Sleeping bag temperature ratings: Comfort vs Lower Limit

The big number on a sleeping bag's label is not a promise that you will be warm at that temperature. Many bags are tested to the international standard ISO 23537, which produces several temperatures, and each brand chooses which one to put in the product name. Two bags both sold as "30°F" can therefore be 4°C apart in real warmth. Knowing which rating you are looking at, and pairing the bag with the right mat, is what turns the label into a useful number.

The four ISO ratings#

ISO 23537 replaced the older European standard EN 13537 in 2016 and was revised in 2022. The test puts a heated manikin dressed in a thermal base layer, socks and a warm hat into the bag, on an insulating mat, in a climate chamber (EN 13537 and ISO 23537). From the heat it loses, the lab calculates:

The "standard" sleepers are 25-year-olds: a woman of 1.60 m and 60 kg, a man of 1.73 m and 73 kg. Comfort uses the female reference in a relaxed position and Lower limit the male one curled up, so Comfort is always the warmer of the two figures. Extreme is not a planning number at all, and Sea to Summit's specification tables list only Comfort and Lower limit.

Same name, different bag: a real comparison#

Sea to Summit labels its unisex bags by the Lower limit and its women's bags by the Comfort rating (Trek specifications). Here are the Regular sizes of two bags that share a "30°F" name:

Sources: Trek and Ascent Women's technical tables.

The names match, but the Ascent Women's is rated comfortable 4°C colder, and it is also lighter and packs smaller. That is not because women's bags are magically warmer: it carries more down of a higher fill power in a different design. The lesson is to ignore the name and compare the same rating across your shortlist, ideally Comfort.

Choosing the rating for your trip#

Start with the lowest temperature you expect at the place you will actually sleep, not the nearest town. Valleys and hollows can be colder than the slopes above them on clear nights.

• If you tend to sleep cold, or want a margin, make the Comfort rating at or below that expected low.
• If you know you sleep warm, a bag whose expected low falls between Comfort and Lower limit may work, but that is the zone where you will be curled up and only just warm.
• Never plan around Extreme.

A worked example with the same family of bags: suppose the forecast low for your campsite is 0°C. The Trek 30°F has a Comfort rating of +3°C, so a cold sleeper would be below its comfort range. The Trek 15°F lists a Comfort rating of −3°C, a Lower limit of −9°C, 1,436 g and 10.6 L compressed, compared with 1,119 g and 8.2 L for the 30°F version (Trek specifications). The warmer bag costs you about 300 g and 2.4 L of pack space, and buys a comfortable 3°C margin. Whether that trade suits you depends on your pack and how you sleep, but the numbers make the choice clear.

The mat changes everything#

The ISO test is done on a well-insulated mat; Sea to Summit puts its R-value at 5.38 (why bags feel cold). Insulation under you is squashed flat by your body weight, so the ground steals heat through the mat, not the bag. Put a bag rated to −1°C on a thin summer mat on frozen ground and it will not perform anywhere near its label.

Sea to Summit's guidance on mat R-values is a useful rule of thumb: 1 to 2 for summer, at least 3 to 4 for spring and autumn, and 5 or more, from one mat or two stacked, for winter camping (mat guidance). R-values of stacked mats add together, so a closed-cell foam pad under an inflatable is a cheap way to winter-proof a three-season system.

Why real nights differ from the lab#

The test manikin does not move. Real sleepers turn over, push warm air out of the bag and draw cold air in around the shoulders, especially in bags without a draught collar (Sea to Summit). A bag that is too roomy has more air to heat; one that is too tight compresses the insulation. A thermal liner adds warmth and keeps the bag clean.

So if you were cold last time, check the mat and the fit before buying a warmer bag. The ratings were designed to compare bags fairly, and they do that well; they cannot account for the ground, your metabolism or a draughty hood. Specifications were checked on the manufacturer's pages on 8 October 2026.