Three numbers, three very different meanings
Most modern sleeping bags are tested to the EN 13537 or ISO 23537 standard. A heated mannequin wearing a base layer is placed in the bag inside a climate chamber, and the test produces three ratings.
The number printed largest on the bag is often the limit rating, because it looks more impressive. That is not the number you should plan around.
- Comfort: a cold-sleeping woman stays comfortable through the night. Use this for planning.
- Limit: a warm-sleeping man is not cold, but is not necessarily comfortable.
- Extreme: survival only. Risk of hypothermia. Never plan around this number.
Which number applies to you
If you sleep cold, plan with the comfort rating and add a few degrees of margin. If you sleep warm, the limit rating is a reasonable guide but still not a target.
Gender matters because the standard uses a female test subject for comfort and a male subject for limit. A bag rated at -3°C comfort is rated comfortable for a cold sleeper at -3°C under standard test conditions, and a bag rated at -3°C limit is only just tolerable for a warm sleeper.
The bag is only part of the system
Your sleeping pad matters as much as the bag. A bag insulates you from the air above; the pad insulates you from the ground below. A -10°C bag on a summer foam pad will often leave you cold from underneath.
Look at the pad’s R-value. Around 2 is fine for summer, 3 to 4 covers three-season use, and 5 or more is for snow. If you sleep cold, add 1 to the R-value you think you need.
A worked example
You are planning a late-September trip with an expected overnight low of 2°C. You sleep cold. Subtract 3°C for safety, subtract 2°C because you are a cold sleeper, and you are looking for a bag with a comfort rating around -3°C. That is a three-season bag, not a summer bag.
If the forecast drops to -2°C, the same math points to a bag rated around -7°C comfort. That is a different purchase, and it is better to know before you drive to the trailhead.
Cheap bags and optimistic labels
Budget bags sometimes print a temperature that is closer to the limit than the comfort rating, or they skip standardized testing entirely. Treat an unlabeled bag as a summer bag regardless of what the packaging claims, and bring an extra layer for the first trip.
