How Sleeping Bag Temperature Ratings Work

If you're looking to buy a sleeping bag, chances are you've seen some numbers on the tag. These temperature ratings may tell you how warm or cold the sleeping bag will keep you, but they don't tell you everything else. So what do they mean? How can we make sense of them? And do they help us choose the right bag when camping in the bush or at home in front of our dying fireplaces?

Let's start with the two popular standard temperature rating systems in Europe, the EN and ISO.

EN and ISO Temperature Rating Systems 

The EN and ISO are the two international temperature rating systems used by sleeping bag manufacturers to describe how cold or warm it is inside a sleeping bag when you're wearing only baselayer thermals. These are identical and are based on users' comfort levels in ambient temperatures.

EN/ISO ratings consider various factors, including a sleeper's metabolism, clothing worn in the bag, and how exposed the bag will be to wind and moisture.

The EN/ISO ratings consider how much insulation your body needs from either air or ground below. Sleeping bags are put through standardized tests where the temperature of each bag's ability is rated in the following three different temperature ratings.

  1. Comfort rating

The comfort rating is a metric that shows what temperature the average female will have a comfortable night's sleep. Any colder and you may need additional layers or a warmer sleeping bag.

  1. Limit rating

The limit rating is based around the lowest temperature in which the average male will have a comfortable night's sleep. This is usually the main number you’ll see advertised on the products title and packaging. Again - if you’re in conditions that exceed your sleeping bags recommended limit of comfort rating, it may be time to layer up, purchase a sleeping bag liner or upgrade to a warmer rated sleeping bag.

  1. Extreme rating

Extreme rating is the lowest temperature at which a sleeping bag is suitable for use. It's not to say you can't use it at lower temperatures, but it's designed for that temperature range and beyond. For example, an 850-fill-down sleeping bag may be rated down to -26°C as its extreme rating. This means that the manufacturer has tested this bag down to -26°C and found that it will keep you warm enough in most cases.

Even though temperature ratings are intended to reflect the warmth of a sleeping bag, they're only a part of the story. A combination of other factors determines the warmth you experience in the field when using a sleeping bag. Some of them include:

  • Weight
  • Fit 
  • Fill and construction


Sleeping bags are also rated by their warmth to weight ratio, which means how much insulation they provide and how much material they use. This is important because, as technology advances, manufacturers can pack more heat-reflecting materials into smaller spaces without sacrificing comfort or safety. For example, 20 years ago, a suitable lightweight sleeping bed might have weighed 2 pounds, while today's may weigh less than half that amount but still keep us warm when temperatures drop below freezing!


The fit of your sleeping bag is just as crucial as its temperature rating. A good-fitting sleeping bag will give you greater comfort, so most people tend to sleep better in them. But how do you know that your sleeping bag fits?

The height and weight of each person vary slightly, so there's no exact formula for what makes an ideal fit. However, generally speaking, If the bag is too big, it can feel cold because there isn't enough insulation between you and the cold air outside. On the other hand, you'll have trouble moving around inside it if it's too small. When trapped by snug walls made from thick synthetic fibres (or down feathers), your body heat has nowhere to go.

When choosing a sleeping bag size for yourself or someone else who may be sharing their space with you d, look for one that allows plenty of room for both arms but also enough room below the waistline so that hips don't overheat or get uncomfortable due to pressure points.

Fill and construction

Once you've decided how much insulation you want, the next thing to consider is the fill and construction of your sleeping bag. These are two very important factors determining how warm a sleeping bag will be.

Synthetic vs Down: Synthetic insulation is made from petroleum products and is less expensive than down. It's also more durable, so it won't clump or lose its loft if it gets wet (a real problem if you camp in moisture exposed conditions). However, synthetic doesn't compress as well as down, so it takes up more space in a backpack or stuff sack when not used—not ideal for hikers who want their gear to take up minimal space in their packs.

Down vs synthetic: Down has many benefits over synthetic insulation, including better compressibility and lighter weight, but they cost more than synthetics. They're usually reserved for high-quality sleeping bags designed to keep you warmer under harsh conditions like mountaineering expeditions and frequent backpacking trips where pack weight matters most. For more information, get in contact with us via email – or give us a call on 0800 25 00 25.