When reporting progress toward containing a large wildfire, I will provide the amount of fire line that has been constructed and an estimate of the fire’s level of containment. Both are expressed as percentages, but having a fire line completely around a fire doesn’t necessarily mean the fire is completely contained.
Containment means there is little chance that a fire will be able generate enough intensity to jump the fire line and cause more trouble. A fire line all the way around a fire isn’t the only factor in declaring a fire contained. The Oregon Department of Forestry likes to have a ring of completely extinguished area inside the fire line, which increases the distance between active flames in the center of the fire and the unburned fuel (vegetation) outside the fire line. On a large fire (100 acres or more) we shoot for having 300 feet inside the fire line completely mopped up before declaring it contained. If the fire has significant amounts of unburned fuel inside the fire line, we may not declare the fire contained until the areas of unburned fuel are burned up (by us, or by nature) or isolated (digging fire lines inside the main fire line to keep active flame from creeping or spotting into unburned areas).
It sometimes takes days or a week or more after a fire line is completed to be able to declare a fire contained.