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Big Agnes Pluton UL 40 Sleeping Bag Review

Big Agnes Pluton UL 40 Sleeping Bag Review

The Big Agnes Pluton UL 40 is an uktralight mummy sleeping bag that weighs 15 oz, which is less than most comparable ultralight backpacking quilts at the same temperature rating. It’s insulated with 850 fill power DownTek water-resistant down and has a full-length, two-way zipper, so you can vent it in warm weather or use it like a quilt or blanket. This kind of flexibility is nice for summer backpacking and into autumn when the nights get colder. But the thing you need to recognize about the Pluton 40 is that it is a minimalist sleeping bag that’s short on comfort and features, even when compared to other ultralight mummy or hoodless sleeping bags. While you can certainly stay warm if you use the Pluton near its lower limit, it’s going to be a tight fit unless you’re petite in height and girth.

Specs at a Glance

  • Type: Mummy
  • Weight: 15 oz (16.6 oz, actually tested, size regular)
  • Fill: 850 fill power DownTek water-resistant down
  • Fill weight: 8 oz
  • Zipper side: left only
  • Zipper length: full length (70″)
  • Sizes: 6′, 6’6″
  • Temperature rating: 40 F, 4C
  • Dimensions (Girth): (Shoulder / Hip / Footbox): 59″ / 53″ / 36 “
  • Shell: 7 denier ripstop nylon with a DWR finish

The Pluton UL 40 is marketed as a minimalist mummy sleeping bag and it certainly is that. While the Pluton UL 40 checks all the boxes, including a mummy hood, a two-way zipper, water-resistant down, and ultralight shell fabrics, it’s not as spacious, robust, or fully featured as one might desire. While I’m impressed with how lightweight the Pluton is and how warm it is, it wouldn’t be my first pick on the basis of comfort alone. I found myself really struggling with the zipper and the hood every time I got in or out of the bag and it’s worth a few more ounces to me to have a sleeping bag that’s easier to use.

The Pluton has a 70 inch full length, bidirectional zipper, so you can vent the lower part of the bag by unzipping it.
The Pluton has a 70 inch full length, bidirectional zipper, so you can vent the lower part of the bag by unzipping it.

The Pluton Zipper

I’m not a particularly tall or stout man, but I found the Pluton UL 40 to be confining in terms of its shoulder girth (59″) and length (72″). While I can lie on my back inside the Pluton with my arms at my sides, it’s quite difficult to zip the bag closed or pull it open again.

Big Agnes Pluton UL 40 Sleeping Bag


Minimalist Sleeping Bag

The Pluton UL 40 is an ultralight, minimalist mummy sleeping bag that trades off comfort for weight. While it is easy to vent in warm weather and true to its temperature rating, be sure you understand the tradeoffs that were made to get the weight so low.

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Using the Pluton zipper also requires two hands to use: one to support the fabric along the zipper and one to open the zipper or pull it shut. This is tough to do from inside the Pluton because there’s not enough room to move your hands into position, especially since you need to reach one across your chest. Trying to operate the zipper one-handed snags frequently. Forget about venting your feet from the inside. That end of the zipper cannot be reached from the interior of the bag when it’s zipped shut because it’s blocked by an interior fabric panel. I’ve found it simpler to zip the Pluton up most of the way and slide in and out of the bag from the top, rather than struggling to open and close the zipper from the inside.

The zipper track is bordered by a fabric stiffener to help reduce snagging
The zipper track is bordered by a fabric stiffener to help reduce snagging.

I think the Pluton UL 40  would be better with a center zipper like the one on the Feathered Friends Flicker 40 instead of a left zipper, since you wouldn’t have to reach across your chest to operate it. This may not be an issue if you’re slimmer than me, but I can’t remember the last time I felt this confined by a sleeping bag and I’ve used a few.

The Pluton hood is extremely minimal and unshaped.
The Pluton hood is extremely minimal and unshaped. It really is just a half-moon-shaped baffle with a cord through the front.

The Pluton Mummy Hood

I also found the minimalist mummy hood counter-intuitive to use. It’s not shaped like a proper mummy hood, although you can get it to wrap around your head like one with some effort. It’s literally a down-filled flap at the top of the bag with an elastic cord in the front. I found that had to scrunch down into the bag, and pull the insulated flap that forms the hood down over my head before I could tighten it around my face. This has the effect of making the length of the Pluton 40 shorter than you’d expect. I normally fit into a 72″ long bag without any problems, but that’s not the case with the Pluton.

The hood cinched down
The hood cinched down to form a mummy enclosure.

The hood is so minimal, it’s hard to even tell if it has fully encircled your face when you put it on. It also requires two hands to position properly. I think I’d punt on the hood if I used the Pluton as my main bag and use a separate fleece hat or down hood instead. It’s just too much of a hassle to use. You have to wonder if it’s a marketing feature, to nudge the Pluton into the more popular “mummy” sleeping bag category, or a technical one. I really can’t decide.

The hood is so lightweight and unshaped, that it’s difficult to tell if its adjusted around your head
The hood is so lightweight and unshaped, that it’s difficult to tell if it’s properly adjusted around your head.

Warmth and Comfort

The Pluton 40 will keep you warm down to 40 if you zip it up all the way and use the hood. I’ve actually taken it down to 35 degrees, but I did the full drill: a sleeping pad with an R-value of 5, heavy wool socks, long underwear, a fleece cap, a big dinner, and a fatty midnight snack, plus I wrapped a wool shirt around my neck as a draft collar, since there isn’t one on the bag to prevent drafts going down your upper chest.

You can unzip the Pluton 40 and use it like a quilt or a blanket
You can unzip the Pluton 40 and use it like a quilt or a blanket

On the flip side, the Pluton is relatively easy to vent in warmer weather. You can unzip the body and flip it over to use like a quilt with a footbox, open up the front and pull it aside like a blanket, or drape it on top of you. The tightness of the shoulder girth is irrelevant in these orientations.

The shell fabric is very comfortable and silky smooth to sleep in. The bag is very quiet. The sewing is pristine, although the bag weight was 1.6 oz higher than its 15 oz spec on my digital scale.

Similar 40 Degree Ultralight Sleeping Bags

Make / ModelWeightHoodFill Wght | PowerShoulder Girth
Big Agnes Pluton UL 4015 ozMummy8 oz | 85059"
Marmot Phase 3017 ozMummy8.5 oz | 85060"
Montbell Down Hugger 900 #516 ozMummy7.0 oz | 90053"-75"
Mtn Hardwear Ghost Whisperer 2017 ozMummy8 oz | 90059"
Therm-a-Rest Hyperion 3216 ozMummy9 oz | 90057"
Western Mtn Everlite 4514.5 ozHoodless6.5 oz | 85060"
Western Mtn Highlite 3516 ozMummy8 oz | 85059"
Western Mtn Summerlite 3219 ozMummy10 oz | 85059"
Feathered Friends Flicker UL 4018.4 ozHoodless8.4 oz | 95062"
Enlightened Equip. Conundrum 4018.4 ozHoodless10.1 oz | 85058"
Zpacks Full Zip 3517.6 ozHoodless9.9 oz | 90061"


I wrestled with the question of whether I should recommend the Big Agnes Pluton UL 40 to my readers or not, since like a lot of backpacking gear, whether something will work for you or not often comes down to personal preferences, your body measurements, and your budget. I wouldn’t use the Pluton 40 because it’s too tight-fitting and minimalist for my tastes. Gear weight is less important than comfort for me, especially when it’s just a difference of a few ounces.

But there are some circumstances where I think the Pluton UL 40 will work for other people. For example, if you’re 5″6″ in height or less, and have a slight build, the Pluton will be a lot more spacious for you than for me. If you like the warm weather flexibility of the Pluton 40, but prefer the “oh shit” assurance of a mummy bag for unexpected temperature drops, the Pluton could also be a good choice. The Pluton can also be a good option if you want a very lightweight and highly compressible sleeping bag instead of a quilt because it’s always draft-free when zipped up. You might also try sizing up to the 6′ 6″ Pluton Long, which weighs 18 oz and has a 62″ shoulder girth, although it takes more body heat to warm an oversized sleeping bag.

Whatever you decide, I would recommend looking at the other ultralight sleeping bag options I’ve listed in the table above, which include shoulder girths. Shoulder girth measures the circumference of a sleeping bag across the shoulders and provides a good approximation of its effective width. More width means more comfort, but it often comes with more gear weight.

Disclosure: Big Agnes donateda sleeping bag for review.

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  1. What would do use instead?

  2. I use Feathered Friends Flicker 40 for ground sleeping. Center zip, draft collar, ventable foot box. It’s really perfect. Roomy. Lightweight.

  3. Fussy zipper and urgent 2:00 AM bladder roll call – not a great combination.

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