Sleeping Bag Temperature Ratings: Comfort, Limit and Extreme
A sleeping bag labeled with a single temperature can be misleading, because people feel cold very differently. Standardized testing gives several numbers instead, and knowing which one applies to you helps avoid long, shivering nights.
The three standard ratings
Many bags are tested to ISO 23537, which replaced the earlier EN 13537. The comfort rating is the temperature at which a typical cold sleeper stays comfortable in a relaxed position. The limit rating is where a typical warm sleeper stays warm curled up. The extreme rating is a survival figure, not a target.
Tests assume the sleeper uses an insulated pad and wears a base layer, so real results depend on the whole system.
Choosing a rating
If you tend to feel cold, choose a bag whose comfort rating is at or below the lowest temperature you expect. Warm sleepers can lean toward the limit rating. Leaving some margin for unexpected cold snaps is usually worth a little extra weight.
A bag liner, dry sleeping clothes and a warm hat add warmth without buying a new bag.
Factors the label cannot show
Wind, humidity, fatigue, hunger and a poorly insulated pad all make nights feel colder. Eating a snack before bed and changing into dry clothes help more than many people expect.
Store bags uncompressed at home so the insulation keeps its loft and its rating.