Understanding ISO Ratings for Sleeping Bags

If you’ve ever wondered why some nights in your sleeping bag feel cozy while others leave you shivering, the answer often comes down to ISO ratings. These ratings help you understand exactly how warm your sleeping bag will keep you and under what conditions. Knowing how to read them can make the difference between a restful night outdoors and a long, chilly one.

Watch this video to learn more about ISO temperature ratings and how they determine your sleeping bag’s performance.

The International Standards Organization (ISO) assigns temperature ratings to sleeping bags through lab tests. In these tests, a dummy equipped with heat sensors wears a baselayer top, bottom, and face mask while lying on a winter-focused sleeping pad. This setup measures how well the bag retains heat. Each sleeping bag is given three ratings: T Comfort, T Limit, and T Extreme. T Comfort tells you the lowest temperature an average cold sleeper can stay comfortable in, while T Limit is for the average warm sleeper. T Extreme represents the lowest temperature a person could survive in—not comfortably, but safely. ISO doesn’t test below -20°C (-4°F), so keep that in mind if you’re planning for extreme cold.

Your sleeping pad also plays a major role in warmth. ISO testing uses a winter pad with a high R-value, but if you’re camping in warmer weather, a lighter pad like an accordion foam one with an R-value of 2.1 might be enough. For colder trips, you’ll need something with more insulation. Even quilts like the Moondance 25 rely heavily on the pad for warmth and can feel cooler since they don’t have hoods. To stay toasty through the night, layer up, do a bit of light exercise before bed, and have a small snack to keep your body generating heat. Understanding ISO ratings and matching your gear to the conditions ensures you stay warm, comfortable, and ready for the next day’s adventure.