Sleeping Bag Temperature Ratings Explained – Choose the Right Warmth Level for Every Season

Learn Which Bag Rating You Need for Any Season


alan jakeman in snow

Alan is our resident expert on all things wild camping. He loves nothing better than to get out of the house with just his ex-Army sleeping gear and a stove, and head off to the remotest spot he can find near his home on the south coast of the UK.
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A key factor to consider when choosing the best sleeping bag for backpacking or camping is its temperature rating.

However, understanding sleeping bag temperature ratings can sometimes be a little tricky, and hard to know which temperature rating to choose, especially if you'll be wearing clothes inside the bag.

In this guide, we're going to explain all the details about sleeping bag temperature ratings and everything you need to know to help buy sleeping bags that have the right warmth level for any season.

Sleeping Bag Season Ratings

Sleeping bags are rated by season. Season rating starts from summer months (season one) to extremely cold seasons (season five).

Season one: ideal for summer camping, the temperature ranges from 5°C and above.

Season two: for late spring and early autumn with temperatures around 0-5°C.

Season three: autumn, early winter with no-frosts, and mid-cold nights. Temperatures between 0 to -5°C.

Season four: best suitable for cold winter nights with frost and high altitude, temperature as low as -10°C.

Season five: suitable for extremely cold, high altitude, and polar conditions. Temperatures as low as -40°C.

To understand sleeping bag ratings better, it will help to know how these bags are tested in the laboratory, the limitations in the process, and the terms used in rating the sleeping bags.

Lab-tested temperature ratings (EN and ISO) and non-EN/ISO are the most important things to consider when shopping for a sleeping bag.

Sleeping Bag Temperature Ratings Explained

What Are Sleeping Bag Temperature Ratings?

A sleeping bag temperature rating is a number by which sleeping bag manufacturers put on their sleeping bags to give people an idea about the best conditions for using a particular sleeping bag.

It basically gives you an idea of the temperature limit and comfort level a sleeping bag can be used in which makes understanding the EN/ISO ratings for sleeping bags important.

The reason for this rating was due to the lack of standard temperature ratings for sleeping bags because the fact that a manufacturer rated a bag for 15F doesn't mean that the bag could safely be used under such conditions.

This has also made it confusing for customers to buy a warmer bag since they have to compare other features like the sleeping bag materials, weight, and price.

So in 2005, the EN 13537 Standard rating for sleeping bags was introduced to help customers compare different sleeping bag brands. A new ISO standard was later introduced in 2017 to serve as an updated version of the EN standards, and as the new protocol for all sleeping bag ratings onward.

Although there's a lot of science, technology, and testing involved in this rating, understanding how it works can help you choose the right sleeping bag for your adventures.

The Sleeping Bag Rating Test

The temperature rating system is quite complex, but you can download the standard (you have to pay) for more guidance regarding the testing procedure. However, here's is a quick version of the guide:

For testing purposes, the testing companies use a heated manikin that's covered in high-tech sensors instead of human beings.

The manikin is placed inside the sleeping bag on a foam sleeping pad and then placed inside a purpose-built cold chamber.

Evaluators will then pay close attention to the data they get from the manikin's sensors. These data include:

  • The time it takes for the manikin's heat to accumulate in the sleeping bag
  • The temperature tog at which heat starts escaping from the sleeping bag
  • Whether the sleeping bag can maintain a relatively steady temperature level
  • The temperature at which the sleeping bag becomes ineffective

This information is then put into a specialized computer which spews out a series of numbers. The numbers are then interpreted by the testing professionals and also used to determine the sleeping bag rating.

Sleeping Bag Temperature Rating Systems

It is important to keep in mind that there's more than one number associated with the sleeping bag's temperature ratings.

Although most sleeping bags are marketed to specific temperatures such as 20°F, these ratings are simplified for marketing purposes. 

The temperature rating system gives a range for sleeping bags instead of a specific number and not all manufacturers use this rating. However, a sleeping bag that uses this rating will have EN13537 written on it next to the rating.

The ISO 23537 standard temperature ratings for sleeping bags is broken into three categories which are:

  • Comfort: this is the temperature at which a person can comfortably sleep inside the sleeping bag
  • Transition/lower limit: this is the highest temperature at which someone can sleep inside the bag with the zip open and their head and arms out. A temperature hotter than this will make a person start sweating and be unable to sleep
  • Risk/extreme: the minimum temperature at which a person would survive at 6 hours without abnormally low body temperature (hypothermia). It is not advisable to use sleeping bags under this condition unless in an emergency.
ISO 23537 standard temperature ratings for sleeping bags

Out of these ratings, the "comfort" level is probably the most useful sleeping bag ratings. You can read the level at which the sleeping bag is rated and make a decision based on how warm you want it to be.

Types Of Sleeping Bag Insulation

Sleeping bag insulation is the final thing to consider when buying a versatile sleeping bag. There are two types of sleeping insulation; down and synthetic sleeping bag insulation.

Down Sleeping Bag Insulation

The down sleeping bag insulation has an excellent warmth to weight ratio and is a go-to choice for lightweight mountaineers and backpackers.

The insulation property of this type of sleeping bag insulation is severely limited once it is wet and can take a long time to dry out.

In recent years, the down sleeping bag insulation has seen a major advancement in water repellent capabilities by companies such as Down Tek.

So if you frequent wet or snowy environments and are longing for the lightweight and warmness offered by down sleeping bags, the Down Tek can be the best solution for you.

Synthetic Sleeping Bag Insulation

Synthetic sleeping bags are usually made from polyester, or a variant and work similar to down sleeping bags but are heavier and less compressible.

The heaviness and less compressible features of these bags make them less appealing to people who are trying to keep their gear small and lightweight.

However, this bag can manage to maintain its insulation when wet, which is an advantage for some mountaineers, campers, and backpackers.

How Do Sleeping Bag Ratings Affect Your Comfort?

Now that you have a solid understanding of sleeping bag ratings, you're probably wondering how it affects your camping experience.

It is important to note that these temperature ratings are not set in stone, but just some guidelines. They are designed to help you make informed decisions when buying sleeping bags, and it is not guaranteed that they will keep you perfectly warm at night. 

So you should keep the following in mind when choosing a sleeping bag based on its temperature rating:

1. What You Wear While Sleeping Matters

The temperature ratings don't take every possible situation into account, so they didn't consider what you wear while sleeping. 

For example, someone wearing a thick jacket that has more fill power while they sleep will feel much warmer in cold weather conditions than someone who's wearing shorts.

This means that you can have a lot of control over how warm you are at night regardless of your sleeping bag temperature ratings.

A man preparing the sleeping pad inside a tent

2. Your Sleeping Pad

One of the primary purposes of sleeping bags is to provide insulation from the ground, and your sleeping pad can be a factor in your sleeping comfort.

Your sleeping pad can make a lot of difference in your sleeping comfort, and it doesn't matter how warm your sleeping bag is if your sleeping pad can't provide you with enough insulation from cold ground temperatures.

How Does Sleeping Bag Design Affect Warmth?

As I mentioned above, many factors contribute to the warmness of a sleeping bag apart from the temperature ratings. These factors can range from the sleeping bag shape, features, fill power, sleeping pad, and age of the bag.

To better understand this, let's first take a look at how a sleeping bag functions. It works just like a thermos; when a warm object (human body) is put inside the bag, it helps that object stay warm.

It is important to note that any extra space inside the bag will take our body more energy to heat up. This means that it is better to find a sleeping bag that fits you perfectly without leaving a ton of extra space. While the extra space can be great for comfort, it can make the bag feel drafty and colder than its actual temperature rating.

A slim mummy-shaped sleeping bag that has a little room will weigh less and take less energy from our body to keep it warm. But it can be a little bit uncomfortable, so it is left for you to decide on the balance.

Another factor that contributes to warmth is the style of the bag. For example, sleeping quilts precede the traditional mummy bag for a minimalist blanket-style design. 

One of the biggest drawbacks of sleeping quilts is; a hood instead of wrapping warmly around your neck, and also cuts off more in the neck/shoulder regions. Also, sleeping quilts don't usually make a complete circle around your body but are instead secured around your sleeping pad. Although these design features save weight, the drawback is that they sacrifice warmth.

Lastly, the age of your sleeping bags can also affect its warmth. An older sleeping bag will lose a certain amount of its warmth and down, which is unavoidable to some extent. However, you can minimize the effects by taking proper care of the bag and storing it in an open place rather than stuffing it in the stuff sack when not in use. Keeping it outside its stuff sack rather than having it compressed for a long period will let it breathe, which will help it to stay firm and healthier.

How Sleeping Pads Affect Sleeping Bag Warmth

The sleeping pad matters a lot more than you might think and can have a great impact on warmth. It doesn't only provide a comfortable sleeping surface, but also provides insulation with the freezing or cold ground.

Sleeping pads are measured in terms of warmth and Insulation and have a key metric (R-value) which ranges from less than 1 for s summer pad, and even up to 8 or higher for a winter pad.

All-season sleeping pads have R-values between 2 to 4 which is enough to provide solid insulation from most non-freezing grounds. And cold seasons require higher R-value sleeping pads.

The most important thing to remember is that you're putting together a sleeping system, this means that focusing only on temperature ratings alone and ignoring other key factors like R-value, fill power, and sleeping bag shape will not guarantee warmth.

mummy sleeping bag

Final Thoughts - Sleeping Bag Temperature Ratings

By now, you have a solid understanding of sleeping bag temperature ratings and the other factors that make a sleeping bag warmth.

The standard sleeping bag temperature rating is broken into 3 categories, comfort, transition/lower limit, and risk/extreme.

It is important to note that temperature rating alone doesn't guarantee warmth, but other factors such as the sleeping bag shape and your sleeping pad also come to play.

Frequently Asked Questions about sleeping bags temperature ratings

How Warm Should My Sleeping Bag Be?

How warm your sleeping bag should be depends on the type of weather conditions you are camping in. 

If you're camping in the middle of winter, you'll want a sleeping bag with a temperature rating of 10°, and more likely 0° or lower. However, a sleeping bag with a temperature rating of 20 degrees can get you through most of the year without discomfort.

Ideally, you should think about how cold or hot it will be while you're on the trip and buy your sleeping bags accordingly.

What Do The Degrees On Sleeping Bags Mean?

The degrees on sleeping bags indicate the lowest temperature at which a sleeping bag can be able to keep you warm (i.e., 20 degrees celsius should be able to keep you comfortable down to 20°C). However, there are no standard measurement ratings in the U.S., and that's why measurement ratings and their real-world meanings vary from one manufacturer to another.

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