Cosy, durable and super warm: The OEX Leviathan EV 900 promises to keep you warm down to -16 degrees Celsius, and all for less than £200 if bought with a GO Outdoors Discount Card. Could this be the best value bag out there for four-season adventuring in the UK?

The OEX Leviathan EV 900

A durable, super warm and cosy sleeping bag, the OEX Leviathan EV 900 is one of the most popular bags out there — and for good reason: Costing only £170 when bought through GO Outdoors, it has a very low comfort rating and performs excellently in a range of conditions, making it an ideal option for four-season UK camping.

The Good

Very affordable

Great warmth to weight

Comes with both stuff and storage sack

Amazing value for money

Reasonably packable

The Bad

The colour is ugly

Temperature ratings are somewhat optimistic

OEX Leviathan EV 900 Review

The OEX Leviathan EV 900 is a 4-season down-filled mummy sleeping bag sold by UK outdoor retailers GO Outdoors, Blacks, Millets and Ultimate Outdoors. Designed and sold under the OEX banner — the most premium of the stores’ stable of own brands — the Leviathan EV 900 has become a flagship product for both the stores and the OEX brand, with hundreds of fans all over YouTube. Hell, just head into the Lake District in mid-January and take a peek into the various tents you spot. We’d bet that you’d see the Leviathan’s unmistakable off-yellow hue emanating from at least a quarter of them. So how good is the OEX Leviathan EV 900, really?

The first thing to be aware of before you buy this bag is that it weighs a relatively heavy 1370 g. For that, however, you get 900gsm of 600 fill power duck down, making the bag lofty, comfy and a joy to sleep in. It also comes with a huge shoulder baffle, a comfortable hood that can be cinched tight around your head, and a handy little pocket on the zipper, suitable for small nick-nacks in the night.

What does fill power actually mean? 

OK, so it’s all well and good to tout the ‘fill power’ of the OEX Leviathan EV 900. But what does that actually mean? Well, ‘fill power’ is simply a marker to understand the quality of the down used versus the volume. It’s a number that’s based on the amount the down in the sleeping bag lofts back up after being compressed, with higher-quality down lofting up more than lower-quality down. In general, the higher the fill power, the better the down.

To illustrate what this actually means, let’s compare the OEX Leviathan EV 900, a budget 4-season down bag, with Rab’s £640 Mythic 600, a high-end bag and the priciest Rab produce in this category. Both bags are supposedly good down to around -12 degrees Celsius, yet there’s a £470 price disparity. So, why the difference?

Production and marketing costs aside, one of the key differences is in the quality of the down used. In the Rab Mythic 600, you get 600 grams of 900 fill power goose down. In the Leviathan, you get 900 grams of 600-fill power duck down. This means that in order to arrive at the same -12 degree Celsius temperature rating, OEX had to use 150% more lower-quality duck down to achieve a similar effect, making the weight of the bag 300 grams heavier off the bat when you compare insulation alone (the OEX is actually almost 500 grams heavier than the Rab due to a host of factors).

In general, then, fill power is a useful marker for understanding how much down you might need to achieve the same insulatory effect. You need fewer feathers the higher the fill power, so down products made with down of a higher fill power (750 or 900) will normally be lighter, more packable and pricier as a result.

The OEX Leviathan EV 900 also comes with a stuff sack and a storage bag that allows the bag to stay lofted when you’re not using it for any extended periods of time. When used on the move, it packs downs well ( to roughly the same size as a standard football) and lofts back up in no time at all (especially if you fluff it up as you toss it into your tent). Oh, and the ethically sourced down is also hydrophobic, meaning that it will maintain some of its loft even when it gets wet, pledging to keep you warmer than non-hydophobic coated feathers (which is always idea in the UK’s soggy climate).

OEX list this bag as having a comfort temperature of -9 degrees Celsius, a lower limit of -16 and an extreme rating of -39. They tested the bag according to EN13537 (which is an industry standard means of testing how warm sleeping bags are), and having used the bag for over three years, we definitely have some thoughts on these numbers (which we cover below).

All in, however, the thing that’s most worth noting is the price: When bought with a GO Outdoors discount card, this bag has an RRP of £170. At numerous times throughout the year, however, they are regularly on sale for as little as £120. We actually bought ours for £105 back in September 2020. And though the price has crept up since then, we still believe that the OEX Levithan EV 900 represents unbeatable value for money.

Our experience using the OEX Leviathan EV 900

We’ve had this bag in our possession for over three years now. As mentioned, we bought it back at the end of summer 2020 and have used it innumerable times over the years. We took it on some freezing winter camps in Germany’s Black Forest National Park, we’ve had it out on numerous summits at various times of year throughout the Bannau Brycheiniog and Eryri National Parks. We’ve slept in it in all weather on Dartmoor National Park (including a super chilly deep winter camp where the mercury without wind chill hit -13 degrees Celsius), and we even used it at the height of the summer on a large meet up when all other bags were taken. So, yeah, we’ve spent some substantial time in this thing and feel pretty informed when it comes to reviewing it. Here’s what we think of it after three years and countless nights.

First of all, let’s look at the warmth: OEX claim that this bag can keep you comfortable down to minus 9 degrees Celsius when wearing nothing but baselayers. Having slept in this thing in temperatures approaching minus 9 numerous times, we absolutely cannot support this claim. Though undeniably warm, for us, the bag tops out at around -5 or -6 degrees Celius. In fact, when we slept in it in -13 degrees C, we had on baselayers, a jumper, a down jacket, thick socks, a hat and gloves. And eventhough we weren’t chilly, we weren’t baking in the Leviathan either.

For UK use, however, we think the bag delivers enough warmth to reasonably be considered a true 4-season sleeping bag, and we’re happy with its performance overall. We wouldn’t take it on a winter arctic expedition, but we’d be happy using this on practically any winter camping trip throughout the UK, and would only upgrade to something warmer if we were camping at elevation or in winter weather that’s uncharacteristically harsh (and cold) for the UK.

Pack size-wise, the OEX Leviathan EV 900 is also a reasonable size (at this price point). If you use the provided stuff sack, you can get it down to the size of a watermelon or a standard football, and weighing just over 1.3 kgs, it’s relatively easy to carry for such a warm winter bag. To maximise the space it takes up in our pack, we’ve taken to packing it in a dry bag. A common trick for making sleeping bags more malleable, this allows you to almost vacuum-pack the sleeping bag by pressing out all of the air inside it. You’re then left with a mouldable pouch that you can manipulate into whatever shape you want — for conveniently fitting into any compartment in your pack.

One thing we have gotten used to after a night in the Leviathan is the inevitable plucking of feathers off our baselayers the morning after. This thing seems to shed like no other down bag we’ve ever used. Presumably, this is down to the soft and thin inner that — despite being a joy to sleep in (and helps keep the weight of the sleeping bag down) — the duck feathers can easily penetrate when you roll around overnight. Initially, we were quite worried about this, wondering how long it would take until the bag was void of insulation altogether. But after three years and over 50 nights in it, the bag hasn’t lost any of its warmth or loft whatsoever.

Finally, it’s also worth mentioning just how comfortable the bag is. For a budget 4-season bag, the materials used are soft, cosy and a joy to sleep in. The outer is also very durable and is reasonably water resistant (as we found out when we spilt a 500 ml pot of cold water over the bag one chilly winter’s night). The zip is also lined with a thick strip of protective material, which minimises snags and ensures you don’t accidentally tear the soft inner while zipping the bag up.

So, what about how it looks? Despite the value for money, this is one thing we’d love to talk to OEX about: What on earth is up with that colour? Have you ever seen a less-appealling shade of yellow? Luckily, you only sleep in this thing, meaning your primary interactions with it will be in the dark. Maybe it’s just us, but the colour makes the bag looks like it needs to see a doctor. That being said, the unusual yellow hue seems to have made this bag instantly recognisable. Either by design, or due to the fact that nobody else is brave enough to make kit in this colour, it’s easy to spot Leviathan EV 900s lurking in the back of many a YouTube video.

OEX Leviathan EV 900 FAQs

Conclusion

All in then, we really rate the OEX Leviathan EV 900. For the price, we think you’d struggle to find anything even remotely comparable on the market right now, especially in regards to how warm the Leviathan is. Additionally, the reasonably small pack size and weight of this bag make it a solid all-rounder for numerous adventures. You can comfortably carry it for miles on winter hikes, and you could even extend its use to winter bikepacking or fastpacking trips if you had enough room in your set-up. Though overkill, you can also use it as a blanket in the summer if you poke a leg out of the side, and the durable outer material is protective and comfortable to sleep in.

After three years of abuse, would we buy another for £170? Absolutely. All we ask is that OEX consider a colour change.