Gear · Sleeping outdoors
A bag labelled “-5°C” does not automatically mean you will sleep comfortably at -5°C. Bags tested under ISO 23537 can show more than one temperature value, and the useful question is whether you are looking at a comfort rating or the lower limit for a warmer sleeper.
The short answer
RatingWhat it meansHow to use itComfortThe lower temperature at which a standardized colder sleeper is expected to remain comfortable.The best starting point if you want normal sleep rather than simply tolerating the night.Lower LimitA lower threshold intended for a standardized warmer sleeper.Useful for product comparison, but more aggressive if you personally sleep cold.ExtremeAn emergency survival-oriented boundary rather than a comfort target.Do not use it as your purchasing temperature.
REI describes ISO 23537 as the current independent testing protocol commonly used for backpacking bags and notes that temperature ratings are guidelines for comparison, not guarantees of warmth. Sea to Summit likewise explains two key ISO values—Comfort and Lower Limit—and stresses that weather, movement and the rest of your sleep system affect real-world warmth.
Why can one bag have two different “low” temperatures?
The test cannot predict exactly how every person will feel. Instead, it provides a consistent basis for comparing products. Comfort and Lower Limit therefore represent different standardized reference points. In a current REI example, one bag has a Tested Comfort value of 40°F (about 4°C) and a Tested Lower Limit of 30°F (about -1°C). That difference is large enough to change a shoulder-season decision.
This is where many buyers go wrong: they see the lower number and treat it as the temperature for comfortable sleep. If you sleep cold, that can leave you with a bag that technically covers the trip but feels marginal in practice.
Which number should you use when buying?
For most people, we would start with Comfort. If the realistic overnight low is around 5°C, it is safer to choose a bag whose Comfort value is around that level or a little lower rather than relying on the Lower Limit matching the forecast.
Practical rule: do not buy with zero margin. Mountain weather, humidity, fatigue and personal cold sensitivity can quickly use up a narrow temperature buffer.
Your sleeping pad is part of the system
A warm bag cannot fully compensate for an under-insulated pad. Your body compresses the insulation underneath you, so much of the protection from the ground comes from the sleeping pad. REI notes that standardized bag testing is performed on an insulated surface, while Sea to Summit explicitly recommends thinking of the bag, pad and other layers as one system.
That means a bag with a 0°C Comfort rating can perform much worse than expected if you place it on a very light summer pad over cold ground.
Personal cold sensitivity matters more than the label
Two people in the same bag can feel very different. Food intake, fatigue, damp clothing, body size, metabolism and acclimatization all matter. If you often feel cold, treat Comfort as your minimum and keep extra margin. If you are consistently a warm sleeper, you may choose closer to Lower Limit, but that remains a personal decision rather than a promise from the standard.
If a bag is called “30°F”, what does that actually mean?
The product name alone is not enough. A bag sold in a “30°F” class can have a Comfort value around 34–40°F while its Lower Limit may sit around 24–30°F, depending on the model. Always open the specification table and identify which standardized number you are reading.
Current REI listings illustrate this clearly: products with similar marketing temperature names can still publish different ISO Comfort and Lower Limit values. Two “30-degree” bags therefore may not offer the same practical warmth for the same sleeper.
How to choose for a real trip
- Estimate the realistic minimum overnight temperature for the place and season.
- Use ISO Comfort as the main reference if you want ordinary sleep without operating at the edge.
- Add a few degrees of margin in changeable weather or if you sleep cold.
- Check your sleeping pad as well; the bag is only one part of the insulation system.
- Do not use Extreme as your target temperature.
- When comparing two bags, compare like with like: Comfort with Comfort, Lower Limit with Lower Limit.
Bottom line: if you want a clear and conservative buying rule, start with Comfort. Lower Limit is useful comparison data, but it is not a promise of pleasant sleep. The most reliable setup combines an appropriate bag, an adequately insulated pad, dry layers and real temperature margin.
