Sleeping Bag Temperature Ratings, Decoded

Before you buy a sleeping bag, find the comfort rating on the spec sheet and compare it with the coldest night you realistically expect. That one habit avoids most cold nights. Here's what the numbers mean and why they don't tell the whole story.

Where the numbers come from

For years, many bags sold in Europe and beyond were rated under EN 13537, introduced in 2005, according to Wikipedia's sleeping bag article. That standard has since been replaced by the international ISO 23537 standard. The current edition, ISO 23537-1:2022, was published in March 2022 and covers adult sleeping bags for sports and leisure use with limit temperatures of -20°C (-4°F) and higher, according to the standard's listing at Genorma. It doesn't apply to bags for children, babies, military use or extreme-climate expeditions.

The ratings come from a lab test, not a person in a tent. A heated, sensor-covered manikin lies in the bag. Cumulus says the manikin it describes has 16 heaters and temperature sensors, and that the standard defines both the thin clothing layer the manikin wears, comparable to lightweight thermal underwear, and the mat beneath it. Rab describes the manikin lying on a standard foam roll mat during testing.

Because every lab follows the same method, the result is a fair way to compare one bag with another. It is not a promise about your night.

Comfort, limit and extreme

ISO-rated bags list up to three cold-side numbers. Here's how Cumulus describes them:

• Comfort: the temperature at which a "standard woman" can sleep in a relaxed position without feeling cold. The standard woman is defined as 25 years old, 60 kg (about 132 lb) and 160 cm (about 5'3") tall.
• Limit: the lowest temperature at which a "standard man" can sleep curled up without feeling cold. The standard man is 25 years old, 70 kg (about 154 lb) and 173 cm (about 5'8") tall.
• Extreme: a survival rating. The standard says these conditions shouldn't be endured for more than six hours, and it assumes severe cold stress with shivering. Rab adds that frostbite is very possible at that temperature.

The space between comfort and limit is where you'll feel cool but should still be able to sleep. Below the limit, you're in survival territory.

Why to buy by the comfort rating

Plan around comfort, and use the limit only if you know you sleep warm. Great Outdoors puts it simply: if you're a cold sleeper, the comfort rating is the best indicator for you; if you sleep warm, the limit rating may be enough. If you're new to camping in the cold, you probably don't know yet which kind of sleeper you are, so comfort is the safer bet.

Two more reasons to lean on comfort:

1. Real nights are harder than lab nights. Cumulus points out that the lab removes things like wind, humidity, fatigue and cold ground. All of those make you feel colder.
2. Brands put different numbers in product names. Rab, for example, says it uses the comfort rating in its product names. Other brands may lead with the limit. A bag called "20" might mean either, so check the spec sheet before you compare names.

And never plan a trip around the extreme rating. It is an emergency number.

Your pad is half of the system

Switchback Travel describes the bag and pad as working "as a system." The ISO test puts the manikin on an insulated mat, and Great Outdoors notes that sleeping bags lose significant warmth when there's no barrier between you and the ground. Put a rated bag on a thin or uninsulated pad and you shouldn't expect the warmth the label suggests.

Check the pad's R-value, a measure of how well it resists heat flowing into the ground. Pad makers began developing a shared test method in 2016, and the standard, ASTM F3340, was published in 2019, according to Cascade Designs. Switchback Travel offers these rough guidelines:

• Summer: R 1 to 3
• Three-season: R 3 to 5
• Winter: R 5 and up

Switchback also notes that R-values are additive when you stack pads, and it reports that Therm-a-Rest's designers say you need a pad of at least R 4 to get the most out of your bag's temperature rating. The National Park Service adds that a pad keeps moisture from accumulating beneath your body, where it would rob you of heat, and that foam pads are the least expensive option.

Liners and what you wear

A sleeping bag liner adds a thin layer inside the bag and keeps body oils and dirt off the bag itself. Retailer Wildfire Sports notes that some liners are designed to add warmth, while a light silk-blend liner "does not add much warmth." Warmth claims vary widely by brand and material, so treat any degree figure on the package as approximate rather than an add-on to the ISO rating.

Clothing works the same way. The manikin wears only a thin base layer and, Great Outdoors notes, wearing thermals can extend a bag's warmth. A dry hat and dry socks help too. The NPS also suggests changing into dry clothes before bed, since damp layers pull heat from your body.

Down vs. synthetic

Both kinds of bag carry the same ISO ratings, but they behave differently in the real world. According to Wikipedia:

• Down is lighter for the warmth it provides and usually costs more. It needs to stay dry; a soaked down bag may insulate less than having no bag at all.
• Synthetic fill absorbs little water, dries easily and still gives some warmth when wet. It's heavier, packs less small and breaks down faster than down.

What to check: if you camp somewhere damp, or you're car camping where weight doesn't matter, synthetic is a forgiving choice. If you need a light, packable bag and can keep it dry, down earns its price.

A quick checklist before you buy

• Find the comfort rating and match it to your expected low.
• Confirm which number, comfort or limit, is in the product name.
• Check that your pad's R-value suits the season, or plan to stack a foam pad.
• Decide on down or synthetic based on how wet your trips usually get.
• Treat liner warmth claims as a bonus, not a substitute for the right bag.

Sources

• Wikipedia: Sleeping bag
• Genorma: ISO 23537-1:2022 listing
• Cumulus: How down sleeping bags are tested
• Rab: Sleeping Bag Temperature Ratings Explained
• Great Outdoors: Sleeping bag temperature ratings explained
• Cascade Designs: The ASTM R-Value Standard
• Switchback Travel: Sleeping Pad R-Value
• National Park Service: What to Bring – Camping
• Wildfire Sports: How to choose a sleeping bag liner

Last verified: November 1, 2026.