An ISO 23537 rating comes from a standardized heated manikin test and publishes two figures: a comfort rating and a lower limit. A single number with no standard named beside it is the manufacturer's own claim, and across the budget end of the category it typically runs 15 to 20 degrees optimistic. Read the label for the word ISO or EN before you read the temperature.
This is not an accusation of dishonesty. ISO testing costs real money per model, and a cheap bag cannot absorb it. But it does mean that a 20 F bag from a maker that tests and a 20 F bag from one that does not are describing different things.
The practical rule: if a bag publishes ISO figures, plan around the comfort number. If it publishes one manufacturer number, add 15 to 20 degrees and plan around that. The full explanation is in sleeping bag temperature ratings explained.
Compared on published numbers
Bag
Rating as published
Standard
Fill
Weight (regular)
Kelty Cosmic Down 20
comfort 31 F / limit 21 F
ISO
550-fill down
2 lb 7 oz
Marmot Trestles Elite Eco 20
limit 20.3 F
ISO
HL-ElixR Eco synthetic
3 lb 8.4 oz
Kelty Cosmic 0 Degree
0 F maker's; 4 F EN
EN, alongside a maker's figure
550-fill DriDown
4.34 lb
Figures as published by each manufacturer.
Note what weight tracks. Roughly the same tested performance costs 2 lb 7 oz in down and 3 lb 8.4 oz in synthetic. That pound is the price of a fill that still works when damp.
Comfort rating and limit rating are not the same
The ISO test produces two figures because it models two different sleepers. The comfort rating is the temperature at which a standard adult woman sleeps comfortably in a relaxed posture. The limit rating is where a standard adult man sleeps curled up, on the edge of being cold, without waking.
Most people should buy on the comfort number, not the limit. A bag advertised as a 20 degree bag with an ISO comfort rating of 31 F is a bag for nights in the thirties, and treating the 21 F limit as a target is how people have bad nights in perfectly good equipment.
The pad decides the night more than the bag does
Body weight crushes the insulation underneath you completely flat, so whatever the bag is rated to, the warmth on the ground side comes from the pad's R-value. A 20 degree bag on an uninsulated pad is a cold night, and a 30 degree bag on an R 4.5 pad is usually a comfortable one. If your budget is fixed, spend it on the pad first — the arithmetic is in best backpacking sleeping pads and why you get cold in a sleeping bag.
Fill power, and what 550 means
Fill power measures how much volume an ounce of down occupies. Higher fill power lofts more per ounce, so a 800-fill bag reaches the same warmth as a 550-fill bag with less down and therefore less weight and bulk.
550 is the entry point for real down and it is a perfectly good place to be. The penalty is pack volume rather than warmth: both Kelty bags here get to their ratings, they just take more room in the pack doing it. Paying for 800-fill buys space and grams, not degrees.
Length, and the most common sizing mistake
A bag that is too long is colder, not roomier. The empty volume at your feet is air your body has to heat, and there is a lot of it in a bag sized four inches above your height. Kelty publishes a fits-to height of 6 ft for the regular Cosmic 0 and 6 ft 6 in for the long; buy the size that matches you rather than the one that sounds generous.
If a bag is already too long, stuff the foot end with tomorrow's clothing. It fills the space, warms the clothes, and costs nothing. A compression sack is the other purchase that makes a bulky bag workable — and see best compression sacks and stuff sacks.
Which to buy
Buy the Kelty Cosmic Down 20 for three-season backpacking; it is the lightest genuinely tested bag at this price. Buy the Marmot Trestles Elite Eco 20 if it will get damp or lent out. Buy the Kelty Cosmic 0 only if your trips genuinely go below freezing, because 4.34 lb is a lot of bag to carry in July.
ISO comfort 31 F and limit 21 F on 550-fill down, at 2 lb 7 oz in the regular.
Kelty publishes an ISO comfort rating of 31 F, an ISO limit of 21 F, 550-fill-power down, a regular weight of 2 lb 7 oz, a regular fill weight of 16.4 oz, a recycled 50D polyester taffeta shell, a recycled 20D nylon taffeta lining and a mummy shape.
Two tested numbers rather than one claimed one is the reason this is the pick. At this price almost nothing else publishes ISO figures, and a comfort and limit pair tells you what the bag actually does rather than what the marketing department chose.
The drawback
550 fill power is modest, so it is bulkier than a premium down bag at the same warmth. Down also loses loft when wet, and no treatment fully prevents that.
An ISO limit of 20.3 F on synthetic HL-ElixR Eco fill, at 3 lb 8.4 oz in the regular, in fully recycled PFC-free materials.
Marmot publishes an ISO limit of 20.3 F / -6.5 C, weights of 3 lb 4.8 oz short and 3 lb 8.4 oz regular, HL-ElixR Eco Micro synthetic insulation, 100 percent PFC-free recycled shell, lining and fill, and a wave-construction top with a blanket-construction bottom and a 3D footbox.
Synthetic fill keeps most of its loft when damp and washes with ordinary detergent, which is why it is the right choice for the Pacific Northwest, for anyone camping near water, and for a bag that gets lent out. The comparison is in down vs synthetic sleeping bags.
The drawback
A pound heavier than the Kelty for a similar tested rating, and it packs noticeably larger. Synthetic always costs you weight and volume.
An EN rating of 4 F against a 0 F maker's figure, on 550-fill DriDown at 4.34 lb in the regular.
Kelty publishes a 0 F / -18 C maker's rating alongside an EN rating of 4 F / -16 C, 550-fill DriDown with a water-resistant treatment, a regular fill weight of 39.5 oz, a regular total weight of 4.34 lb, a fits-to height of 6 ft in the regular and 6 ft 6 in in the long, and a full draft collar, draft tube and thermal comfort hood.
Publishing both numbers is unusually transparent and it illustrates the point of this page: the tested figure and the label figure are four degrees apart on the same bag.
The drawback
4.34 lb is nearly twice the Cosmic Down 20 for a bag you will not need most of the year. The gap between the 0 F claim and the 4 F EN result is also worth reading carefully.
No product photo: we don't take our own
Published specs
Maker's rating
0 F / -18 C
EN rating
4 F / -16 C
Fill
550-fill DriDown, water-resistant treated
Fill weight (regular)
39.5 oz / 1,120 g
Total weight (regular)
4.34 lb / 1.93 kg
Fits to
6 ft 0 in regular; 6 ft 6 in long
Features
full draft collar, draft tube, thermal comfort hood
QWhat temperature rating do I need in a backpacking sleeping bag?
Buy on the ISO comfort rating rather than the limit, and match it to the coldest night you expect. A bag sold as 20 degrees with an ISO comfort rating of 31 F is a bag for nights in the thirties, not for 20 F.
QWhat is the difference between ISO comfort and limit ratings?
The comfort rating is where a standard adult woman sleeps comfortably in a relaxed posture; the limit is where a standard adult man sleeps curled up on the edge of being cold. Most people should buy on comfort.
QIs 550 fill power good enough?
Yes, for warmth. Fill power measures loft per ounce, so a 550-fill bag reaches its rating with more down and therefore more weight and bulk than an 800-fill bag. Paying for higher fill power buys pack space and grams, not degrees.
QShould a backpacking sleeping bag be down or synthetic?
Down for weight and pack size if you can keep it dry; synthetic if it will get damp, get lent out or get washed often. In these picks the same tested rating costs 2 lb 7 oz in down and 3 lb 8.4 oz in synthetic.
How we picked
Everyone else in this category will tell you they tested twenty of these. We have not tested any of them, and we would rather say so than pretend. There is no gear closet here and no test lab.
What we did instead: pulled the published specifications from each manufacturer's own spec sheet or manual, named the standard behind every rating so you can check what it actually promises, did the arithmetic that turns a spec into a decision, and pulled today's real retail price rather than quoting one from memory. Every pick states a drawback, because a list where nothing is wrong with anything is an advert.
The limits of that method, stated plainly: published specs do not capture zipper quality, seam durability, or how a tent behaves at 2am in a storm. Where that matters, we say so on the pick. The full method is here.
