Sleeping bags are one of the biggest investments you’ll make in your camping or hiking kit – and getting the temperature rating right can be the difference between a great night’s sleep and a long, cold, regret-filled one.
The tricky part? Temperature ratings can be confusing. You’ll often see three numbers on the tag – comfort, limit, and extreme – and it’s not always clear which one matters. To make things even more complicated, some brands test to international standards, while others don’t test at all.
Let’s break down how these ratings actually work, why the ISO 23537 standard matters, and how to choose the right bag for your next trip.
Why Sleeping Bag Temperature Ratings Exist
Temperature ratings are there to give you a guide – a baseline – for how warm a sleeping bag should be in specific conditions.
The problem is that not all ratings are equal. Some brands create their own “in-house” numbers, which might be based on lab tests, field testing, or just experience. That means one brand’s “-5°C bag” might feel warmer or colder than another’s.
That’s where ISO 23537 comes in.
What is ISO 23537?
ISO 23537 is the international testing standard for sleeping bags. It replaced the older EN 13537 system, but works in a similar way.
The idea is simple: if two bags are tested under the same conditions using the same method, their results are directly comparable. It removes guesswork and marketing fluff, giving you confidence that a -2°C bag from one brand should perform similarly to another -2°C bag tested to the same standard.
In short – if you’re comparing sleeping bags, look for ISO 23537 ratings.
Anyone can print a temperature on a tag, but ISO-tested bags have been through a controlled lab test.
How ISO 23537 Testing Works
ISO testing uses a thermal mannequin – basically a heated dummy covered in temperature sensors – dressed in a base layer and placed inside the sleeping bag on a sleeping mat, in a climate-controlled chamber.
By monitoring how much energy the mannequin needs to maintain body temperature, testers can calculate three key values:
- Comfort – the lowest temperature where a standard woman (warm sleeper) can expect to sleep comfortably.
- Limit – the lowest temperature where a standard man (cold sleeper) can expect to sleep comfortably curled up.
- Extreme – the survival rating. This is not a comfort number – it’s the absolute lowest temperature where a sleeper might avoid hypothermia. It’s there as a reference only.
Most people should use the Comfort or Limit rating as their guide – not the Extreme.
Comfort vs Limit: Which One Should You Use?
If you sleep cold, choose your bag based on the comfort rating.
If you sleep warm, or plan to use a good sleeping mat and wear thermals, you might go closer to the limit rating.
As a rule of thumb:
Add 5°C to the rating if you’re not sure. It’s always easier to cool down by unzipping than to warm up when you’re freezing.
That said, don’t go too warm either. Picking a bag that’s way warmer than your conditions can actually make you less comfortable.
If you buy a -5°C bag “just in case” but you’re always camping at +5°C, you’ll likely overheat. You’ll unzip or stick your arms out, but it’s still only 5°C outside – so now you’re too cold again. This constant on-and-off dance is what keeps people awake. The sweet spot is a bag that matches your real conditions, not the extreme ones that might happen once in a blue moon.
Why Your Sleeping Mat Matters More Than You Think
Even if you buy the warmest sleeping bag in the world, it can only trap the heat you keep. If you’re losing warmth to the ground, no bag can fix that.
That’s why R-value (the insulation rating of your sleeping mat) plays a huge role. Your mat stops conductive heat loss to the ground, while your sleeping bag traps radiant heat around your body.
And remember – when you lie in your sleeping bag, the insulation underneath you gets squashed flat. Once it’s compressed, it can’t trap warm air, which means it provides almost no insulation underneath you. Your mat is doing all the work there.
Think of them as a team – bag = air insulation, mat = ground insulation.
If you want to go deep on this, check out our full guide on R-Value and Sleeping Pads (coming soon).
Other Factors That Affect Warmth
Temperature ratings are a guide – not a guarantee. You might feel warmer or colder depending on:
- Your metabolism: Some people just run hot or cold.
- Your tent: A small, enclosed tent traps more heat than a big open one.
- What you wear: Layers and dry thermals help massively.
- Your dinner: A warm meal before bed can lift your core temp.
- Fatigue and hydration: Being tired or dehydrated can make you feel colder.
So don’t rely solely on numbers. Use ratings as a starting point, and factor in your experience, your gear, and your destination.
Quick Tip: Trust Tested Ratings
If a bag’s temperature range looks too good to be true, and it doesn’t mention ISO 23537, it probably isn’t tested.
Reputable brands like Sea to Summit, Nemo, Therm-a-Rest, Mont, and Rab all use ISO ratings – and it’s worth prioritising that when shopping.
Key Takeaways
- Check for ISO 23537 certification – it’s the only way to compare apples with apples.
- Use Comfort or Limit – never buy based on Extreme.
- Match your mat – your sleeping pad is half the equation.
- Don’t go too warm – overheating makes for a rough night.
- Know yourself – cold sleeper? Go warmer.
- Numbers are guides – conditions and habits matter too.
Final Thoughts
Choosing a sleeping bag is about more than chasing the lowest number on the tag. It’s about matching your bag to your conditions, your body, and your setup.
So next time you’re comparing -5°C bags, don’t just look at the number – look for the ISO 23537 label, check your R-value, and know how you sleep.
That’s how you’ll end up warm, comfortable, and actually enjoying your nights in the outdoors.
