A bag marked 20°F does not mean you will be comfortable at 20°F. The number usually refers to the limit rating, which is the temperature at which a standardised warm sleeper survives the night curled up — not comfortable, just not in danger.
- 1
Learn the three numbers
ISO and EN testing produce three figures. Comfort is where a standardised cold sleeper rests comfortably. Limit is where a standardised warm sleeper is just holding on. Extreme is a survival-only figure where hypothermia is a real risk. Most bags are marketed on the limit rating.
- 2
Assume you need the comfort rating
For an actual comfortable night, use the comfort figure and treat the marketed number as optimistic by roughly 10-15°F. A bag sold as a 20°F bag typically has a comfort rating around 30-32°F, which is the honest temperature to plan around.
- 3
Adjust for yourself
The standard assumes an average sleeper in base layers on an adequate pad. People who sleep cold should add 10°F to whatever rating they think they need. Fatigue, dehydration, and going to bed underfed all make you sleep substantially colder than the same bag would suggest.
- 4
Remember the pad does half the work
Insulation compresses under you and stops working, so the sleeping pad's R-value determines a large share of your warmth. A 15°F bag on a summer pad will be cold at 30°F. Matching pad R-value to conditions is not optional.
Sleeping pad R-valuesWhy the pad matters as much as the bagRead Guide → - 5
Buy for your coldest realistic night, then vent
It is easy to cool a bag down by unzipping and hard to make a bag warmer than it is. Buy for the coldest trip you actually take and manage warm nights with the zipper, rather than owning a bag that fails in spring.
What the Ratings Mean
| Rating | Meaning | Use it for |
|---|---|---|
| Comfort | Standard cold sleeper is comfortable | Planning your real range |
| Limit | Standard warm sleeper is curled up, just coping | The marketed number |
| Extreme | Survival only — hypothermia risk | Never plan around this |