If you're not sure which bag is for you then read our guide on How to choose a sleeping bag. If you've read that and want to be able to better compare the bags on offer or want to know some more details on the plethora of numbers that come with each sleeping bag then you're in the right place, read on.

Interpreting the numbers in the table

Each sleeping bag page (except OMM see below) has a table which will include at least some of the following information:

EN Comfort	EN Limit	EN Extreme	Brand temp guide	Fill weight Fill power (down only)

Below is a description of what each heading means and the best way to use the information to your advantage when picking a sleeping bag.

The EN 13537 rating

This is the standardized European test for sleeping bags and is the primary way of comparing sleeping bags temperature ratings. The EN13537 test gives 4 numbers for each bag tested:

• Upper Limit: Based on the highest temperature which a standard man can have a comfortable nights sleep at without excessive sweating.
• Comfort: Based on a standard woman having a comfortable nights sleep.
• Lower Limit: Based on the lowest temperature which a standard man can have a comfortable nights sleep.
• Extreme: Based on a survival rating for 6 hours without risk of death for a standard man.

At Facewest we don't show the Upper Limit temperature, only the Comfort, Lower Limit and Extreme ratings.

In each case it is assumes that the person is wearing full base layer cover and that you have had a hot meal for tea. It also assumes that you are using a suitable mattress.

There are two main issues with this rating system: Firstly it doesn't work for extremely low temperatures so none of the bags designed for extreme conditions have been tested. Secondly, as of 2012 the test was standardized across the different European test centres, this has meant that temperatures given in the new test are not as cold as before; so newer bags may have a temperature rating that perhaps isn't as good as their older counterparts, even though the actual performance of the bag is the same or better. Due to the expense of testing brands are not resubmitting any bags that have already been tested, so it is only newly designed bags that have a rating from the new test. Older bags still feature their original rating.

Just because a bag shows a lower limit temperature of -10°C doesn't necessarily mean you will be comfortable in that bag on a particular night with an outside air temperature of -10°C as there are far too many other factors to consider. The EN rating temperatures should be used in conjunction with the other information available with the bag and your own experience.

Brand Temperature Guide

Some brands have got around the two issues above by providing their own rating on the temperatures they have designed the bag to perform at; this includes various tests and their own experience. This is great when comparing bags within a certain brand, but as the tests are set by the brand they all different, so cannot be used to accurately compare between them. They are a good way of getting an idea of the temperatures the bag is suitable for and they are the only temperature comparison available for bags designed for extreme conditions.

Fill Weight

The weight of the insulation in the bag is shown in either grams per m2 or as a total weight. When using this to compare between bags make sure that if it's referring to down bags the down is of the same fill power, and if it's synthetic it's the same type of synthetic insulation. This cannot be used to accurately compare between different insulation types.

Fill Power

Fill Power is a measure of how well the down will expand after being compressed. This correlates to warmth for weight and packability. It is NOT a measure of how much down is in a product. For more information on Fill Power read Down Fill Power Explained.

OMM

The OMM Mountain Raid sleeping bags have not been submitted by OMM for testing, this is because OMM feel that the difference in results between EU and US testing is too wide to be conclusive and is therefore not of value to their products. They also say that as their bags are designed specifically for mountain marathon competitors they would expect anyone who would buys their product to have gained enough experience to be able to use the insulation weights and their own knowledge of what they require when racing to know which bag is suitable for them.

Conclusion

So, there is not one magic number that will allow you to see which bag will work for you at a specific temperature. There are a number of figures that when applied together, with the addition of your own experience and individual needs, can give a good idea of the performance a sleeping bag will provide.