How do firefighters contain a fire, and what does containment mean exactly?

Most news stories about large wildfires report some level of “containment” reached by firefighters. What does it mean to contain a fire, how is the percentage calculated, and when is a fire “controlled”?

To prevent a blaze from spreading, firefighters dig a “fire line” around its circumference. If three miles of fire line have been built around a fire that is 10 miles in circumference, then 30 percent of the fire is contained.

Fire lines are trenches dug to create a “fuel break” around the fire. Fires need fuel, oxygen, and heat to burn, and the easiest of the three to eliminate is fuel. Fire lines can also include “natural” barriers such as roads, rock bluffs, or streams.

Once a fire is fully contained, firefighters work on “controlling” it by battling it inside the containment line. A controlled fire is one that has no risk of expanding beyond the fire line.