Walk into any outdoor store and you'll see sleeping bags labelled with one cheerful number: "-5°C", "-10°C", "0°C". That number is almost always wrong for you, and learning why is the difference between sleeping well and lying awake at 2am wishing you'd packed differently.
The three numbers that matter
European standard EN 13537 (now ISO 23537) tests every certified bag at three temperatures:
- Comfort — temperature at which a "standard woman" can sleep comfortably in a relaxed position
- Limit — temperature at which a "standard man" can sleep for eight hours in a curled position without waking
- Extreme — survival rating; you will be cold, but the bag should prevent death from hypothermia for six hours
Which one is on the box?
Almost every brand prints the Limit rating in big numbers on the front of the bag, because it sounds more impressive. The Comfort rating is what you should actually buy by — and it's usually 5–7°C warmer than the Limit.
Our rule of thumb for Indian Himalayan trekking
Take the coldest night-time temperature you expect, subtract 5°C, and buy a bag whose Comfort rating matches that number. For a Ladakh trek in October, that means a -10°C Comfort bag (which most brands will sell you as a "-15°C bag" on the label).
The five degrees of padding accounts for: a tired body that's been hiking for 8 hours, dehydration, the fact that the temperature inside a single-wall tent can drop 3–4°C below ambient, and the very real possibility that the weather is worse than forecast.
Down vs synthetic — a quick word
This is a separate post, but the short version: down is warmer per gram, packs smaller, lasts longer if you look after it, and is useless when wet. Synthetic is heavier, bulkier, cheaper, and keeps insulating when damp. For Indian conditions where humidity is high and a single rainstorm can soak everything, we lean synthetic for anything below the snowline.
Up next: why we still pick Supra-Loft synthetic over 850-fill goose down for the Himalaya.