How Do You Rate a Forest Fire?

Most of the news stories about the 43 large wildfires in the Western United States 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.


Bonus Explainer: CNN.com reports that the National Interagency Fire Center is on “National Preparedness Level Four,” and that Level 5 is the highest. What does that mean?

The NIFC measures its preparedness on a scale from one to five. Level 1 preparedness means there are virtually no fires, and full resources are available.

Level 5 means that at least 550 crews are on the fire line and that fires are burning in several geographic regions of the country. Level 5 also means that the nation’s personnel and equipment resources are stretched, and they could be exhausted. Level 5 is a trigger point to ask foreign partners (primarily Canada) for equipment or fire specialists and to ask the U.S. military for troops to help with firefighting.

Thursday, the NIFC plans to bump its preparedness to Level 5.

Next question?

Explainer thanks Donald Smurthwaite of theNationalInteragencyFireCenter.