GlossaryFire and EMS operations
Fire containment
Fire containment describes the extent to which established control lines are expected to prevent a wildfire from spreading.
Fire containment describes the extent to which established control lines are expected to prevent a wildfire from spreading. A reported containment percentage concerns the fire's perimeter and containment assessment, not the percentage of burned acres that are extinguished. A contained fire can still require extensive monitoring and work.
Interpreting an incident update
The official ICS-209 reporting guide explains containment as the percentage of fire perimeter contained. Read the figure with the update's date, time, and geographic scope. It is an operational assessment at a particular point in the incident.
Fire size and containment percentage measure different things. Acres describe area affected. Containment describes a boundary-related assessment. Neither should be converted directly into the other, and neither alone establishes when residents may return to an evacuated area.
Example
Hypothetical example: A wildfire update reports 2,000 acres and 40% containment. A reader says that 800 acres are out and 1,200 are still burning.
That calculation misapplies the percentage. Forty percent containment does not divide the acreage into burning and extinguished sections. The update needs its own perimeter and operational information; the arithmetic does not supply those facts.
Common confusion
Containment is different from extinguishment. A fire can retain heat within its boundary after lines are established. “Under control” may also have a particular agency or structural-fire meaning, so it should not automatically replace a wildfire containment statement.
A percentage decrease in a later update does not by itself establish reporting error; the perimeter and assessment may have changed.
Local and source differences
Use the responsible incident team's explanation for the reported percentage. For structure fires, agencies may use containment language without publishing a wildfire-style percentage. Follow the actual incident terminology rather than import one calculation into every fire report.