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Ultralight sleeping bag: compare grams and the right rating

By the Plain Prepper editors. Updated 2026-10-15.

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Quick answer

Ultralight sleeping bags save weight with high fill power down and, in some designs, by dropping the zipper or using a quilt. Compare the weight against the comfort rating, not the name. Sierra Designs' Cloud 20 weighs 2 lb with an ISO comfort rating of 26°F. Therm-a-Rest's Vesper 20F quilt weighs 1 lb 3 oz but lists 32°F for comfort. Naturehike's CW400 weighs 2 lb and lists 40.4°F comfort. The number in a name can be the limit rating, as with the Vesper 20F.

Three light options compared

Spec	Therm-a-Rest Vesper 20F quilt	Sierra Designs Cloud 800F 20	Naturehike CW400
Weight (regular)	1 lb 3 oz (550 g)	2 lb (0.91 kg)	2 lb (0.9 kg)
Comfort rating	32°F (EN)	26°F (ISO)	40.4°F
Limit rating	20°F (EN)	15°F	31.2°F
Fill	900-fill goose down, 12.5 oz	800-fill DriDown, 14.8 oz	650-fill duck down, about 400 g
Packed	5.5 x 8 in	7.5 x 7.5 x 16 in	14 x 7 x 7 in
Fits	To 72 in	To 6 ft 0 in	87 x 33 in bag
Design	Quilt with pad connector	Zipperless mummy with built-in comforter	Mummy top, rectangular foot

The Vesper weighs about 60% of the Cloud (550 / 910 g), but its comfort rating is 6 degrees warmer, so the two are not equal on a cold night.

Comfort vs limit: read the right number

Lab ratings give two key numbers. REI explains that the comfort rating is where a cold sleeper might feel comfortable, and the lower limit is where a warm sleeper might. Naturehike adds that at the limit, a warm sleeper may stay in thermal balance only by curling up, and calls its extreme figure survival-only.

• Therm-a-Rest's Vesper 20F is named for its 20°F limit; its comfort rating is 32°F.
• Sierra Designs' Cloud 20 lists a 15°F limit and a 26°F comfort rating.
• Naturehike's CW400 lists 40.4°F comfort and 31.2°F limit, plus a 3.5°F extreme rating.

REI's rule is to take the coldest night you expect and add a 10 to 15 degree buffer when picking a rating.

Fill power vs fill weight

Two down numbers matter, and Naturehike notes they describe different things:

• Fill power (650, 800, 900) measures how much volume down lofts to; Naturehike says higher fill power traps more air for its weight and usually gives a better warmth-to-weight ratio.
• Fill weight is how much down is inside. The Vesper holds 12.5 oz, the Cloud 14.8 oz, and Naturehike says the CW400 holds about 400 g, which is what its name refers to.

Naturehike's point is that higher fill power alone does not make one bag warmer than another; compare both numbers with the comfort rating. REI notes hydrophobic down resists water and humidity better; Therm-a-Rest lists hydrophobic down, and Sierra Designs uses its DriDown.

Quilt or bag?

• Quilt: Therm-a-Rest builds the Vesper as a quilt with a SynergyLink connector and snap-neck closure, and says it pairs with a NeoAir pad as one sleep system. It is the lightest option here.
• Zipperless bag: Sierra Designs' Cloud has no zipper, an oversized built-in comforter, a self-sealing foot vent and a sleeve that holds your pad in place, aimed at side and hot sleepers.
• Traditional zip bag: Naturehike's CW400 uses a two-way YKK zipper and a roomier rectangular foot.

REI notes your sleeping pad affects real-world warmth, so choose the pad with the bag or quilt, not as an afterthought.

Keeping an ultralight bag ready

• Storage: REI suggests a large mesh or cotton sack that lets the insulation expand, or hanging the bag in a closet.
• Length: check the fit height; the Cloud and Vesper regulars fit to 72 in, with longs to 78 in.
• Layers: on nights near the limit rating, wear dry insulated layers and use a warm pad.
• Moisture: REI warns down loses insulating ability when wet, even as hydrophobic treatments improve resistance; keep it dry.

What to look for when you buy

Questions people ask

How much does an ultralight sleeping bag weigh?

The three here run from 1 lb 3 oz (Therm-a-Rest Vesper quilt) to 2 lb (Sierra Designs Cloud and Naturehike CW400).

Is a 20 degree ultralight bag good for 20°F?

Check the comfort rating; Therm-a-Rest's Vesper 20F lists 32°F comfort, and Sierra Designs' Cloud 20 lists 26°F.

Are quilts warmer than sleeping bags?

Compare ratings, not shapes; Therm-a-Rest's Vesper quilt lists 32°F comfort and is designed to work with a NeoAir pad.

What fill power is best for ultralight?

Higher fill power lofts more per ounce, as with 800 to 900-fill down here, but compare fill weight and comfort rating too.

Sources

• Vesper 20F/-6C Down Quilt, Therm-a-Rest (Cascade Designs)
• Cloud Zipperless 800F 20 Deg Sleeping Bag, Sierra Designs
• CW400 Down Ultralight Sleeping Bag, Naturehike
• Sleeping bags for camping: how to choose, REI Co-op Expert Advice

Sources checked 2026-10-15.