A sleeping bag's rating has three numbers from the EN/ISO test: comfort (a cold sleeper stays warm), limit (a warm sleeper isn't shivering), and extreme (survival only). Buy by the comfort rating, and choose one about 10°F below the coldest night you expect. The ratings assume you're on an insulated pad in a tent — so your pad's R-value matters just as much.
Sleeping bag warmth is standardized, which is genuinely useful — but only if you know which number you're reading. Get it right and you sleep warm; get it wrong and a “20-degree bag” leaves you shivering. Ready to shop? See our best sleeping bags and best cold-weather bags.
The three temperature ratings
Every EN/ISO-tested bag lists up to three numbers. Only one is the right planning number for most people:
| Rating | What it means | How to use it |
|---|---|---|
| Comfort | A cold sleeper stays comfortably warm | The number to trust for most people |
| Limit | A warm sleeper is curled up but not shivering | Often the headline number on men's/unisex bags |
| Extreme | Survival only — real risk of hypothermia | Never a planning number; emergencies only |
The takeaway: the comfort rating is the one to trust. The limit rating makes a bag look warmer than it sleeps for a cold sleeper, and the extreme rating is never a number you plan a trip around.
What EN/ISO 23537 actually tests
The numbers come from a lab standard — ISO 23537 (which replaced the older EN 13537 in 2017; the two are effectively the same). A heated mannequin wearing a base layer, lying on an insulated pad, is cooled in a chamber while sensors measure how much warmth the bag holds. Because every brand runs the same test, an ISO comfort rating lets you compare a bag from one company against another fairly — look for “EN” or “ISO” on the tag.
Men's vs women's ratings (an easy trap)
People sleep at different temperatures, and women on average sleep colder, so many brands print the comfort rating on women's bags and the limit rating on men's and unisex bags. That means two bags showing “20°F” can be meaningfully different in real warmth. Always compare the same rating type — ideally comfort — across the bags you're weighing.
How much margin to build in
Ratings are lab numbers under ideal conditions, so give yourself room: pick a bag whose comfort rating is ~10°F (5°C) below the coldest night you realistically expect. Then remember what the test assumes — an insulated pad, a tent, and dry base layers. Skip any of those and the bag sleeps colder than its number.
Ratings by season
| Season | Typical rating | Best for |
|---|---|---|
| Summer | 30°F and above (0°C+) | Warm nights, lightweight bags |
| 3-season | 15–30°F (−9 to −1°C) | Spring–fall, the most versatile choice |
| Winter | 0°F and below (−18°C+) | Freezing nights, snow camping |
A 3-season bag (around 15–30°F) is the most versatile single bag for most campers. Going colder? Pair the right rating with a warm pad — see what R-value you need, and whether down or synthetic fits your climate.
FAQ
What do sleeping bag temperature ratings mean?
They tell you the coldest conditions a bag is built for, using standardized EN/ISO tests. The comfort rating is the lowest temperature a cold sleeper stays comfortable; the limit rating is where a warm sleeper is curled up but not shivering; the extreme rating is survival-only. For most people, buy by the comfort rating.
Should I use the comfort or limit rating?
Use the comfort rating — it's the honest one for a good night's sleep, especially if you sleep cold. The limit rating is a 'not-shivering' number that suits warm sleepers, and it's often the headline figure on men's and unisex bags, which makes them look warmer than they sleep. When in doubt, go by comfort.
How much colder than the rating can I go?
Don't. If anything, add a margin: pick a bag with a comfort rating about 10°F (5°C) below the coldest night you expect. Ratings assume you're on an insulated pad, in a tent, wearing base layers — miss any of those and the real-world warmth drops. It's safer to be slightly too warm and vent than too cold.
What is EN/ISO 23537?
It's the international lab standard for testing sleeping bag warmth (ISO 23537 replaced the older EN 13537 in 2017; the two are near-identical). A heated mannequin with sensors measures how much insulation the bag provides, producing the comfort, limit, and extreme numbers — so you can compare bags from different brands on the same scale.
Why are men's and women's ratings different?
Women, on average, sleep colder, so many brands label women's bags with the comfort rating and men's/unisex bags with the limit rating. That means a women's and a men's bag showing the same number aren't equally warm. Compare the same rating type (ideally comfort) across bags to get a fair comparison.
Do temperature ratings account for the sleeping pad?
The test assumes you're on an insulated pad — but the pad's warmth is on you. A bag can only trap the heat your pad lets you keep, so a low R-value pad will leave you cold even in a well-rated bag. Match your pad's R-value to the season alongside the bag rating.
Next: our best sleeping bags, and keep the warmth by learning how to wash a sleeping bag without flattening the loft.
