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5.4 Perimeter and Area Estimation Charts

The perimeter and area of a fire can be determined by using estimation charts as shown below. These charts require knowledge of the spread distance in chains and the effective windspeed in miles per hour. A fire area/size worksheet is provided to be filled out with the data collected.

EFFECTIVE WINDSPEED

Effective windspeed is the midflame windspeed, corrected for the effects of slope on fire spread. Changes in slope affect how wind influences a fire and the overall fire spread. For slopes above 20%, each additional 20% increase in slope translates to an additional 1 mph increase in the effective equivalent “upslope” component of the wind. For slopes between 20% and 40%, for example, a 1-mph slope-equivalent upslope component would be added to the flame-level wind. Between 40% and 60%, a 2-mph slope-equivalent upslope component would be added, and above 60%, flame-level winds would increase by 3 mph.
 

screengrab of animation of flame heights increasing as slope increases
Click to view animation of slope effects on fire

Example 1 - A fire starts at point A. Given the following values, determine the fire perimeter. 
rate of spread = 5 ch/h, projection time = 3 h, effective windspeed = 10 mi/h

Step 1. Use the equation for determining the spread distance (see Section 5.3).
ROS × PT = 5 ch/h × 3 h = 15 chains spread distance.

Step 2.> Look at the perimeter estimation chart. Find a spread distance of 15 chains on the vertical axis. Move your gaze across horizontally.

Step 3. Find an effective windspeed of 10 miles/hour on the horizontal axis, which is between 9 miles/hour and 11 miles/hour. Follow the column down vertically.

Step 4. Where the two lines intersect, read the number. In this case there are two numbers.
34 chains and 33 chains

Step 5. Take an average of the two values.
34.0 chains + 33.0 chains / 2 = 33.5 chains

The perimeter of the fire is 33.5 chains.

Example 2 - Determine the area of the fire in Example 1 and complete the fire area/size worksheet. The spread distance is the same, 15 chains.

Step 1. Reference the area estimation chart for a spread distance of 15 chains. Follow the row across horizontally.

Step 2. Look for an effective windspeed of 10 miles/hour, which is between 9 miles/hour and 11 miles/hour. Follow the column down vertically and read the numbers from the two windspeeds.
4.9 acres and 5.7 acres

Step 3. Take an average of the two values.
4.9 acres + 5.7 acres / 2 = 5.3 acres

The area of the fire is 5.3 acres.

Perimeter estimation for point source fires.
Table showing Perimeter Estimation

Area estimation for point source fires.
point source fires

Fire Area/Size Worksheet

Line Input    
0 PP Projection point A
1 ROS Rate of spread, ch/h 5
2 EWS Effective windspeed, mi/h 10
3 PT Projection time, h 3
4 SDF Spread distance, ch 15
  Output    
  PER Perimeter, ch 33.5
  AC Area, ac 5.3

NWCG Latest Announcements

Incident Position Standards and the Next Generation Position Task Books Now Available for UASD, UASM, UASL and UASP

Date: July 31, 2025
Questions?  Please contact:
Interagency Fire Unmanned Aircraft Systems Subcommittee
 

NWCG is excited to announce that Incident Position Standards and the Next Generation Position Task Books are now available for all four Unmanned Aircraft Systems positions:

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The Performance Support Packages for these positions were developed as part of the Incident Performance and Training Modernization effort. These resources support trainees, qualified personnel, and evaluators in their respective roles.

Any changes to qualification pathways will take effect with the next update of the NWCG Standards for Wildland Fire Position Qualifications, PMS 310-1, scheduled for January 2026.

References:

NWCG Unmanned Aircraft Systems, Data Specialist Position Page

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Incident Position Standards and the Next Generation Position Task Books Now Available for RADO and INCM

Date: July 30, 2025
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Incident Logistics Subcommittee
 

NWCG is excited to announce that Incident Position Standards and the Next Generation Position Task Books are now available for Radio Operator (RADO) and Incident Communications Center Manager (INCM).

The Performance Support Packages for these positions were developed as part of the Incident Performance and Training Modernization effort. These resources support trainees, qualified personnel, and evaluators in their respective roles.

Any changes to qualification pathways will take effect with the next update of the NWCG Standards for Wildland Fire Position Qualifications, PMS 310-1, scheduled for January 2026.

References:

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ETC Equipment Bulletin: 25-004 NEW Aquatic Invasive Species – Golden Mussels

Date: July 29, 2025
Questions?  Please contact:
Equipment Technology Committee
 

The Equipment Technology Committee (ETC) issued Equipment Bulletin: 25-004, notifying the wildland fire community about a newly discovered invasive species and actions to take if found. Golden mussels, a highly invasive species recently identified in California, pose a significant risk to native ecosystems, infrastructure, and fire equipment. These mussels rapidly colonize hard and soft surfaces, including aquatic plants and fire equipment, clogging pipes, fouling motors, disrupting water systems, and impacting native species.

To help prevent their spread, fire personnel must follow the decontamination procedures outlined in the NWCG Guide to Preventing Aquatic Invasive Species Transport by Wildland Fire Operations, PMS 444. Additionally, report any sightings immediately to your Lead Resource Advisor and include clear, close-up photos and location details.

Read the complete ETC Equipment Bulletin: 25-004 to learn more. 

References:

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ETC Equipment Bulletin: 25-004

NWCG Guide to Preventing Aquatic Invasive Species Transport by Wildland Fire Operations, PMS 444

Incident Position Standards and the Next Generation Position Task Books Now Available for ICT5 and FFT1

Date: July 29, 2025
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NWCG is excited to announce that Incident Position Standards and the Next Generation Position Task Books are now available for Incident Commander Type 5 (ICT5) and Firefighter Type 1 Squad Boss (FFT1).

The Performance Support Packages for these positions were developed as part of the Incident Performance and Training Modernization effort. These resources support trainees, qualified personnel, and evaluators in their respective roles.

Any changes to qualification pathways will take effect with the next update of the NWCG Standards for Wildland Fire Position Qualifications, PMS 310-1, scheduled for January 2026.

References:

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