Sleeping-Bag Temperature Ratings: Comfort, Limit and Extreme
Do not plan a night around the Extreme rating on a sleeping bag. It is a danger point in the test model, not a temperature at which a person should expect safe, restful sleep. The ISO definition warns that hypothermia can cause harm at this temperature and that it can lead to death.
The other numbers are useful—but only after you identify the test standard and the exact label being shown.
The three cold-side ISO reference temperatures
| Label | What it is useful for | What it does not promise |
|---|---|---|
| Comfort | The lower edge of the comfort range for the standard model sleeper in a relaxed posture, at the threshold of feeling cold. | That every user will be comfortable at that temperature. |
| Lower Limit | A lower reference for the standard model sleeper in a curled posture, at the threshold of feeling cold. | A universal comfort point or a sensible target for all buyers. |
| Extreme | A danger point at which the test model indicates a risk of harm from hypothermia. | Normal use, comfort or survival without injury. |
The standard also defines a Maximum temperature at the warm end of the comfort range. It is not the cold-weather number to use when choosing a bag.
What ISO 23537 covers
The current published standard is ISO 23537-1:2022. It provides a controlled method and labelling framework for adult-sized leisure sleeping bags whose limit temperature is −20°C or higher. It does not cover children’s or babies’ sleeping bags, military bags or bags intended for extreme-climate expeditions. The standard says its mannequin method is suitable for mummy-shaped bags that fit the mannequin appropriately, and it flags test issues with some non-uniform insulation layouts. Do not assume a quilt or other non-standard construction has a directly comparable ISO result unless the maker identifies the test used.
In the laboratory, a dressed, heated mannequin is placed inside the bag on a specified foam mattress in a controlled atmosphere. The bag’s thermal resistance is measured, then a physiological model is applied to calculate the reference temperatures. This makes results from the same standard more comparable; it does not recreate your exact campsite or body.
Read the product label in this order
- Find the declared standard and test version.
- Identify whether the prominent number is Comfort, Lower Limit or something else.
- Check that you are reading the rating for the exact model and size.
- Ignore an Extreme number when choosing for normal use.
- Add margin for uncertainty rather than selecting a bag that only just meets the forecast.
If a bag shows one low number without naming the rating or test, it should not be assumed equivalent to an ISO Comfort or Lower Limit result.
Why your night can be colder than the test
The mat, tent, site, wind, humidity and condition of the bag all affect the result. So do fit, clothing, fatigue, food, hydration, age, metabolism and personal cold tolerance. Damp or compressed insulation will not perform like a clean, dry laboratory sample.
Ground insulation deserves special attention: the bag beneath the body is compressed, so a suitable sleeping mat must limit heat loss below. Read the R-value guide and choose the whole sleep system, not the bag in isolation.
A practical decision rule
Start from the coldest credible overnight conditions for the location and season. Use Comfort if you know you sleep cold or want a conservative reference. Treat Lower Limit cautiously and only in the context of the test definition. If the forecast is uncertain, the trip is remote or the consequence of being wrong is serious, increase the margin or change the plan.
Browse sleeping bags and compare like-for-like ratings. Ask what test supports a number when the product information is unclear.