One-alarm fires, two-alarm fires, three-alarm fires, etc., are categories classifying the seriousness of fires, commonly used in the United States and in Canada,[citation needed] particularly indicating the level of response by local authorities. The term multiple-alarm is a quick way of indicating that a fire is severe and is difficult to contain. This system of classification is used by both fire departments and news agencies.[4]
The most widely used formula for multi-alarm designation is based on the number of units, (for example firetrucks, tankers, rescue vehicles and command vehicles) and firefighters responding to a fire; the more vehicles and firefighters responding, the higher the alarm designation.[4]
In terms of understanding the relative severity of an incident, the government of Rochester, New Hampshire has reported in a statement that a "typical fire in a small structure will usually require only one or two alarms". In contrast, however, a "large-scale fire in a commercial building would require three alarms or more."[5]