Containment is the status of a wildfire when a control line has been completed around the fire, and any spot fires, that can reasonably be expected to stop its spread. Percent contained is the approximate share of the fire's perimeter that is surrounded by such line, whether built by crews or formed by natural barriers.
What counts as line
A control line includes all constructed or natural barriers and treated fire edges used to control a fire. Fireline is the part scraped or dug down to mineral soil.
Natural barriers such as roads, rock outcrops and streams can serve as line alongside line built by firefighters. The percentage tracks how much of the edge is secured, not how much area has burned.
Contained, controlled and out
These are separate steps. Under the National Wildfire Coordinating Group (NWCG) glossary, a fire is controlled only after line surrounds it and its spot fires and interior islands, unburned fuel next to the line has been burned out, and hot spots threatening the line are cooled, until the line can be expected to hold.
Mop-up follows, making the fire safe by extinguishing or removing burning material near the control line. A University of Florida fire ecologist notes that a fully contained fire is likely still burning and smoldering, and it may take weeks before it is declared out.
Reading the percentage
A low figure early in a fire can simply mean line is still being built or has not yet been judged able to hold. It is not a direct measure of how fast the fire is growing.
Some naturally ignited fires are managed with a strategy that does not prioritize full suppression, so containment is not the only goal on every incident. Read the incident's own update for the strategy in use.
What it does not tell you
Even 100 percent containment is not a guarantee. Depending on weather and fuel conditions, fires can still escape across containment lines, so wind and humidity forecasts matter until the fire is out.