The temperature rating for each sleeping bag is derived from a combination of factors. First, there’s a formula for the amount of down in a given size bag to reach a certain warmth rating. That’s just a place to start. Next, bags are tested in the lab according to EN 13537. This test procedure provides a lower limit and a comfort rating which are used to rate our men’s and women’s bags, respectively. Lastly, we rely upon user feedback from our employees, test team, and ambassadors.