Choosing a Temperature Rating
The insulating qualities of a sleeping bag depend on much more than simply its temperature rating. Bag shape, the type of sleeping pad, and your physical condition and metabolism all contribute to the overall level of insulation. If you are a cold sleeper, you may consider a bag rated five- to ten-degrees below the anticipated temperature. If you are a warm sleeper, choose a bag with a rating equivalent to the ambient temperature.
All temperature ratings refer to the outside air temperature in degrees Fahrenheit.
Courtesy of Kelty