Sleeping Bags in 2026: How to Buy Warmth by the Gram
How to read a temperature rating, why fill weight matters more than fill power, per-size prices, and what the makers publish for the Kelty Cosmic 20, the Western Mountaineering line, NEMO, Sea to Summit, Mountain Hardwear and Therm-a-Rest.
On this page10 sections
Three numbers decide a sleeping bag#
Kelty publishes three numbers for the Cosmic 20 that explain how sleeping bags are made and sold: a total weight of 1,106 g (2 lb 7 oz, Regular), a fill weight of 464 g of 550-fill-power down, and a 20°F / -7°C rating. The rating is Kelty's own; it publishes no ISO 23537 comfort or limit figure.
Every bag gives you some version of these numbers, and makers publish them unevenly. This guide covers the bags in the RidgeNote catalogue, how to read a temperature rating, why fill weight matters more than fill power, the per-size price trap, and why the pad matters as much as the bag.
The short version#
The Kelty Cosmic 20 (1,106 g, 39.0 oz Regular, MSRP $189.95) is a 550-fill-power down mummy with a recycled 20D taffeta shell and a published fill weight of 464 g. Kelty lists three lengths at three prices: Short $179.95, Regular $189.95, Long $209.95. Plan on it for nights somewhat warmer than its 20°F rating.
Skip it if pack weight is your main constraint: at 1,106 g it weighs 487 g more than the Sea to Summit Spark Pro 30 (619 g, Regular), though Sea to Summit states 30°F for it against Kelty's 20°F, so it is a bag for milder nights and not a like-for-like swap. Whatever bag you buy, choose the pad with it. A pad like the NEMO Tensor All-Season (522 g packed, MSRP $219.95, R 5.4) keeps frosty ground from drawing off the warmth the bag's rating assumes.
How to read a temperature rating#
A bag rated under ISO 23537 carries two numbers. Comfort is the temperature at which a standard cold sleeper stays comfortable. Limit is where a standard warm sleeper can sleep curled up for eight hours. Buy to the comfort figure.
A bag sold on a single number with no standard named, like the Cosmic 20, carries a maker's claim, not a test result. Two bags with the same number can differ in warmth. When a maker publishes ISO figures, use those; when it does not, allow a margin and read the bare number as the optimistic end.
Fill weight beats fill power#
Fill power measures loft per ounce of down: one ounce of 550-fill lofts to 550 cubic inches, one ounce of 850-fill to 850. It describes down quality, not bag warmth. A bag with plenty of 550-fill can be warmer than a bag with a little 850-fill; it will be heavier and bulkier for the same warmth. That is the Cosmic 20's trade.
Fill weight, the grams of down inside, says more about warmth. Not every maker publishes it, and of these bags this guide gives it only for the Cosmic 20. Kelty publishes 464 g in the Regular, inside a 1,106 g bag. When you compare bags, look for the fill weight before the fill power.
One bag, three prices: check the size the price belongs to#
Most bags come in two or three lengths, each with its own price and weight. The Cosmic 20 lists Short at $179.95, Regular at $189.95 and Long at $209.95, and its 1,106 g figure is the Regular's. Many retail pages show one price and one weight for the whole family, so the number on screen may belong to a size you are not buying.
The check is quick: find the maker's per-size price list and confirm that your retailer's price matches the size in your cart. The Cosmic 20 listing on RidgeNote is pinned to the Regular: its price matches Kelty's Regular list price and neither other size.
Western Mountaineering, by rating#
The catalogue carries twenty-two Western Mountaineering bags and quilts. All use 850+ fill-power goose down, per WM. WM states its temperature ratings as its own figures and publishes no ISO 23537 comfort or limit results, so read each with the margin described above.
For the warm-weather end, WM states 45°F for the Everlite, 38°F for the Nanolite Top Quilt, 35°F for the Caribou MF and 32°F for the Summerlite. For three-season use, WM states 30°F for the Megalite and 20°F for both the Ultralite and the wider-cut Alpinlite. For colder trips, WM states 15°F for the Apache, 10°F for the Versalite, 5°F for the Antelope, 0°F for the Kodiak MF, -10°F for the Lynx, -25°F for the Puma MF and -40°F for the expedition Bison GWS. For a roomier cut, WM states 25°F for the Alder, Sycamore and Terralite, 15°F for the Ponderosa and 5°F for the Sequoia.
WM publishes a weight and a price for each size, and each WM product page on RidgeNote lists both size by size, so compare the length you will buy rather than one headline figure.
NEMO, Sea to Summit, Mountain Hardwear and Therm-a-Rest#
Seven of the catalogue's bags and quilts from these four makers are below. Every temperature figure below is the maker's own rating. Sea to Summit's two are ISO 23537 lower limits from its own testing; read the others as the makers' stated ratings.
NEMO's Tempo and Forte are synthetic bags in NEMO's Spoon shape, cut wider at the elbows and knees. NEMO states 20°F for both. The Riff is an 800-fill-power hydrophobic down bag, also in NEMO's Spoon shape, and NEMO states 15°F for it.
Sea to Summit states 30°F for the Spark Pro (900+ fill-power goose down, 619 g in the Regular), the lightest bag in this group, and 30°F for the tapered-rectangular Trek (650+ fill, 1,197 g packed). Mountain Hardwear states 30°F for the Bishop Pass (650-fill down, about 810 g per Mountain Hardwear).
Therm-a-Rest's Vesper is the only quilt among these seven; Therm-a-Rest states 32°F for it, with 900-fill Nikwax Hydrophobic Down. Like any quilt it has no back panel, so the pad's R-value is your only insulation from below.
The sub-2-pound question#
A common target for light sleep insulation is 2 lb, or 907 g. The Cosmic 20 at 1,106 g is over it. Bags get under the line three ways: higher fill power (more loft per gram), quilt construction (no hood, zip or back panel), and thinner shell fabric. Each costs something: price, draft control, and durability.
In this catalogue, Sea to Summit publishes the Spark Pro 30 at 619 g in the Regular, using 900+ fill-power down, and the Therm-a-Rest Vesper takes the quilt route. Western Mountaineering publishes the Summerlite at 539 g (6 ft 0 in), with 850+ fill down; check WM's per-size weights for the others. Wherever you shop: look for ISO 23537 figures or treat the rating as a claim, find the fill weight, and confirm the price and weight belong to the length you are buying.
A bag is half of a sleep system#
The fill under you is flattened by your weight and insulates little, so on cold ground the pad's R-value matters as much as the bag's rating. For frosty nights with the Cosmic 20, a pad in the R 4 to 5.4 shoulder-season band fits, such as the NEMO Tensor All-Season (MSRP $219.95, R 5.4). The sleeping pad guide covers the full range, including how a foam pad like the Therm-a-Rest Z Lite Sol (R 2.0, MSRP $59.95) under an air pad adds its R-value.
If your current bag keeps you warm, you may not need a new one. A clean, well-stored bag keeps its loft for years; the down-care guide covers washing and storage.
FAQ#
Is 550-fill down bad? No. It insulates the same way as 850-fill but needs more grams for the same loft, one reason the Cosmic 20 weighs 1,106 g. It usually costs less, so it suits buyers for whom weight is not the main constraint.
Quilt or mummy bag? A quilt drops the back panel and hood to save weight and relies on the pad and straps to seal out drafts, so pad R-value matters more. A mummy like the Cosmic 20 has a hood and full zip and does not need straps to the pad to seal out drafts.
Down or synthetic? Down gives more warmth per gram and packs smaller; synthetic keeps more of its warmth when wet and usually costs less. Water-resistant down treatments help, but treated down still loses more warmth when soaked than synthetic fill does.