The Problem with Temperature Ratings
Walk into any gear shop and you'll see sleeping bags labeled with temperature ratings: 32°F, 15°F, -10°F. The numbers look definitive. In practice, two people sleeping in identically rated bags on the same night can have very different experiences — one sleeping comfortably, the other shivering. Understanding what these numbers actually measure is essential to choosing the right bag.
The EN/ISO Standard: What It Actually Tests
Since the early 2000s, most reputable sleeping bag manufacturers have adopted the EN 13537 standard (now updated to ISO 23537) for temperature testing. This standardized approach finally made it possible to compare bags across brands on an equal footing.
The test uses an instrumented, heated manikin placed inside the bag in a controlled environment. It generates four key ratings:
- Comfort Rating: The temperature at which a "standard woman" can sleep comfortably in a relaxed position. This is the conservative, safer number.
- Lower Limit Rating: The temperature at which a "standard man" can sleep in a curled position for eight hours without waking due to cold.
- Extreme Rating: The temperature at which a "standard woman" can survive (not sleep comfortably — survive) for six hours. This is a survival threshold, not a comfort threshold.
- Thermoneutral (Upper) Rating: The temperature above which the bag becomes too warm.
How to Use These Ratings Practically
A common mistake is choosing a bag based on the lower limit rating and assuming it means "comfortable at that temperature." It doesn't — it means you'll survive without waking, in ideal conditions, after sleeping eight hours straight. For genuine comfort, use the comfort rating as your benchmark.
General Rules of Thumb
- Choose a bag rated 10–15°F below the coldest temperature you expect to encounter.
- If you sleep cold (common among many people, particularly women), lean toward the comfort rating rather than the lower limit.
- If you sleep warm, the lower limit rating is more relevant to your needs.
Factors That Affect Real-World Performance
Temperature ratings are tested under controlled laboratory conditions. In the real world, multiple variables shift performance:
- Sleeping pad R-value: Your bag's insulation does almost nothing underneath you — your body weight compresses the down. A high-R-value sleeping pad is critical for cold-weather camping.
- Humidity & moisture: A slightly damp bag performs well below its rated temperature. Keep your bag dry.
- Clothing layers: Wearing a base layer and hat inside your bag can add several degrees of effective warmth.
- Tent vs. bivy vs. open air: A tent provides meaningful wind protection and reduces the effective cold you experience.
- Caloric intake: Eating a calorie-dense snack before bed genuinely helps your body generate heat overnight.
- Individual metabolism: This varies significantly from person to person and is the primary reason two people in identical bags can feel so differently.
Down Sleeping Bags vs. Quilts: A Note on Ratings
Camping quilts — open-backed down blankets used by ultralight backpackers — are generally rated similarly to mummy bags, but the absence of back insulation means the pad R-value becomes even more critical. A quality quilt rated to 20°F paired with a high-R-value pad can outperform a lower-quality mummy bag on a thin foam pad.
Choosing Your Temperature Rating
| Camping Season | Typical Low Temps | Recommended Rating |
|---|---|---|
| Summer (lowland) | 45–60°F / 7–15°C | 35–45°F comfort |
| Three-season | 25–45°F / -4–7°C | 15–25°F comfort |
| Winter / Alpine | Below 20°F / -7°C | 0°F or colder comfort |
The Bottom Line
Temperature ratings are a useful starting point, not a guarantee. Use the EN/ISO comfort rating as your primary guide, factor in your personal sleep temperature tendencies, and always build in a buffer. A bag that's slightly too warm can be vented; a bag that's too cold is a serious problem.