In the United States and Canada, the term "alarm" is used to classify the seriousness of fires, indicating the level of response by local authorities. A four-alarm fire is a catastrophic fire event that happens only a couple of times a year for most city fire departments. The number of alarms in a fire refers to the number of resources, including fire engines, trucks, chiefs, specialty units, and personnel, needed to manage an emergency incident. The more vehicles and firefighters responding, the higher the alarm designation. The following is a general guide to the number of resources dispatched for different alarm levels, but these can vary by department:
- One-alarm fire: 20 firefighters, 3 pumper trucks, 2 other trucks, and 2 members of command staff.
- Two-alarm fire: 40 firefighters, double the trucks, and 4 members of command staff.
- Three-alarm fire: 60 firefighters, 6 members of command staff, and additional engines and trucks.
- Four-alarm fire: up to 21 emergency vehicles, including 6 battalion chiefs, and additional engines and trucks.
- Five-alarm fire: at least 100 firefighters, 10 members of command staff, and additional engines and trucks.
- Six-alarm fire: at least 120 firefighters, 12 members of command staff, and additional engines and trucks.
Its important to note that the number of firehouses responding is not the rule behind the naming convention, although some cities may use the number of firehouses responding for multi-alarm designations because that is the simplest way.
