Sleeping bag temperature ratings explained

What is ISO 23537-1:2016?

ISO 23537-1:2016 is an international standard for testing the warmth of sleeping bags. This international standard replaces EN 13537 which was introduced in the European market in the early 2000s and was a compulsory requirement for sleeping bags sold in Europe after 1 January 2005. Testing is conducted in laboratory conditions with a thermal manikin dressed in a thin thermal top, long johns and socks. The testing procedure generates three temperatures:

  • Comfort Temperature
  • Limit Temperature
  • Extreme Temperature

    Comfort temperature

    The comfort temperature is defined as: “Lower limit of comfort range, down to which a sleeping bag user with a relaxed posture, such as lying on their back, is globally in thermal equilibrium and just not feeling cold. ” Basically the Comfort Temperature is a good guide for the coldest temperature a ‘cold sleeper’ would want to plan to use their sleeping bag for.

    Limit temperature

    The limit temperature is defined as: “Lower limit at which a sleeping bag user with a curled body posture is globally in thermal equilibrium and just not feeling cold.” The Limit Temperature is a good guide for the coldest temperature a ‘warm sleeper’ would want to plan to use their sleeping bag for and what we mark on our bags.

    Extreme temperature

    The extreme temperature is defined as: “Very low temperature where the risk of health damage by hypothermia is possible.” No one should usually plan to use their sleeping bag at this low temperature - when it's between the lower limit and extreme rating the user will feel cold to very cold and is a survival rating only.

    Transition range

    The transition range is the temperature between the Comfort Temperature and Limit Temperature.

    Risk range

    The Risk Range is the range of temperatures between Limit Temperature and Extreme Temperature.

    What do these temperatures really mean?

    Firstly, they mean you can compare sleeping bags that have been tested to this standard within the BlackWolf product range or across brands and be confident you are comparing apples with apples. Bags that are tested to these standards have to meet strict criteria and are a better guide than not being tested.

    Despite being tested, all temperature ratings should be used as a guide. Everyone sleeps differently. Some people are warm sleepers whilst others feel the cold. Most people will know how they react to the cold and should take this into account when choosing a sleeping bag. It’s always better to be cautious and choose a warmer sleeping bag than you need, you can always open the sleeping bag up to cool down but a cold, uncomfortable night can ruin a camping trip.

    Sleep system: Shelter, Sleeping Bag, Mat

    A sleeping bag is only one part of your camping sleep system - other components that play a significant part are what you sleep on (mat) and what you sleep in (shelter).


    Tent, swag, caravan, indoors or under the stars – where you sleep will affect the performance of your sleeping bag. A small tent will be warmer than a large tent. A swag will be warmer than sleeping out under the stars. Sleeping indoors will generally be warmer than outdoors especially if there is some form of heating.

    Sleeping bag

    Choose a sleeping bag that is suitable for the coldest conditions you expect to use it for.


    It is vital to sleep on an insulated surface especially in cold conditions. BlackWolf mats have an R-Value which is a tested measurement indicating how much insulation they provide, a larger number means more insulation and a warmer night.

    You can shop our range of BlackWolf sleeping bags.