# Sleeping Bag Temperature Ratings (2026): How to Read ISO 23537 Comfort, Limit and Extreme > How to read an ISO 23537 or EN 13537 sleeping bag label: Comfort vs Limit vs Extreme, which number to plan around, and four bags whose makers publish both ratings. - Source: https://wildprooflab.com/cold-weather/sleeping-bag-temperature-ratings - Publisher: WildProofLab (https://wildprooflab.com) - Published: 2026-09-29 - Category: cold-weather - Tags: sleeping bag, temperature rating, ISO 23537, EN 13537, comfort rating, lower limit, winter camping, Sea to Summit, Therm-a-Rest - Method: picks chosen from Amazon listings and brand specs; not hands-on tested; current Amazon prices are on the page; content checked 2026-09-29. - Quick answer: Plan around the ISO Comfort rating if you sleep cold, the Limit if you sleep warm, and never the Extreme. For cold nights, buy the Therm-a-Rest Boost 650 20F (Comfort 32°F, Limit 20°F, sold by Amazon.com); for a women's bag near 15°F, the Sea to Summit Ascent Women's 15 (Comfort 16°F). ## Key takeaways - A bag's name is usually its Limit for men's and unisex bags and its Comfort for women's bags, so two '15-degree' bags can be 10°F apart. - ISO ratings assume one base layer and an insulated pad under the bag; a thin pad makes any bag colder than its label. - The Extreme figure is a six-hour survival number with a risk of hypothermia, not a camping temperature. ## Products (chosen from listings, not tested) 1. **Therm-a-Rest Boost 650 20F/-6C Down Sleeping Bag, Regular** (details: https://wildprooflab.com/cold-weather/sleeping-bag-temperature-ratings#product-B0DN3RRNNB) — Buy it if you sleep on your side or feel trapped in a mummy bag, for nights near freezing. - Pros: EN Comfort 32°F, Limit 20°F, Extreme -9°F, per Therm-a-Rest; 649 g of 650-fill hydrophobic duck down; 2 lb 11 oz (1,205 g) total; WarmZip center zipper narrows the semi-rectangular bag into a mummy shape, which Therm-a-Rest says adds up to 10°F - Cons: Heavier than a mummy bag of the same rating; low stock at Amazon when we checked 2. **Sea to Summit Ascent Women's Down Sleeping Bag, 15°F, Regular** (details: https://wildprooflab.com/cold-weather/sleeping-bag-temperature-ratings#product-sea-to-summit-ascent-women-s-down-sleeping-bag-15-f-regular) — Buy it for cold-weather backpacking if you want the lightest bag here rated near 15°F Comfort. - Pros: ISO Comfort 16°F, Lower Limit 5°F, per Sea to Summit; 720 g of 750+ fill goose down with Ultra-Dry treatment; 1,251 g total; Narrower shoulders, wider hips; Free Flow zips open it into a quilt - Cons: Low stock when we checked; fits up to 5 ft 7 in in Regular 3. **Sea to Summit Trek Women's Down Sleeping Bag, 15°F, Regular** (details: https://wildprooflab.com/cold-weather/sleeping-bag-temperature-ratings#product-B0CTPNTPVH) — Buy it for car camping and short hikes to camp, where the extra weight matters less. - Pros: ISO Comfort 16°F, Lower Limit 3°F, per Sea to Summit; 800 g of 650+ fill duck down with Ultra-Dry treatment; recycled 20D shell; Women's cut with room at the hips for side sleeping - Cons: 1,612 g total per Sea to Summit, about 360 g more than the Ascent 4. **Therm-a-Rest Boost 650 32F/0C Down Sleeping Bag, Regular Wide** (details: https://wildprooflab.com/cold-weather/sleeping-bag-temperature-ratings#product-B0DN3VN63Q) — Buy it for spring and fall if a standard bag is tight across the shoulders. - Pros: EN Comfort 41°F, Limit 32°F, Extreme 5°F, per Therm-a-Rest; 76 in shoulder girth in Regular Wide; 537 g of 650-fill down, 1,121 g total; Same WarmZip and arm holes as the 20F version - Cons: A 41°F Comfort rating is spring-and-fall warmth, not winter 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 | Bag | Comfort | Limit | Extreme | Fill | Weight | Sold by | Amazon price | |---|---|---|---|---|---|---|---| | [Therm-a-Rest Boost 650 20F](https://wildprooflab.com/cold-weather/sleeping-bag-temperature-ratings#product-B0DN3RRNNB) | 32°F | 20°F | -9°F | 649 g, 650-fill duck | 1,205 g | Amazon.com | see the current Amazon price on the page | | [Sea to Summit Ascent Women's 15](https://wildprooflab.com/cold-weather/sleeping-bag-temperature-ratings) | 16°F | 5°F | Not published | 720 g, 750+ goose | 1,251 g | Sea to Summit | see the current Amazon price on the page | | [Sea to Summit Trek Women's 15](https://wildprooflab.com/cold-weather/sleeping-bag-temperature-ratings#product-B0CTPNTPVH) | 16°F | 3°F | Not published | 800 g, 650+ duck | 1,612 g | Sea to Summit | see the current Amazon price on the page | | [Therm-a-Rest Boost 650 32F, Reg Wide](https://wildprooflab.com/cold-weather/sleeping-bag-temperature-ratings#product-B0DN3VN63Q) | 41°F | 32°F | 5°F | 537 g, 650-fill duck | 1,121 g | Amazon.com | see the current Amazon price on the page | Ratings are the makers' own, from Therm-a-Rest's pages for the [Boost 650 20F](https://www.cascadedesigns.com/products/boost-650-20f-6c-sleeping-bag) and [Boost 650 32F](https://www.cascadedesigns.com/products/boost-650-32f-0c-sleeping-bag) and Sea to Summit's pages for the [Ascent Women's](https://seatosummit.com/products/ascent-womens-down-sleeping-bag) and [Trek Women's](https://seatosummit.com/products/trek-womens-down-sleeping-bag). 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](https://rab.equipment/us/rab-lab/sleeping-bag-temperature-ratings-explained) 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](https://www.rei.com/learn/expert-advice/understanding-sleeping-bag-temperature-ratings.html) 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](https://www.rei.com/learn/expert-advice/sleeping-pads.html) gives for extremely cold weather. Our [sleeping pad R-value guide](https://wildprooflab.com/cold-weather/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](https://wildprooflab.com/cold-weather/merino-vs-synthetic-base-layers). 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](https://www.nemoequipment.com/products/disco-mens-endless-promise-down-sleeping-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](https://wildprooflab.com/cold-weather/sleeping-bag-temperature-ratings#product-B0DN3RRNNB). 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](https://wildprooflab.com/cold-weather/sleeping-bag-temperature-ratings), 16°F Comfort in 1,251 g. - **Same rating, heavier down**: the [Sea to Summit Trek Women's 15](https://wildprooflab.com/cold-weather/sleeping-bag-temperature-ratings#product-B0CTPNTPVH), 650+ duck down, for car camping and short walks in. - **Spring and fall, broad shoulders**: the [Boost 650 32F Regular Wide](https://wildprooflab.com/cold-weather/sleeping-bag-temperature-ratings#product-B0DN3VN63Q). ## 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](https://wildprooflab.com/cold-weather/hand-warmers-disposable-vs-rechargeable)). Know the signs. The [CDC's hypothermia guidance](https://www.cdc.gov/winter-weather/prevention/index.html) 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](https://wildprooflab.com/blog/layering-for-cold-weather-hiking) and [down vs synthetic insulated jackets](https://wildprooflab.com/cold-weather/insulated-jackets-down-vs-synthetic). ## Frequently Asked Questions ### Is EN 13537 the same as ISO 23537? Close enough to compare. [REI](https://www.rei.com/learn/expert-advice/understanding-sleeping-bag-temperature-ratings.html) 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](https://seatosummit.com/products/trek-womens-down-sleeping-bag), 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.* ## Sources - Cascade Designs (Therm-a-Rest, MSR): [Boost 650 20F](https://www.cascadedesigns.com/products/boost-650-20f-6c-sleeping-bag) - Cascade Designs (Therm-a-Rest, MSR): [Boost 650 32F](https://www.cascadedesigns.com/products/boost-650-32f-0c-sleeping-bag) - seatosummit.com: [Ascent Women's](https://seatosummit.com/products/ascent-womens-down-sleeping-bag) - seatosummit.com: [Trek Women's](https://seatosummit.com/products/trek-womens-down-sleeping-bag) - rab.equipment: [Rab's explainer](https://rab.equipment/us/rab-lab/sleeping-bag-temperature-ratings-explained) - rei.com: [Understanding sleeping bag temperature ratings](https://www.rei.com/learn/expert-advice/understanding-sleeping-bag-temperature-ratings.html) - rei.com: [REI's pad guide](https://www.rei.com/learn/expert-advice/sleeping-pads.html) - nemoequipment.com: [Disco men's bag](https://www.nemoequipment.com/products/disco-mens-endless-promise-down-sleeping-bag) - Centers for Disease Control and Prevention: [CDC's hypothermia guidance](https://www.cdc.gov/winter-weather/prevention/index.html) --- Last updated: 2026-09-29 Canonical URL: https://wildprooflab.com/cold-weather/sleeping-bag-temperature-ratings How to cite: "Sleeping Bag Temperature Ratings (2026): How to Read ISO 23537 Comfort, Limit and Extreme", WildProofLab, https://wildprooflab.com/cold-weather/sleeping-bag-temperature-ratings (checked 2026-09-29). Disclosure: as an Amazon Associate WildProofLab earns from qualifying purchases. Safety information summarizes public guidance and is not medical advice.