EN/ISO temperature rating

What the standard replaced

Before the standard existed, a bag's rating was whatever the brand felt like printing. EN 13537, since updated as ISO 23537, replaced that with a heated mannequin in a climate chamber and a defined method, which is why ratings are broadly trustworthy now.

Comfort, limit and extreme

The test produces three numbers people care about. Comfort is the temperature at which a standard adult woman sleeps comfortably. Limit is where a standard adult man sleeps curled up but not shivering. Extreme is a survival figure for six hours without hypothermia, and it should be ignored entirely when shopping, because it describes surviving a night rather than sleeping through one.

Why the number on the label is usually the limit

Here's the catch. Most bags are named after the limit number, so a bag sold as a 20 degree bag typically has a comfort rating closer to 30. If you sleep cold, buy on the comfort number, and expect the bag on the shelf labelled for your conditions to be roughly ten degrees optimistic for you.

What the test actually does

A heated mannequin dressed in a base layer and lying on a standardised pad inside the bag, in a climate chamber. The heaters hold it at a fixed skin temperature and the machine measures how much power that takes at various air temperatures, from which the ratings are derived.

That is why the numbers are comparable at all, and also why they read as abstract. A mannequin does not sleep badly, does not have a bad hydration day, and never crawls into the bag already cold.

Why your experience differs from the number

The test assumes a pad doing its job, and a cold pad will make any bag feel ten degrees worse. It assumes a base layer, so sleeping in nothing changes the result. It assumes an adult of standard build, so a small, lean or older sleeper will typically feel colder than the rating implies.

Then there are the things nobody accounts for: going to bed dehydrated, going to bed on a large calorie deficit, a draughty pitch, or a damp bag that has lost some loft. Treat the rating as a laboratory comparison between bags, not a promise about a specific night.