In heating calculations, the outside design temperature is the coldest outside temperature expected for a normal heating season. It is not the coldest temperature on record, but rather the lowest one recorded for a particular locale over a three to five year period.

Lists of outside design temperatures are published for selected localities throughout the United States. If a locality is not included on the list of winter outside design temperatures, check with a local weather bureau. Using an outside design temperature from the nearest locality on the list can be misleading, because even closely located cities can differ widely in weather conditions as a result of different altitudes, the effects of large bodies of water, and other variables.