Comfort rating

Comfort rating comes from the EN 13537 (now ISO 23537) sleeping bag test standard, which also produces the lower limit and extreme figures printed on the same tag. It estimates the temperature at which a standard cold-sensitive sleeper can rest in a relaxed position without waking from cold, using a heated manikin inside a climate chamber rather than a real trail or campsite.

For car camping, comfort rating matters more than the other two numbers because most trips involve mild to cool nights where the goal is actual rest, not survival. The lab test assumes a base layer, a liner, and an insulated pad underneath, so a bag's real performance shifts once you swap in a thin foam pad, sleep on a hard cargo floor, or camp somewhere windy or damp.

When shopping, pick a bag with a comfort rating 10 to 15 degrees below the coldest night you realistically expect, since cold tolerance varies and vehicles rarely stay as warm as open ground with good airflow. A common mistake is shopping by the lower limit or extreme number because it sounds like extra safety margin, then feeling cold on nights that were nowhere near that temperature.