It’s important to understand the temperature ratings of lightweight sleeping bags because they can be confusing and are different for men versus woman.
Your Sleeping Bag Has Been Lying To You
Unfortunately most sleeping bags purchased in the past twenty years relied upon the manufacturers own internal testing standards to determine the temperature ratings. The tests were up to the manufacturers and there was no consistency of testing or results reported. Due to this most sleeping bags purchased in the past two decades probably have a temperature rating that is not accurate.
Thankfully large retail stores and select manufacturers realized this was causing problems for people trying to decide which lightweight sleeping bag to purchase. Beginning in 2009 these companies standardized on a third party independent standard called EN 13537 (EN). These tests were consistent and allowed consumers to make accurate comparisons between sleeping bags.
Understanding The EN Temperature Ratings
There are four ratings outlined in the EN ratings. Each one is explained in the table below.
EN Rating Description
Upper Limit This is the highest temperature a standard man can sleep without an excessive amount of sweat.
This assumes the man is sleeping with his arms outside the bag, the zipper completely open, and the hood is not being used.
Comfort This is the temperature that the average woman can comfortably sleep in the bag.
Lower Limit This is the temperature that the average man can comfortably sleep in the bag.
Extreme This is the lowest temperature that the average woman can remain in the bag without expiring from hypothermia.
This does not assume sleeping as the woman will be shivering to stay alive. In addition, frostbite is still possible.
As you can see the ratings for men and women are quite different. As a rule of thumb the advertised rating on a bag is the lower limit rating. For example, my Marmot Flathead 20 would translate to an EN lower limit rating of 20.
It’s important to note that the EN testing assumes that the person is wearing a thermal layer (long underwear) while sleeping.
Be sure to checkout our Lightweight Sleeping Bag Guide.