinely light enough for big hill days and even winter climbing, I can see myself using one or other of these bottles throughout the year.

GSI say:

The GSI Outdoors Microlite 1000 Twist insulated vacuum bottle is the perfect bottle to keep your tea steaming hot, your water ice cold and your soup that perfect, piping hot temperature. The bottle keeps beverages hot for 18 hours and cold for 32 hours, while being 1/3rd lighter and holding 25% more liquid for the same size traditional vacuum bottles.

  • Capacity: 1 litre
  • Weight: 370g
  • Material: Stainless steel
  • Incredibly lightweight
  • 2mm walls are actually thinner than most single wall plastic bottles for an amazingly compact footprint that fits anywhere
  • 18/8 stainless steel boasts unrivaled purity and durability
  • Secure, screw top cap includes leverage enhancing lobe which makes opening frozen bottles easy even with mittens or gloves
  • Leash provides convenient attachment point

For more info see

Support UKC

As climbers we strive to make the kind of website we would love to visit, with the most up-to-date news, diverse and interesting articles, comprehensive gear reviews, breathtaking photographs and a vast and useful logbook system. As a result, an incredible community has formed around the site - we’ve provided the framework but it’s you who make the website what it is today. If you appreciate the content we offer then you can help us by becoming an official UKC Supporter. This can be a one-off single annual payment or a more substantial payment paid monthly or yearly which includes full access to Rockfax Digital and discounts on Rockfax print publications.

If you appreciate then please help us by becoming a UKC Supporter.

UKC Supporter

  • Support the website we all know and love
  • Access to a year's subscription to Rockfax Digital.
  • Plus 30% off Rockfax guidebooks
  • Plus Show your support UKC Supporter badge on your profile and forum posts
UKC/UKH/Rockfax logo

4 Sep, 2018

Like this review, particularly the hand drawn graph of temperature drop vs time, I am definitely going to do this next time I have to present some data instead of spending hours wrestling with spreadsheets in Powerpoint to get some fancy 3D multi coloured chart.

4 Sep, 2018

Needs to label his axes mind!

4 Sep, 2018

I like lighter ansd smaller things, but why is it a thing to not have a build in lid/cup anymore? Now you have to bring a cup or risk burning your mouth? (or wait untill the entire content is cooler, but isn't the point you can have a hot drink over a extended periode).

UKC/UKH: we do the hard science so that you don't have to. Probably the first time I've drawn a graph of any sort since leaving school... can you tell?!

4 Sep, 2018

Better if temp retention tests performed in the field as you will be more likely using on cold days, not 20C ones.

Do an outdoors test on a cooler day?

More Comments

Loading Notifications...
Facebook Twitter Copy Email LinkedIn Pinterest