During the design process, our bags are all field tested for warmth, fit, durability, and usage before they are finalized, but it’s important to note that determining a sleeping bag’s temperature rating is not an exact science. We do not use EN standard ratings. While we’ve tried testing our bags against the EN rating systems in the past, the test is expensive to perform and doesn’t provide crucial real-world metrics. We have over 45 years of experience building, testing, and rating bags, and rely heavily on the decades of feedback we have from users, guides, and employees, from hiking in the Northern Cascades to alpine ascents in the Himalayas. That said, when we compare the specs of our bags (loft, down quality, construction, etc.) against the bags of competitors who did EN test and our bags exceed theirs in every measure, we feel pretty confident in labeling our bags at the same temperature rating.