Sleeping Bag Temperature Ratings Explained (2026)

The number in a sleeping bag's name tells you less than the label on its stuff sack. That label, if the maker tested to ISO 23537 or the older EN 13537, gives a Comfort, a Limit and often an Extreme temperature, and they can be 10°F or more apart. Which one you plan around decides whether you sleep or shiver.

How we chose: we did not test these bags; we kept only bags whose makers publish both Comfort and Limit ratings on their own spec pages, sold by Amazon.com or the brand's own store and in stock on September 29, 2026.

Four bags and their published ratings

Prices are Amazon's, as of the time shown, and subject to change. Details

Ratings are the makers' own, from Therm-a-Rest's pages for the Boost 650 20F and Boost 650 32F and Sea to Summit's pages for the Ascent Women's and Trek Women's. Therm-a-Rest labels its figures EN; Sea to Summit labels its ISO 23537.

The three numbers on an ISO label

Under ISO 23537 (and EN 13537 before it), a lab puts a heated manikin in the bag, on an insulating mat, dressed in a base layer, and measures how much heat the bag holds in. Rab's explainer defines the three results:

• Comfort: "the minimum temperature where an individual can sleep in a relaxed position and get a good night's sleep."
• Limit: "the lowest temperature where a person sleeping in a curled position and fighting against the cold can still sleep through the night."
• Extreme: "for an emergency scenario – it's the minimum temperature a person can remain in the sleeping bag for up to six hours in a curled position while really fighting against the cold."

Plan trips around the first two only. The Extreme figure is a survival number. Rab also notes that the standard does not cover quilts or expedition bags below about -4°F (-20°C), so those ratings are the maker's own method.

Which number to plan around

REI's temperature ratings guide gives a simple rule: use the lower limit if you are a warm sleeper and the comfort rating if you are a cold sleeper. Therm-a-Rest's Boost pages say the same, recommending that "most women and people who sleep colder" choose on the Comfort rating.

The same guides explain the naming trap. Men's and unisex bags are usually named for their Limit; women's bags for their Comfort. Sea to Summit says so on its own pages: unisex bags are listed by the Lower Limit, women's-specific bags by the Comfort rating.

So the Therm-a-Rest Boost 650 "20F" is a 20°F Limit and a 32°F Comfort. The Sea to Summit Trek Women's "15" is a 16°F Comfort and a 3°F Limit. Both are called cold-weather bags, but the Sea to Summit is far warmer.

Our rule: take the coldest night you expect, then match it to the Comfort rating if you sleep cold, are tired, or will be at altitude. Use the Limit only if you know you sleep warm, and keep a margin of 5 to 10°F either way.

What the test assumes: pad and clothing

The ISO test puts the manikin on an insulating mat and dresses it in a base layer. Your real setup changes the result. A bag rated to 20°F on a thin summer pad on frozen ground will feel much colder than its label, because the pad, not the bag, stops heat flowing into the ground under you.

For winter nights, pair the bag with a pad of R 5.5 or higher, the range REI's pad guide gives for extremely cold weather. Our sleeping pad R-value guide explains ASTM R-values and how to stack two pads.

Clothing counts too. A dry base layer and a warm hat are part of the system the test assumes; see our merino vs synthetic base layer guide. Sleeping in your damp hiking layers is the fastest way to lose the rating you paid for.

Ratings you cannot compare

Not every bag has an ISO or EN label. Budget bags often carry a single "20 degree" in the name with no test behind it. The TETON Sports Trailhead 20, sold by Amazon.com, is a common example: its listing gives no Comfort or Limit rating, so there is nothing to compare with the four bags above.

Some makers publish only one ISO figure. NEMO's own page for the Disco men's bag lists an ISO Limit of 16°F for the 15°F version but no Comfort rating, which is why we could not rank it here. Ask the maker, or check a retailer that lists both.

Sleeping bag liners are another case. Sea to Summit says its Reactor insulated liner adds up to 11°F, but that is the maker's figure, not an ISO rating of the bag and liner together.

Which bag to buy

• Cold three-season nights, roomy fit: the Therm-a-Rest Boost 650 20F. Comfortable to about freezing for cold sleepers, lower for warm ones.
• Women's bag for real cold: the Sea to Summit Ascent Women's 15, 16°F Comfort in 1,251 g.
• Same rating, heavier down: the Sea to Summit Trek Women's 15, 650+ duck down, for car camping and short walks in.
• Spring and fall, broad shoulders: the Boost 650 32F Regular Wide.

Staying warm and safe on a cold night

Eat before bed, go to sleep warm rather than cold, and keep tomorrow's damp clothes out of the bag. A warm water bottle in a sock helps; a chemical hand warmer against skin does not, as HotHands warns against direct skin contact (see our hand warmer guide).

Know the signs. The CDC's hypothermia guidance lists shivering, exhaustion, confusion, fumbling hands, memory loss, slurred speech and drowsiness, and calls a body temperature below 95°F a medical emergency. If a partner cannot get warm in their bag, get them dry, share warmth, and get help. This page is not medical advice.

Related: how to layer for a cold hike and down vs synthetic insulated jackets.

Frequently Asked Questions

Is EN 13537 the same as ISO 23537?

Close enough to compare. REI says EN was the original standard, ISO now oversees testing, and the method is almost identical, so an older EN-rated bag can be compared with a newer ISO-rated one.

Why are women's bags rated warmer?

Because the Comfort rating is based on a standard woman and the Limit on a standard man, as Sea to Summit explains, and brands name women's bags by Comfort. Women's bags often carry more fill as well: Sea to Summit's Trek Women's 15 has 800 g of down in Regular.

Does the Therm-a-Rest WarmZip change the rating?

The EN figures are for the bag as tested. Therm-a-Rest says zipping the WarmZip boosts warmth by up to 10°F, which is its own claim, not part of the EN rating.

What sleeping bag do I need for 0°F?

One whose Comfort rating (if you sleep cold) or Limit (if you sleep warm) is at or below 0°F, on a pad of R 5.5 or more. None of the bags above reaches that; look at expedition bags, whose ratings fall outside ISO 23537's range.

---

WildProofLab is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com. Temperature ratings and fill weights are the makers' own.