Understanding Sleeping Bag Temperature Ratings

Article by NEMO

Understanding Sleeping Bag Temperature Ratings

Waking up well-rested can make all the difference in maximizing the next day’s adventures. Here’s a simplified guide to picking the right sleeping bag temperature rating for you and your plans. 


Man and woman sitting outside of a tent with two sleeping bags inside.


The standardized testing used to assign temperature ratings to sleeping bags (the ISO Test) provides three temperature thresholds to help guide users, a COMFORT rating, a LIMIT rating, and an EXTREME rating. The names can be a bit misleading, but the reason for offering all three is because sleep comfort is a very personal preference, and whether you sleep warm or cold at night determines how much insulation you need in your bag. 

Typically, sleeping bags for women have more insulation in them, making them roughly 10–15 degrees Fahrenheit warmer than similarly rated men’s bags based on the premise that the average woman sleeps colder than the average man. But many of us live to break the rules, so these stereotypes shouldn’t blindly determine your own comfort and safety. It’s best to know how the ratings work to ensure that you’re choosing the bag that best matches the mood of your hypothalamus at 3:00 a.m. 


Man inside a tent, unzipping his sleeping bag.


According to the ISO Test, the COMFORT rating is based on the air temperature at which the “average female” can sleep comfortably through the night (i.e. not curled up in a ball for warmth). The way we like to think of it is this: If you tend to sleep cold — you’re the blanket hog who’s always chilly — then you should pay closest attention to a bag’s COMFORT rating. That’s the lowest temperature at which your bag is likely to provide a cozy night’s sleep. 

The LIMIT rating is based on the temperature at which the “average man” can maintain his thermal equilibrium in a curled-up position. Basically, according to the test, the “average man” won’t necessarily be comfortable at this temperature, but he can sleep without expending energy shivering and is not in danger of hypothermia (the dream scenario, right?!). A more general way of interpreting this data is to apply it to warm sleepers. If you’re a warm sleeper — the fewer layers, the better! — then the LIMIT temperature is likely most relevant for you. That's the temperature at which your bag shouldn’t be too hot or too cold. (Be prepared to supplement your bag with extra layers if this is your first time testing it out!) 

The EXTREME rating gives an idea of how cold the air could be before there’s serious risk of death from hypothermia in a matter of hours. Realistically, you want to avoid counting solely on the bag for survival at that temperature; extra layers, a fire, or sharing body warmth are likely necessary, as well.  

No matter which temperature bag you choose, you should always properly prepare before a night sleeping outside. Check the weather before you go and leave a safe margin between expected nighttime lows and your bag’s rating. 


Woman sitting outside while huddled inside her sleeping bag


Sleeping bags today are rated according to the International Organization for Standardization standard, using ISO 23537 (“the ISO Test” we reference above). The test involves placing a heavily instrumented electronic manikin, clad in long underwear, inside the sleeping bag in question, and positioning both on top of a foam sleeping pad. The temperature of the air is then cooled, and the amount of electrical energy needed to maintain the manikin’s initial temperature is measured. (If it takes a lot of energy input to keep the manikin toasty, then the bag isn’t very warm.) 

Through rounds of experimentation, these results have been correlated to the real-world outcome and the test can help predict what kind of comfort a person can expect from the bag. Here’s where it gets tricky, though: All our internal heat-generating engines run a little differently. Some of us pile on the blankets at night and some of us sleep in our skivvies with the window open in February. Individual preference, metabolism, and age can all throw a wrench into the simple-sounding outline above.  

We’ve done our best to translate the information into easier-to-understand guidelines, but it’s important to keep in mind that they’re not hard and fast rules. If you need further guidance in choosing your sleeping bag, our Customer Service Team is happy to help!