Ultralight Sleeping Bags in 2026: How to Buy Warmth by the Gram

The Kelty Cosmic 20 in our catalogue publishes three numbers that between them explain how sleep insulation is made and sold: a finished weight of 1106 g (2 lb 7 oz, Regular), a fill weight of 464 g of 550-fill-power down inside it, and a 20°F / -7°C figure that Kelty states as its own rating — the maker publishes no ISO 23537 comfort or limit result for it. Weight is measured, fill weight predicts warmth, and the temperature number is a claim until a standard is named. Every bag on the market hands you some version of those three numbers, published with wildly uneven honesty.

This guide is built on reading them. It covers the bags we stock and the numbers behind them, how to read a temperature rating anywhere, why fill weight beats fill power as the number to hunt for, the per-size pricing trap that catches online buyers, and the pad pairing that decides whether any bag performs to its label.

The bag we carry: the Kelty Cosmic 20 (1106 g, 39.0 oz Regular, MSRP $189.95) is a 550-fill-power down mummy with a recycled 20D nylon taffeta shell, a published fill weight of 464 g, and per-size pricing Kelty publishes plainly — Short at $179.95, Regular at $189.95, Long at $209.95 MSRP. Kelty states its 20°F figure as the manufacturer's own rating rather than an ISO 23537 test, so plan a real margin below it. At its list price it is one of the least expensive routes to genuine down warmth on the market, and the published fill weight — a figure most brands omit — is the reason we can say that with a straight face.

Skip it if you are building a single-digit-pound base weight: at 39.0 oz it is roughly double the weight of what high-fill-power quilt construction achieves, and the sub-2-pound section below tells you how to vet that upgrade wherever you shop. And whatever bag you buy, budget for the ground first — the NEMO Tensor All-Season pad (500 g, MSRP $219.95, an R-value NEMO publishes as 5.4) is the shoulder-season pairing that lets a three-season bag actually deliver its rating.

A bag rated under the ISO 23537 standard carries two numbers: a comfort rating, the temperature at which a cold sleeper sleeps, and a limit rating, roughly where a warm sleeper is surviving rather than sleeping. Buy to comfort. A bag sold on a single number with no standard named — like the Cosmic 20's manufacturer-stated 20°F — is making a claim, not reporting a test, and the practical response is to add margin: treat a manufacturer's 20 as a tested 30 until your own nights tell you otherwise.

The corollary is that two bags wearing the same number are not the same warmth. An ISO 23537 comfort 20 and a marketing 20 can sit a full season apart in practice. When a maker does publish ISO figures, they are the most comparable numbers in this market — demand them first, and read a bare number as the optimistic end of the range.

Fill power is the most misread spec on the hang tag. It measures loft per ounce of down — 550-fill lofts to 550 cubic inches per ounce, 850-fill to 850 — so it describes down quality, not bag warmth. A bag with a lot of 550-fill can be warmer than a skimpy 850-fill bag; it will just be heavier and bulkier at the same warmth. That is precisely the Cosmic 20 trade: inexpensive warmth at a weight penalty, honestly labelled.

The number that settles warmth arguments is fill weight — how many grams of down are actually inside — and most brands do not publish it. Kelty does for this bag: 464 g in the Regular, inside a 1106 g total. When you comparison-shop, hunt the fill weight before the fill power, and treat a maker that publishes it as one point up on candour.

Nearly every bag on the market comes in two or three lengths, each with its own price and its own weight — the Cosmic 20 lists Short at $179.95, Regular at $189.95 and Long at $209.95 MSRP, and its 1106 g figure is the Regular's, not the family's. Most product pages quote one headline price and one headline weight for the whole family, which means the number on your screen may belong to a size you are not buying. The 38.6 oz one retailer quotes and the 39.0 oz another quotes can both be true — of different trims and different lengths.

The check takes thirty seconds: find the maker's own per-size price list, and confirm the price at your retailer matches the size you put in the cart. Every listing on this site is verified to a single configuration before it may appear — the Cosmic 20 page links a listing whose price matches Kelty's Regular exactly — and that same one-price-one-size match is worth demanding anywhere you shop.

The catalogue now carries twenty-two Western Mountaineering bags and quilts, every one verified size by size against WM's own per-size price list and WM's own per-size metric weights — the exact one-price-one-size discipline the previous section told you to demand, applied to a whole line. Two things to know before the figures: every WM bag here is filled with 850+ fill power goose down, and WM states its temperature ratings as its own figures — it publishes no ISO 23537 comfort or limit results — so read each number with the margin the ratings section above prescribes.

The warm-weather end is where the line is lightest. The Everlite (WM's stated 45°F) weighs 410 g in its 180 cm size and 495 g in the 200 cm, at $415 and $440 list respectively. The Nanolite Top Quilt (stated 38°F) is the lightest sleep insulation in the catalogue at 370–402 g across its two sizes, $455–470. The Summerlite (stated 32°F) runs 510–595 g across six sizes at $535–570, and the Caribou MF (stated 35°F) 565–650 g at $490–515. All four sit comfortably under the classic 2 lb line in every size WM sells.

The three-season core is the stated-20°F pair: the Ultralite (795–880 g across six sizes, $660–705) and the wider-cut Alpinlite (820–995 g across seven, $740–815) — same rating, same fill, the difference is girth. The Megalite (stated 30°F, 650–710 g, $615–650) trades a season of warmth for another 100 g saved. Below 20°F the weights and prices climb honestly with the fill: Versalite (stated 10°F, $780–815), Apache MF (15°F, $715–755), Kodiak MF (0°F, $920–980), Antelope MF (5°F, $820–870), the Lynx pair (−10°F, from $1030), the Puma MF (−25°F, $1125–1200) and the expedition Bison GWS (stated −40°F, 2095–2180 g, $1400–1440). Semi-rectangular sleepers get the same fill in a roomier cut: Alder, Sycamore, Terralite, Ponderosa and Sequoia, stated 25°F down to 5°F.

One honest gap: WM publishes per-size total weights for these bags, and our records carry those; it does not publish a per-bag fill weight on the pages we verified, so this guide does not quote one. Each product page links the size-specific listing it was verified against — the price you see is the price of the size you click.

Beyond the Western Mountaineering line and the Kelty value anchor, the catalogue now carries seven bags and quilts from big names you can buy on Amazon today, each verified to a single Amazon listing at its own price: NEMO, Sea to Summit, Mountain Hardwear and Therm-a-Rest. Every temperature figure below is the manufacturer's own rating — none of these makers publishes an ISO 23537 comfort or limit result on the listings we verified — so read each with the margin the ratings section prescribes.

NEMO builds the roomiest bags here on its Spoon shape, cut wider at the knees and elbows for side sleepers. NEMO states 20°F for the synthetic Tempo ($199.95) and the synthetic Forte ($259.95), and 15°F for the down Riff ($449.95). The Tempo and Forte use recycled synthetic fill that keeps insulating when damp; the Riff's 800-fill-power down trades that wet-weather tolerance for less weight and bulk at the same stated warmth.

Sea to Summit and Mountain Hardwear anchor the down field. Sea to Summit rates the Spark Pro ($549.00) at 30°F on 900+ fill-power goose down — about the highest-lofting down in production, which is why it is the lightest bag in this group — and rates the value-cut Trek ($468.02) at the same 30°F on 650+ fill. Mountain Hardwear states 30°F for the Bishop Pass ($314.99), a 650-fill down mummy and the least expensive name-brand down bag in the catalogue.

For quilt sleepers, Therm-a-Rest's Vesper ($439.95) is the one Amazon-sourced quilt here: Therm-a-Rest states 32°F on 900-fill Nikwax hydrophobic down, with an insulated footbox and pad-connector straps. Like every quilt it makes the pad's R-value non-negotiable — there is no back panel between you and the ground. Down or synthetic, premium or value, the discipline is the one this guide keeps returning to: the price you see is the price of the size on that listing, and a bare temperature number is a claim until a standard is named.

The classic target for ultralight sleep insulation is 2 lb — 907 g — and the Cosmic 20 at 1106 g is not that bag; it is the value anchor those builds are measured against. Bags and quilts get under the line three ways: higher fill power (more loft per gram of down), quilt construction (deleting the hood, zipper, and the back panel you compress flat anyway), and thinner shell fabrics. Each is a real trade — cost, draft management, and durability respectively — not a free lunch.

That kit is no longer hypothetical here: six Western Mountaineering families in the section above sit under the line in every size WM sells — Nanolite Top Quilt (to 402 g), Everlite (to 495 g), Summerlite (to 595 g), Caribou MF (to 650 g), Megalite (to 710 g) and even the stated-20°F Ultralite (to 880 g) — and the 850+ fill power against the Cosmic 20's 550 is precisely the first of the three routes in action, at two to four times the Cosmic's list price. If you are shopping the class elsewhere, the checklist stands: demand ISO 23537 figures or treat the rating as a claim, find the fill weight, confirm the price and weight belong to the length you are buying, and weigh the finished product against its published spec.

Whatever bag you buy, the pad under it sets your floor: compressed down insulates almost nothing, so on cold ground the pad's R-value — its published resistance to heat loss into the ground — matters as much as the bag's rating. Pair the Cosmic 20 with a pad in the published R 4 to 5.5 class, like the NEMO Tensor All-Season (MSRP $219.95, published R 5.4), for shoulder-season frost, and see our sleeping pad guide for the full field including the stacking math that gets you to winter numbers for MSRP $49.95 of foam.

Skip the upgrade cycle entirely if your current bag keeps you warm: a clean, well-stored bag holds its loft for years, and our down-care guide covers the washing and storage routine that protects the loft you already paid for.

Is 550-fill down bad? No — it is inexpensive down. It insulates by the same mechanism as 850-fill and simply needs more grams of feathers to build the same loft, which is why the Cosmic 20 weighs 39.0 oz where premium bags in its warmth class weigh less. If pack weight is not your binding constraint, 550-fill is the value play.

Quilt or mummy bag? A quilt deletes the back panel and hood to save weight and relies on your pad and a strap system for the seal, which makes pad R-value non-negotiable and adds a learning curve of a night or two. A mummy like the Cosmic 20 is the forgiving option: hood, full zip, no draft management to learn.

Down or synthetic? Down gives more warmth per gram and lasts longer; synthetic keeps more of its warmth when soaked and costs less. Water-resistant down treatments are the industry's hedge — a coating on the plumes — but no treatment makes down shrug off a wet night the way synthetic fill does.