Western Mountaineering UltraLite 20 Evaluate

Bottom Line

The Western Mountaineering UltraLite 20 is one of our top choices for almost any backpacking adventure. It ranks highly on our Best Backpacking Sleeping Bags Guide as one of the cushiest and highest-quality down bags we tested. It boasts a stellar warmth-to-weight ratio, massive down draft collar, full-length draft tube, and a reliable no-snag zipper. Most importantly, it’s as comfortable as down bags get. Gear Analyst Ian Krammer used this bag during many 30-40°F fall and spring nights across Colorado and Utah, and it was also the bag of choice for Gear Analyst Casey Handley’s thru-hike of the Arizona Trail.

The downsides of this sleeping bag? You’ll pay top dollar. But for ultralight backpackers with an eye on durability and performance, it is worth every penny. It’s also worth noting the UltraLite’s fit is a classic, snug mummy bag, so it may feel a bit narrow for some users. And, it doesn’t have many features beyond the basics which is a pro for minimalists and long-distance hikers, but may be underwhelming for folks who want a more feature-forward sleep system.

Warmth

The Western Mountaineering UltraLite 20 offers one of the best warmth to weight ratios we’ve tested. It’s exceptional at retaining warmth with its premium 850+ fill power down and soft 12D nylon shell. The generous draft collar – which extends 360° around the neck region of the bag – combined with the plush full-length draft tube along the zipper guarantees that cold air stays out and warmth is locked in, making it perfect for cold nights. The no-snag zipper design enhances the bags ability to maintain consistent warmth throughout the night. The snug mummy design effectively traps heat close to your body, maximizing insulation.

And, we like Western Mountaineering’s conservative temperature rating for the UltraLite. While most manufacturers overstate their comfort level claims, the UltraLite 20 is a welcome exception. It’s absolutely stuffed with down fill – boasting 16 ounces, or a full pound, of feathers – so we’ve found it can go quite a bit lower than the recommended 20°F comfort level. To test this, we slept through frigid October and November nights in the Arizona and Utah deserts, where temps hovered around 10°F. Paired with base layers and a warm sleeping pad, our analysts stayed toasty warm all night long.

The only downsides when it comes to warmth? The Ultralites’ large baffles. Its down tends to settle fairly quickly in these bigger spaces after about a dozen times unpacking and repacking the bag. Luckily, the feathers are easy to redistribute with manual massaging to eliminate cold spots, but it can become a bit of a task to prepare for colder nights. And, the sacrifice you’ll make for such exceptional warmth is the fit is narrow through – which is typical for a mummy bag, since it eliminates dead spaces that can get cold – although the snug fit limits how many layers you can wear. Last, while the UltraLite is impressively warm, it is not considered a 4-season or winter bag. If your next backpacking trip involves overnight temps that consistently 20°F or below, you’ll want to consider adding a liner, another quilt, or swapping to a warmer bag to stay as warm as possible.

Comfort

The Western Mountaineering UltraLite 20 is as comfortable as down bags get. Thanks to premium materials and a practical, time-tested design, we keep reaching for it year round for backpacking trips. The generous down collar wraps 360° around the neck, which makes the bag feel extra cushy on cold nights. Its hood is wide enough to accommodate a small to medium pillow and offers two cinch cords, one on the inside and one on the outside, to get the exact fit around your face and head, fitting like a snug down hat. 12D ripstop nylon feels exceptionally soft on the skin and is relatively quiet. The snug fit of the mummy bag design retains heat and offers just enough room to toss and turn. And, the high-quality down used in the UltraLite molds nicely to the body.

However, the UltraLite 20 is a true mummy bag, which means it offers a fairly narrow fit throughout. If you toss and turn, switching into multiple different positions overnight, this may not be the bag for you, and there isn’t much extra space for broad shoulders. Our only other complaint is that its high-quality construction demands a bit of extra care and maintenance to ensure longevity – we find that the thinner ultralight material is prone to snags and punctures – which might be a drawback for folks who are hard on their gear.

Weight & Packability

The Western Mountaineering UltraLite 20 leads the pack for its warmth to weight ratio, so it’s perfect for backpackers who want outstanding packability, top-tier insulation, and minimal weight. At 1 pound, 13 ounces for the 6 foot model, the UltraLite lands in the middle of the pack for overall weight, but keep in mind 16 ounces, or one pound – more than half of its total weight – is nothing but down feathers. That’s a lot of insulation, giving it around five inches of loft that’s super warm on cold nights but compresses remarkably well. On longer trips we tend to stash our sleeping bag loosely around our other gear to prevent shifting while hiking, but it does fit effortlessly into a compact stuff sack. The minimalist design helps keep weight down without compromising performance, so it’s a convenient choice for multi-day trips, balancing weight and functionality.

That said, if you’re an ultralight aficionado who’s mostly camping in summer or conditions where overnight temps stay above 40°F, this bag is a bit heavy, and we’d recommend going with a lighter option. But for backpackers who are trekking in shoulder seasons, taking on mild winter conditions, or are spending time in the high alpine, you won’t find many more packable or lightweight bags anywhere.

Features

The UltraLite is built for ultralight enthusiasts, so it has a straightforward feature set. The hood is spacious enough for most backpacking pillows, and the internal and external hood cinch cords are perfect for blocking out cold air. Durable, lightweight two-way zippers with thick full-length zipper baffles and a full-circumference neck baffles makes this bag effective and warm.

But that’s where the features end. Since the UltraLite 20 is focused on shedding every unnecessary ounce, you won’t find elements like a stash pocket, ventilating mesh, additional zippers, or other conveniences that heavier, more feature-forward bags offer. This might not hit the mark for everyone, but we admire its streamlined design: by cutting out the extras, the UltraLite offers the best of the best for a high-performance, lightweight sleeping experience.

Should You Buy the Western Mountaineering UltraLite 20?

The Western Mountaineering UltraLite 20 is perfect for ultralight backpackers who want a technical sleeping bag with an exceptional warmth-to-weight ratio and high-quality construction. It’s the gold standard for those willing to invest in a premium sleeping bag that will last for decades if well-maintained. With a no-frills design that offers a generous down collar, a full-length draft tube, and a no-snag zipper, this bag excels at keeping you warm and comfortable down to 20°F. Its design, optimized for warmth and efficiency, makes the UltraLite 20 an excellent choice for cold-weather camping and long-distance hiking.

However, the UltraLite 20 isn’t for everyone. The high price point can be a barrier for budget-conscious hikers. Also, its snug mummy fit may be a bit too narrow for users who prefer more space. The bag’s minimalist design, while perfect for weight-conscious adventurers, lacks extra features that some might find useful. However, for high-mileage adventures where every gram counts and reliable performance matters, you won’t find a higher quality bag anywhere.

What Other Backpacking Sleeping Bags Should You Consider?

Feathered Friends Swallow YF 20 Review:  This bag offers a similar weight, warmth, comfort rating, and features to the UltraLite 20. But it’s more affordable, available in both women’s and men’s versions, and boasts a slightly higher fill power and fill weight that gram counters and minimalists might prefer.

Kelty Cosmic Ultra 20 Review: The Ultra 20 offers a comparable fill power and a straightforward feature set as the Ultralite 20, although this bag is quite a bit heavier – half a pound for the men’s version, and a pound heavier for the women’s. But for roughly a third of the price, the Ultra is a better option for beginners or those on a budget.

Zpacks Classic 20 Review: The Classic 20 bag shares elements with the Ultralite 20 like a similar comfort rating, high-quality down and a simple, well-executed design. However, it’s significantly lighter and more compressible, comes with a higher fill power, and for a lower price –  although it’s not as warm or versatile in cold weather.