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When it comes to sleeping bags, temperature ratings are not an exact science. These ratings are not a guarantee of warmth, but rather a guideline to help you compare different products. A sleeping bag doesn’t generate heat on its own—your body does. The job of the bag is to keep that warmth around you as efficiently as possible.
With that in mind, every human body is different, every night is different, and many factors combine to create a warm night’s sleep.
How to Use Western Mountaineering Temperature Ratings
All Western Mountaineering bags have a temperature rating based on extensive field use and carefully calculated fill weights. These ratings also assume:
- A properly insulated ground pad (around R 5.0+ for colder conditions)
- A set of dry base layers
- A pair of socks
We also have EN/ISO testing data available for most of our models, and you’ll find that the WM rating usually falls somewhere between the “limit” and “comfort” ratings derived from that protocol.
We recommend you treat the WM temperature rating as a goal post for the lowest temperatures you realistically expect to encounter, not as a comfort guarantee for every situation.
In practice, it’s usually smart to “aim a little lower” than the coldest forecast you expect. For many people, choosing a bag rated about 5–10°F (3–5°C) colder than the lowest temperature they anticipate will provide a comfortable margin, especially if they:
- Tend to sleep cold
- Expect long or wet days on the trail
- Are camping at higher elevations or in windy, exposed locations
Factors That Affect Warmth
A few common variables that can make you feel warmer or colder in the same bag:
- Individual metabolism (warm vs. cold sleepers)
- Hydration and nutrition
- Fatigue level and recovery
- Altitude, humidity, and wind exposure
- Shelter choice (fully enclosed tent vs. tarp or open bivy)
Because of these variables, we always encourage customers to choose a bag that gives them a bit of extra warmth in reserve, rather than aiming right at their limit.
Tips to Stay Warm
Base layers
A dry light–midweight thermal base layer can easily add warmth to your sleeping system, plus it helps shield your sleeping bag from body oils and grime, keeping it cleaner and extending its lifespan.
Warm head and feet
A warm pair of sleeping socks and/or a cozy hat can go a long way toward staying comfortable. The head and feet are major areas of heat loss, and covering them is an effective way to lock in warmth.
Go to bed warm and dry
Maintaining a warm core temperature before you get into your bag is crucial. If you go to bed cold and damp, your body may struggle to warm up the insulation around you. Do a few light exercises, change into dry layers, and avoid crawling in chilled if you can.
Calories & hydration
Without sufficient caloric intake, your metabolism may not generate enough heat throughout the night. A high-fat snack before bed and a warm drink can make a noticeable difference. A classic trick is filling a Nalgene with hot water, sealing it tightly, and placing it in your bag (or at your feet) before zipping up.
Sleeping pad choice
Always use a sleeping pad, and choose one with an appropriate R-value for the conditions you expect:
- Around R 3–5: suitable for most summer and shoulder-season trips
- Around R 7+: a better choice for winter and snow camping
Remember: your sleeping bag insulates you from the air around you, but most of your conductive heat loss is to the ground. Even the warmest bag will underperform without a properly insulated pad.
| Model Description | T Comfort | T Limit | T Extreme | WM Rating |
|---|---|---|---|---|
| Caribou MF | 39°F (4°C) | 30°F (-1°C) | 2°F (-17°C) | 35°F (2°C) |
| HighLite | 39°F (4°C) | 30°F (-1°C) | 2°F (-17°C) | 35°F (2°C) |
| DreamLite | 36°F (2°C) | 27°F (-3°C) | 2°F (-17°C) | 35°F (2°C) |
| SummerLite | 41°F (5°C) | 32°F (0°C) | 5°F (-15°C) | 32°F (0°C) |
| MegaLite | 32°F (0°C) | 22°F (-6°C) | -8°F (-22°C) | 30°F (-1°C) |
| Sycamore MF | 32°F (0°C) | 22°F (-6°C) | -9°F (-23°C) | 25°F (-4°C) |
| TerraLite | 28°F (-2°C) | 18°F (-8°C) | -15°F (-26°C) | 25°F (-4°C) |
| UltraLite | 25°F (-4°C) | 14°F (-10°C) | -21°F (-29°C) | 20°F (-7°C) |
| AlpinLite | 25°F (-4°C) | 14°F (-10°C) | -21°F (-29°C) | 20°F (-7°C) |
| Apache MF | 23°F (-5°C) | 11°F (-12°C) | -25°F (-32°C) | 15°F (-10°C) |
| Apache STS/GWS | 23°F (-5°C) | 11°F (-12°C) | -25°F (-32°C) | 15°F (-10°C) |
| Badger MF | 23°F (-5°C) | 11°F (-12°C) | -25°F (-32°C) | 15°F (-10°C) |
| Badger STS/GWS | 23°F (-5°C) | 11°F (-12°C) | -25°F (-32°C) | 15°F (-10°C) |
| CloudLite | 22°F (-5°C) | 9°F (-13°C) | -22°F (-30°C) | 15°F (-10°C) |
| VersaLite | 21°F (-6°C) | 9°F (-13°C) | -22°F (-30°C) | 10°F (-12°C) |
| Sequoia MF | 18°F (-8°C) | 5°F (-15°C) | -33°F (-36°C) | 5°F (-15°C) |
| Sequoia STS/GWS | 14°F (-10°C) | 0°F (-18°C) | -40°F (-40°C) | 5°F (-15°C) |
| Antelope STS/GWS | 10°F (-12°C) | -2°F (-19°C) | -43°F (-42°C) | 5°F (-15°C) |
| Antelope MF | 13°F (-11°C) | -1°F (-18°C) | -41°F (-41°C) | 5°F (-15°C) |
| Kodiak MF | 13°F (-11°C) | -1°F (-18°C) | -41°F (-41°C) | 0°F (-18°C) |
| Kodiak STS/GWS | 10°F (-12°C) | -2°F (-19°C) | -43°F (-42°C) | 0°F (-18°C) |