Temperature ratings

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