A comfort rating is a number that helps consumers determine the temperature tolerances of a sleeping bag. There are actually several different measurements involved in temperature ratings. The ratings are divided between conditions for men and conditions for women based on assumptions about differences in biology. The EN 13537 European standard has helped to provide clear guidelines for comparing the temperature ratings of different sleeping bags. Four measurements make up most temperature ratings.
The comfort rating is the lowest outside temperature that the sleeping bag can handle for an average woman. This rating is determined in a laboratory using a mannequin that measures conditions for an average woman. This assumes the woman is wearing some type of clothing and is not in contact with the ground. The sleeping bag will not protect people inside from temperatures below this figure. A number of sleeping bags list only the comfort rating on the label.
The lower limit rating is the lowest temperature where a man in the sleeping bag will be able to sleep. The rating requires a man to be wearing clothes and to be sleeping on a mat that is at least an inch thick. The level of comfort will vary as temperatures drop. Although a man might be able to sleep when temperatures meet the rating, the conditions inside of the bag might be uncomfortable.
The upper limit of a sleeping bag is a measurement of the maximum temperature in which an average man can sleep without sweating through the night. Using a sleeping bag in temperatures above this rating will mean hot conditions and an uncomfortable sleeping experience. This rating is not always listed on sleeping bags. The reason is that opening the bag, wearing light clothes or removing an underlying pad can all make conditions more comfortable. Upper limits are most important when camping or traveling through consistently hot climates.
The extreme rating lists the lowest possible temperature at which the sleeping bag will still provide some protection. This figure can be below freezing for some sleeping bags. An issue with extreme ratings is that they are not always accurate because bodies respond differently to incredibly cold temperatures. The main reason to choose a bag with a good extreme rating is to provide some protection in the event of an emergency. Bags with good extreme ratings can keep a person warm if a severe snowstorm or avalanche occurs.