Sleeping bag season ratings (1 through 5) are a British-calibrated shorthand for typical temperature ranges; for accurate US buying decisions, you need the ISO 23537 Comfort and Limit numbers instead.
A tag that says “3-season” sounds straightforward until you camp a Colorado shoulder season and freeze. The problem: season ratings were built around British nights, not the Rockies. Most US buyers pick the wrong bag because one label—”Extreme”—looks like a spec but is really a survival limit. The table below shows what each season number actually expects, and the ISO system that replaces guesswork with three specific numbers.
What The 1–5 Season Ratings Actually Mean
Season numbers map loosely to temperature bands. Season 1 is for muggy summer nights. Here is the standard breakdown as published by Mountain Warehouse and Alpkit.
| Season | Typical Use (UK/Calibration) | Approx. Temperature Range |
|---|---|---|
| 1 | Summer camp / indoor shelter | Above 32°F (0°C) |
| 2 | Late spring, early autumn | 32°F to 23°F (0°C to –5°C) |
| 3 | Three-season (spring–fall) | 23°F to –4°F (–5°C to –20°C) |
| 4 | Winter / high altitude | –4°F to –22°F (–20°C to –30°C) |
| 5 | Expedition / extreme cold | Below –22°F (–30°C) |
A “3-season” bag typically benchmarks around 20°F / –6°C. A winter bag targets 0°F / –18°C. Those are useful starting points, but the season number alone won’t save you from a bad night. You need the ISO three-number system.
ISO 23537: Comfort, Limit, Extreme — The Only Numbers That Matter
The global standard (formerly EN 13537) gives every bag three lab-tested ratings. They are not interchangeable, and buying on the wrong one is the most common mistake in camping gear.
- Comfort Rating: The lowest temp where an average cold sleeper (modeled on a “standard woman”) can sleep relaxed without feeling cold. If you sleep cold or are female, buy to this number.
- Limit Rating: The lowest temp where an average warm sleeper (modeled on a “standard man”) can sleep curled for 8 hours without waking. Most men who sleep warm can use this as their buying threshold.
- Extreme Rating: A survival statistic. The coldest temp an average person can survive 6 hours without dying of hypothermia—but frostbite risk is high. Never use this to choose a bag.
Women’s bags typically carry ~10–15°F more insulation than a men’s bag with the same number, because women generally sleep colder. A bag that lists only one temperature without specifying Comfort or Limit is a brand estimate, not a standardized result.
How To Pick The Right Bag For Your Trips
The BushBuck and Mountain Equipment selection guides agree on a five-step process. Follow it and you skip the guesswork.
- Know your coldest night. Check the actual low for your destination and season, not the average. A June night in the Sierra can dip to 30°F.
- Pick your strategy. If you want one bag for “most trips,” buy a Comfort or Limit rating 5–10°F below your expected low. If you need a “worst case” bag, add more margin.
- Match your sleeping mat. The ISO test assumes an R-value of roughly 5–6. Summer needs R 2–3, three-season needs R 3–4+, winter needs R 4+. Without the right mat, none of the bag numbers protect you.
- Adjust for your personal thermostat. Cold sleeper? Target the Comfort rating. Warm sleeper? The Limit rating is your number.
- Verify the label. Read whether the printed temp is Comfort, Limit, or Extreme. If it says only “20°F,” treat it as a rough brand estimate until you see the full ISO table.
Who This Does And Doesn’t Work For
These steps work for every camper and backpacker using an ISO-tested bag from a major brand (Marmot, REI, Sea to Summit, Therm-a-Rest, NEMO). If you buy a budget bag with no ISO label, the season number is all you get—and in that case, size up one full season from what you think you need.
Common Mistakes That Leave You Cold
- Buying on Extreme. This is the most dangerous error. The Extreme number is a survival threshold, not a comfort guideline. Choosing a bag whose Extreme matches your expected low guarantees a freezing, unsafe night.
- Skipping the mat check. A 20°F bag loses all its warmth if your sleeping mat lacks the R-value to insulate you from the ground. The ISO test includes a test pad; your trip needs a real one.
- Treating ratings as guarantees. Ratings are lab benchmarks under controlled conditions. Your metabolism, clothing, tent, and wind exposure change the real-world performance.
- Assuming a 1-season bag works for cool summer nights. A summer bag is for warm weather only. If the forecast dips near 32°F, even “summer” can freeze you out.
How Season Ratings Stack Up For US Campers
The season numbering system was calibrated for British nights, which are milder than much of the US. A UK 3-season bag might fail in a Montana October. The fix is simple: ignore the season number and read the ISO Comfort and Limit ratings. The table below shows how a range of real bags perform.
| Bag Type | ISO Comfort / Limit | Best For |
|---|---|---|
| Summer / 1-season | 35°F / 28°F (approx.) | Warm nights, indoor shelter, car camping in July |
| 3-season (20°F bag) | ~30°F Comfort / ~20°F Limit | Spring through fall; most backpacking trips |
| Winter (0°F bag) | ~10°F Comfort / ~0°F Limit | Snow camping, high elevation, deep cold |
| Expedition (–20°F bag) | ~–10°F Comfort / ~–20°F Limit | Mountaineering, arctic conditions |
If you camp across multiple seasons—spring trips in the Appalachians, summer in the desert, fall in the Smokies—a single 3-season bag at 20°F Limit handles most of it. For dedicated winter camping, step up to a 0°F bag. Our tested roundup of all-season sleeping bags shows the real-world best options for variable conditions.
Your Final Selection Checklist
- Confirm your coldest expected night (not the average).
- Find the ISO Comfort and Limit on the bag tag—ignore the Extreme number.
- Choose: Comfort rating if you sleep cold, Limit if you sleep warm.
- Buy a sleeping mat with R-value that matches the season (R 4+ for winter, R 2–3 for summer).
- If the bag has no ISO label, assume the single number is optimistic and buy one season warmer than you planned.
FAQs
Can I use a 3-season bag in winter?
Only if your winter temperatures stay above the bag’s Limit rating. A 20°F Limit bag works for a mild 25°F night but fails in single digits. In real winter weather, you need a bag with a Comfort rating below your expected low, plus a high-R-value mat.
What does “Extreme” mean on a sleeping bag label?
The Extreme rating is the temperature at which an average person can survive about six hours without fatal hypothermia—but will likely suffer frostbite. It is not a comfort or usability target. Never choose a bag based on this number.
Do women need a different sleeping bag rating than men?
Yes. Women generally sleep colder than men, so ISO standards model the Comfort rating on a “standard woman” and the Limit on a “standard man.” A women’s bag with the same season number as a men’s bag typically carries 10–15°F more insulation. If you sleep cold regardless of gender, buy to the Comfort number.
Why does my sleeping mat matter for temperature ratings?
If your mat has a lower R-value, ground heat loss can make a 20°F bag feel like 35°F. Always match your mat’s insulation to the same conditions your bag is rated for.
References & Sources
- Mountain Warehouse. “Sleeping Bag Guide.” Defines season 1–4 ratings and British temperature calibration.
- Alpkit. “How Warm Should Your Sleeping Bag Be?” Temperature ranges, Comfort/Limit/Extreme definitions.
- Sea to Summit. “Sleeping Bag Temperature Ratings Explained.” ISO test requirements and R-value importance.
- Therm-a-Rest / Cascade Designs. “EN/ISO Sleeping Bag Ratings.” Comfort/Limit/Extreme clarity from a major manufacturer.
- BushBuck. “Sleeping Bag Temperature Ratings Explained.” Step-by-step selection guide and Extreme rating warning.