Knowledge is (Fill) Power: How to Interpret Sleeping Bag Warmth and Weight

Table of contents

Sleeping Bag Warmth

Sleeping Bag Weight

What About Sleeping Quilts?

Warmth-to-Weight Ratio

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Sleeping bags may look like simple pieces of gear—they don’t require setup like a tent, and they’re not riddled with mysterious pockets like a backpack. But the sleeping bag category as a whole is just as wide and varied as that of any other major hiking product, and having a working knowledge of how to interpret a bag’s two main traits—warmth and weight—is essential to choosing the right model for you. Here’s a handy explainer of what all those numbers cited by sleeping bag manufacturers actually mean.

Sleeping Bag Warmth

Let’s start with the most important stats to look at when considering a sleeping bag: its ISO temperature ratings, formerly known as EN ratings. Most sleeping bag manufacturers use these standardized ratings, which indicate at which temperatures a bag will keep you comfortable, and which are determined by testing a bag to rigorous International Organization for Standardization (ISO) requirements. Thus, a model rated to a 15-degree lower limit from Therm-a-Rest should, in theory, be just as warm as a 15-degree lower limit model from Nemo, and so on. This isn’t always the case, though, and if a bag exceeds or doesn’t hit its temp rating, we’ll mention that in our review. (Some brands, such as Feathered Friends, don’t subscribe to ISO standards and test their bags’ warmth independently.)

After ISO testing is completed, a sleeping bag is assigned three ratings:

Comfort: The lowest temperature at which an average cold sleeper will sleep comfortably in the bag.
Lower Limit: The lowest temp at which an average warm sleeper will sleep comfortably.
Extreme: The lowest temp at which this bag will keep you alive. (Note: Do not test this yourself!) Few manufacturers list Extreme in their product specs.

It’s important to understand that these ratings are for “average” cold and warm sleepers, so your experience with the bag may differ slightly from what the ratings indicate, but they’re still extremely accurate guidelines. Also, ISO ratings assume that the sleeper is wearing a midweight base layer. If you like sleeping au naturale (or, you know, with heavier clothing on), your experience may change.

Temperature ratings on the Marmot Trestles Elite Eco 20 sleeping bag

Many sleeping bags are named for their lower limit rating (i.e. the Sea to Summit Spark 15 has a lower limit of 15°F), but that’s not always the case. For example, the REI Co-op Magma 15 is named for a temperature that falls between its tested comfort (21°F) and lower limit (9°F) ratings, which seems practical. Another popular bag, the Nemo Riff 15, is even wonkier. It has an ISO comfort rating of 28 degrees and a limit rating of 17 degrees, meaning that the bag’s name doesn’t accurately reflect its limit, and not in a good way. (If Nemo followed REI’s convention of splitting the difference the bag would be called the Riff 22.5, which doesn’t exactly roll off the tongue.)

The larger point is that a sleeping bag’s product name should give you a general idea of its warmth. That information is not always accurate, though, so make sure to check the ISO comfort and limit ratings for confirmation. When available, we provide these specs for all products in our guide to the best backpacking sleeping bags.

The answer depends on several factors, of course, including the environment in which you’ll be backpacking (sleeping at 9,000 feet in the alpine of the Rockies is a whole lot different than fair-weather backpacking at low elevations on the East Coast) and how warm or cold you run. That said, here’s a general guide. All of these temperatures refer to ISO lower limit ratings.

20°F: This is the most versatile temperature rating for bags that will excel on most three-season backpacking trips in the mountains (think the Western U.S.) without causing you to stress over the forecast. It gives you enough of a buffer should the weather turn chilly, although you might be too warm on some particularly balmy summer nights.
30°F: This is the classic three-season rating for hikers who camp at lower elevations, sleep warm, or want to gamble with their warmth vs. weight tradeoff. We’ve been cold in 30-degree bags at altitude even in summer, but these bags should perform well the majority of the time while weighing and costing less than 20-degree models.
40°F: This temperature rating is for summer backpacking in moderate to warm weather—the Appalachian Trail in July comes to mind—and sticking to lower elevations.

In terms of planning, remember that even if the weather generally looks good for your trip, there are a few hours in the middle of the night and early morning when the temperature bottoms out. This is the temp you should be looking at when considering your sleeping bag choice unless you plan on counting stars while shivering through the night.

Wrapping the Sea to Summit Spark 15 Sleeping Bag around shoulders

Throw aside dimensions and colorways, there’s one key difference between men’s and women’s sleeping bags worth knowing: temperature ratings. Most women’s sleeping bags are advertised using the ISO "Comfort" rating; in contrast, men’s sleeping bags often use the ISO "Limit" rating. This means that a women’s 20-degree bag is often warmer than a men’s 20-degree bag, even if both claim the same rating. For example, take the Nemo Disco 15: The men’s version has 1 pound 7 ounces of 650-fill down and a Comfort rating of 27 degrees, while the women’s version has 2 pounds (9 oz. more!) of 650-fill down and a Comfort rating of 17 degrees. Thus, if you want more warmth for the price and don’t mind carrying a little extra weight and bulk, a women’s sleeping bag can be a sneaky way to get a better deal on insulation.

Nemo Disco sleeping bag (Endless Promise)

A sleeping bag is just one half of your sleep system, and if you neglect the other half you might end up cold no matter which bag you choose. Because you sleep on the ground when backpacking, your sleeping pad is the second key factor in keeping you warm. The heat from your body wants to transfer to the cold ground beneath you, and a pad goes a long way toward preventing that heat from escaping.

The ability of a sleeping pad to resist heat transfer is measured by a statistic called R-value. As with sleeping bags, there’s a standardized testing system devised by the American Society for Testing and Materials (ASTM) that most pad manufacturers adhere to. The R-value scale runs from roughly 1.0 to 8.5 for backpacking sleeping pads (it’s even higher for car camping mattresses). R-value measurement is linear, so a pad with a 4.0 R-value will be twice as warm as one with 2.0 R-value. Pads warm enough for three-season use generally start around an R-value of 3.0—any lower and a pad is really only good for warm-weather summer camping—and go up to about 5.0. For winter camping, you would be wise to use a pad with a minimum R-value of 5.0, although we’d recommend going higher than that.

Sea to Summit Spark 15 Sleeping Bag on sleeping pad

Choose a sleeping pad that roughly matches your sleeping bag’s warmth, and is worthy of the conditions you’ll encounter on your adventures. If you grab a simple closed-cell foam model such as the Therm-a-Rest Z Lite, which checks in with an R-value of 1.7, don’t be surprised when your bag feels chillier than you expected (remember, it’s all part of a system). For reference, our top sleeping pad pick, the Nemo Tensor All-Season, has a healthy 5.4 R-value—perfect for the gamut of three-season backpacking, and even for pushing into light winter use—and weighs just over a pound, while the popular Therm-a-Rest NeoAir XLite has an R-value of 4.5 while coming in at just 13 ounces (read our NeoAir XLite review).

Setting up the Nemo Disco Endless Promise 30 Sleeping Bag

Sleeping Bag Weight

Now that we’ve discussed how to stay warm in your sleeping bag, it’s time to address another critical factor: weight. Buying a bag that’s too heavy and bulky can negatively impact your trip almost as much as being too cold can, and packability is a crucial quality for most backpackers.

Just as your intended camping environment determines how warm of a sleeping bag you require, considering the type of backpacker you are and the nature of the trips you plan most often is important when choosing what sleeping bag weight you should aim for.

If you’re a hiker who likes to head out with a minimalist kit—think frameless pack, tarp shelter, beefy trail-running shoes—then an ultralight sleeping bag is probably, well, your bag. Ultralight models generally weigh around 1.5 pounds or less (the Therm-a-Rest Hyperion 20, for example, checks in at 1 lb. 4 oz.) and leverage extremely high-loft down and thin materials to achieve supreme packability. These bags are also the most expensive models on the market, so they might not be the best choice for hikers who aren’t fully steeped in the ultralight ethos.

For most average backpackers—those who can put away plenty of miles over a long weekend, but don’t necessarily need or want the lightest kit possible—sleeping bags in the 1.5- to 2-pound range are ideal. These models offer the best blend between weight, roominess (that Hyperion bag above is a tight squeeze), and warmth, and will get you through most backcountry missions. A top model in this category is the Feathered Friends Swallow UL 20, one of our favorite sleeping bags for serious backpackers, which comes in at an extremely portable 1 pound 11 ounces and packs down quite small. (Feathered Friends doesn’t do ISO testing, but we can tell you that their 20-degree rating is conservative.)

Montbell Seamless Down Hugger 800 #2 Sleeping Bag's packed size

Nemo Riff Endless Promise 15 Sleeping Bag packed size

If casual, low-mileage backpacking mixed with car camping is more your speed, using a sleeping bag that weighs well over 2 pounds is totally fine. These models usually cost a lot less due to less-than-premium materials, so they’re great for backpackers on a budget. The Kelty Cosmic Down 20 is stuffed with 550-fill-power duck down and comes in at 2 pounds 7 ounces, and costs a mere $160 (although it has a fairly narrow shape). The Therm-a-Rest Boost 650 20 ($320) uses 650-fill-power duck down and comes in a hefty 2 pounds 11 ounces; however, it's much roomier and has a ton of features—including vents and armholes—and is a great choice for traveling and frontcountry camping (read our review of the Boost 650 20).

Now, what are the factors that go into making a bag lighter or heavier? Let’s get into it:

We’ll keep it quick here: The majority of backpacking sleeping bags are filled with down—the insulating plumage of geese or ducks—which is considerably warmer, lighter, and more packable than synthetic fill. Synthetic insulation is much cheaper than down, though, and retains its ability to insulate when wet. Buying a synthetic bag such as the Marmot Trestles Elite Eco ($170) or REI Co-op Trailmade 20 ($100) is primarily a cost-cutting measure, but it will add a lot of weight to your kit. The vast majority of backpackers use a down sleeping bag, and for good reason.

Nemo Disco Endless Promise 30 Sleeping Bag leaking down

Fill power measures the loft and warmth that down provides—essentially, it indicates overall quality. At the low end of the quality—and price—spectrum, budget backpacking bags such as Kelty Cosmic use 550-fill-power duck down, mid-range bags run more in the 650- to 750-fill-power range, and premium models start at 800-fill power. At the very high end, the Feathered Friends Swallow UL employs ultra-lofty 950-fill-power down, and you’ll pay for the privilege.

Fill power measures the quality of down and is the more promoted metric, but fill weight is just as important—it measures the amount of down in a bag and is a better indicator of how warm a specific model is (along with the temp rating, of course). Just because two sleeping bags contain 800-fill power down doesn’t mean they insulate equally; the model stuffed with 1 pound of down will be warmer than the one that has a fill weight of 12 ounces. (If two bags have different fill powers but also different fill weights, the comparison is a little harder to make.) We provide both the fill power and fill weight for all sleeping bags we review when possible.

In addition to down quality, the shell that keeps it all enclosed is another factor in overall sleeping bag weight. As with many categories of outdoor gear, shell fabrics run the gamut from thin and light to thick and heavy. Backpackers who want a light sleeping bag should target one with a 10- or 12-denier shell and accept the tradeoff that they’ll have to treat it as carefully as possible. Budget bags often use thicker and more durable materials, such as the 50-denier fabric on the Kelty Cosmic. For packability, choose a bag with a light and thin fabric; for durability and saving money, go thicker and heavier.

Hood of the Kelty Cosmic 20 Sleeping Bag

Sleeping bag shapes—also called silhouettes—vary widely across the category. On one end, you have narrow mummy bags meant for people who don’t toss and turn and don’t mind being bundled in a tight cocoon. These bags are the most thermally efficient shapes around—less space to fill means less heat lost—and so you can take advantage of a warm bag that might not weigh all that much.

Conversely, roomier bags with semi-rectangular or even rectangular shapes are better for side-sleepers or folks who move around a lot at night. These larger models require more fabric and insulation to build, so they’re by and large heavier than mummy bags (and less thermally efficient).

A sleeping bag’s dimensions are commonly measured at the shoulders, hips, and footbox (some manufacturers skip this last one). The given measurement refers to the circumference of the interior of the bag at that point. In our specs, you’ll see these shoulder / hip / footbox girths referenced in this format: 60 / 56 / 38 inches.

Jumping for joy in the Therm-a-Rest Boost 650 20°F Sleeping Bag

Sitting in Sea to Summit Spark 15 sleeping bag watching sunset

What About Sleeping Quilts?

Sleeping quilts are a favorite among ultralight backpackers and a great option for those who sleep warm or camp in mild climates. Unlike traditional sleeping bags, quilts replace the full-length zipper with an open-back design that drapes over the body, while still featuring an enclosed footbox for warmth. The idea is simple: Any down compressed beneath you doesn’t provide insulation—loft is key to warmth—so eliminating it reduces weight without sacrificing much heat retention. While this design might seem prone to drafts, most quilts stay surprisingly secure thanks to sleeping pad attachment straps and a snug closure around the neck.

Quilts aren’t subjected to ISO testing, so it’s difficult to give an apples-to-apples comparison between a 20-degree sleeping bag and a 20-degree quilt. However, looking at the weight specs does give a juicy take: The Enlightened Equipment Enigma 20 packs 13.8 ounces of 950-fill down into a 1 pound 3.3 ounce package, while an ultra-efficient bag like the Feathered Friends Swallow UL 20 features 16.8 ounces of 950-fill down in a 1 pound 11.3 ounces. A little math reveals that the quilt offers a better warmth-to-weight ratio, if the advertised weight of 20 degrees is correct for both models. Not only that, but the referenced model of the Enigma will cost you $430, while the Swallow UL is a whopping $629.

Choosing between a quilt and a sleeping bag ultimately comes down to preference. Quilts are lightweight, allow better airflow on warm nights, and eliminate unnecessary insulation underneath, as your pad provides that warmth. However, they require some adjustments—such as wearing a hat for warmth, managing potential gaps where cold air can seep in, and ensuring a secure attachment to the sleeping pad. For many backpackers, a mummy bag remains the simplest and coziest option, but quilts are an excellent alternative for those looking to reduce weight, especially in mild conditions.

Warmth-to-Weight Ratio

This metric rolls all the factors we discussed above into one easily scannable tidbit. We’ve included a warmth-to-weight score for every sleeping bag in our guide, and you can use our handy “Sort By” function to rearrange all of the products by this rating.

In terms of specific sleeping models, it’s no surprise that you’ll see bags with high fill powers, thin shell fabrics, and minimalist shapes at the top. On the other end of the spectrum are more budget-oriented bags with lower-quality down, thicker materials, and roomier cuts. Below is a preview of some of the highest-rated models we’ve tested in warmth-to-weight ratio, and you can read the full guide for more.