Explained: Sleeping Bag Temperature Ratings

Can your "zero-degree" bag really keep you warm at zero degrees?

Q. What do sleeping bag temperature ratings mean?

A. You may notice when shopping for a sleeping bag that most of the product names include a temperature. For example: the Marmot 20°F Flathead down sleeping bag. This number is the temperature rating. Generally speaking, it’s the lowest temperature (Fahrenheit in the U.S.) at which the bag is supposed to keep you warm.

Were it that simple.

With the exception of a few companies, there’s no independent standard for arriving at that rating, so it’s usually a ballpark figure that doesn’t take into account whether you’re a cold sleeper (i.e. you have a low metabolism when sleeping and require extra insulation) or a hot sleeper. The difference between the two can be ten degrees or more, meaning a bag rated to 30 degrees might work for a hot sleeper in 30-degree weather, but leave a cold sleeper shivering. On average, women tend to be colder sleepers than men.

However, some American companies are beginning to adopt what is known as the European Norm for sleeping bag ratings—you might see it referred to as “EN 13537”—which requires standardized testing and a uniform rating system. This list includes REI, Marmot, Mountain Hardwear and The North Face.

Let’s take a look at a generic EN label:

(Image via DonsNotes.com.)

Instead of one rating, there are four:

Upper limit: Not all EN labels show this—some companies just have an infinity symbol—but it’s the highest temperature at which an average man can have a comfortable night’s sleep without excess sweating.

Comfort: This is the coldest temperature at which an average woman can sleep comfortably.

Lower limit: The coldest temperature at which an average man can sleep comfortably.

Extreme rating: The coldest temperature at which an average woman can survive the night.

You may notice that this chart has separate ratings for men and women. This is because of the aforementioned fact that women tend to require more insulation than men. These ratings are based on the “standard man” and “standard woman”—both 25 years old and of average build—and don’t account for the range of metabolisms within each sex. That said, the European Norm is more accurate than the other ratings manufacturers slap on their labels.

But wait, you say: If REI, Marmot et al. use European Norm ratings, then why do I still see product names like REI’s "Lumen +25?"

Unfortunately, standard practice dies hard in a marketplace where competitors can set their own ratings. In most cases the advertised rating is the lower limit rounded to the nearest 5-degree increment. As REI explains on its website, the Lumen +25 actually has a lower limit of 27 degrees.

If you’re a “standard” woman or a cold-sleeping man, you’d do well to add 10 to 15 degrees to this rating.

It’s worth noting that there are many other factors that affect your body temperature when sleeping outdoors, from the type of sleeping base you use—sleeping pad, cot, hammock, etc.—to whether or not you use a sleeping bag liner, to what you wear to sleep.

But getting your bag's rating right is the first step.