Sleeping Bags & Quilts
The bag is the part of your sleep system everyone thinks about first — but the spec that matters is the ISO 23537 rating, not the headline number on the box. This silo compares backpacking sleeping bags and quilts on the things that decide warmth and weight: Comfort vs Limit rating, down vs synthetic fill, fill power, mummy vs quilt, and how light you can go without sleeping cold. Specs are verified against manufacturer details and current retailer listings; no hands-on testing is claimed.
Remember the brand's whole hook: warmth is the bag and the pad as one system. A great bag on a cold pad still sleeps cold. Read this silo alongside the pads hub, and run your trip through the sleep-system matcher to size the bag rating and pad R-value together.
The spec that matters: the ISO 23537 Comfort rating
If you compare bags on one number, make it the ISO 23537 Comfort rating. The standard puts a sensor-covered, heated mannequin in the bag and measures the insulation, then reports two figures: the Comfort rating (the lowest temperature a cold sleeper stays comfortable) and the Limit rating (the lowest a warm sleeper avoids shivering). The box almost always headlines the warmer-sounding Limit number, which is why so many first bags feel colder than expected. Shop by Comfort, pick a bag whose Comfort figure sits at or below your coldest realistic overnight low with a little margin, and treat the Limit as marketing. Cold sleepers and most women should ignore the Limit entirely. These are lab figures — real-world warmth varies with the sleeper, the pad, the shelter and the conditions, so a rating compares bags but never guarantees a night.
Down vs synthetic fill
The fill decides most of a bag's weight, pack size, price and wet-weather behaviour. Down — the soft under-plumage of ducks and geese — traps the most air for its weight, so it gives the best warmth-to-weight and packs down smallest, and it lasts for years if kept dry. Its weakness is moisture: soaked down clumps, loses loft and stops insulating until it dries. Synthetic fill is heavier and bulkier for the same warmth and loses loft faster over the years, but it keeps insulating when damp, dries quickly and costs less. The decision is really about how wet your trips get: dry alpine and high-desert nights favour down; persistent rain, heavy condensation and snow-melt shoulder seasons favour synthetic. There is no universal winner, which is why a lot of us end up owning one of each.
Fill power: down quality, not warmth
For down bags you will see a fill power number — 650, 800, 900. It measures the down's quality, not the amount: how many cubic inches one ounce of that down lofts to. Higher fill power traps more air per ounce, so a higher-fill-power bag reaches the same warmth with less weight and packs smaller — at a higher price. Do not read fill power as a warmth rating, though: total warmth depends on how much down is in the bag, which is why two bags at the same fill power can carry different ratings. Judge warmth by the Comfort rating and read fill power as how much weight and pack volume you are paying to save. For most people 650–750 fill is the value sweet spot; 800-plus earns its premium mainly for dedicated gram-counters and cold-weather trips.
Mummy vs quilt: the shape decision
A mummy bag wraps fully around with a snug fit, a draft collar and an insulated hood, which traps the most heat and makes it the cold-night choice. A quilt deletes the crushed-flat underside (and usually the hood and zipper) and straps to your pad instead, saving real weight and pack size — but it leaves all the under-body warmth to the pad, so it only works when the pad's R-value is genuinely up to the season. Quilts shine for warm-to-3-season ultralight trips and suit restless and side sleepers who hate feeling wrapped up; mummies win when it is genuinely cold and you want a sealed hood and collar. Start from the trip's coldest night and the shape usually picks itself.
Length, width and fit
The same rating sleeps warmer in a bag that fits. Too long and you have a cold, empty footbox your body has to heat; too short or too narrow and the insulation compresses against you into cold spots. Most bags come in regular and long lengths and many in a women's-specific cut with extra insulation in the footbox and torso where colder sleepers feel it. Side sleepers and anyone who tosses and turns should look hard at quilts and roomier "spoon"-shaped bags. Getting length and width right is a free upgrade — no extra weight, more warmth.
The bag is only half the system
Worth repeating because it is the brand's whole hook: warmth is the bag and the pad as one system. When you lie down, the fill beneath you crushes flat and does almost nothing, so the cold ground pulls heat out of you through the pad — which is why a warm bag on a cold pad still sleeps cold. Buy the bag and the pad as a matched pair, and let the sleep-system matcher set the bag Comfort rating and the pad R-value together. To compare candidate bags by how much warmth they deliver per gram, run their rating and packed weight through the warmth-to-weight comparator. And to see the full beginner walkthrough, read the sleep-system guide.
Featured guides
The current published guides in this silo. More land each batch.
Landing next: Down vs synthetic sleeping bags, Sleeping bag vs quilt, Best down sleeping bags, and Mummy sleeping bag guide.
Frequently asked questions
What is the difference between the Comfort and Limit temperature rating?
The ISO 23537 Comfort rating is the lowest temperature a cold sleeper stays comfortable; the Limit rating is the lowest a warm sleeper avoids shivering. Both are lab figures. Most people, and cold sleepers in particular, should buy to the Comfort rating, not the optimistic Limit.
Down or synthetic fill?
Down packs smaller, weighs less and lasts longer for the same warmth, which is why it dominates ultralight kits. Synthetic keeps more of its warmth when damp and costs less, making it the safer pick for wet shoulder-season trips. It is a trade-off between weight and wet-weather reliability.
Mummy bag or quilt?
A mummy bag wraps all the way around and has a hood, so it is warmer in the cold. A quilt drops the crushed-flat underside and the hood to save weight, relying on your pad for the underside warmth. Quilts shine for warm-to-3-season ultralight use; mummies win when it is genuinely cold.