All sleeping bags have temperature ratings, which indicate the minimum temperature that the bag is designed to handle. Temperature ratings should be taken with a grain of salt however, because as of now, there is no standard, universal method for determining ratings. This means that manufacturers are free to make their own claims about their bags’ warmth. In most cases, these claims are realistic, but in some cases, temperature ratings are optimistic.
A new standardized rating system has been widely adopted in Europe, called EN 13537. Some U.S. bag makers have also adopted the system (and others are planning to). The system assigns bags a series of ratings:
Upper limit = the highest air temperature at which the average man can sleep comfortably.
Comfort = the lowest air temperature at which the average woman can sleep comfortably.
Lower limit = the lowest air temp at which the average man can sleep comfortably.
The ratings are established after a set of standardized tests using a heat-sensored mannequin.
Until a universal temperature standard is adopted, your best bet is to use temperature ratings on the hangtag as a starting point and then use your gut to guide you. If you know you’re a cold sleeper and a bag looks too thin for you, go with a plumper one.