Skip to main content

Firefighter Math: 5.3 Spread Distance

Spread distance (SD) is the forward distance a fire spreads in a given amount of time. SD can be calculated from rate of spread (ROS) and projected time (PT).

Spread Distance = Rate of Spread × Projected Time
SD = ROS × PT

Example 1 - What is the spread distance, in feet, for a fire that has a rate of spread of 6 chains per hour for a 3-hour time span? 

Step 1. Use the equation for determining the spread distance.

spread distance

Step 2. The answer needs to be in feet. Set up the cancellation table so all units will cancel, except the
desired unit, feet.

cancellation table

The spread distance is 1,188 feet.

Example 2 - Using the information in Example 1, find the map distance of the fire spread. The map scale is 1:31,680 (inch/inch).

Step 1. Convert inch/inch to feet/inch. Set up the cancellation table so all units will cancel, except the desired unit, feet/inch.

cancellation table

Step 2. Convert the ground spread distance to a map spread distance.

spread distance

The map distance is 0.45 inches.
 

MAP SPREAD WORKSHEET

These values can be summarized in a Map Spread Worksheet.

Map Spread Worksheet (blank version)

Line Input      
0 PP Projection point ______ ______
1 ROS Rate of spread, ch/h ______ ______
2 PT Projection time, h ______ ______
3 SDCM Spread distance, ch (line 1 x line 2) ______ ______
4 SDFT Spread distance, ft (line 3 x 66 ft/ch) ______ ______
5 SCL Map scale ______ ______
6 CF Conversion factor, ft/in (see map scale conversion) ______ ______
  Output      
1 MD Map spread distance, in (line 4 divided by line 6) ______ ______


Map Spread Worksheet (completed)
 

Line Input      
0 PP Projection point A Given
1 ROS Rate of spread, ch/h 6 Given
2 PT Projection time, h 3 From ex. 1
3 SDCM Spread distance, ch (line 1 x line 2) 18 From ex. 1
4 SDFT Spread distance, ft (line 3 x 66 ft/ch) 1188 From ex. 9
5 SCL Map scale 1:31,680 From ex. 2
6 CF Conversion factor, ft/in (see map scale conversion) 2640 From ex. 2
  Output      
1 MD Map spread distance, in (line 4 divided by line 6) .45 from ex. 2


When completing the map spread worksheet, notice that the projection point is line zero. Line 1 begins with the rate of spread.

Fire Area Size Worksheet
 

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

FORWARD RATE OF SPREAD

The shape of a fire is dependent on slope and effective windspeed. The higher the windspeed and slope percent, the longer, more cigar-shaped the burn area.
 

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


 

Safety dictates that firefighters be positioned at the flanks (sides) or back of a fire. A firefighter should never be uphill (at the head) of a fire where the rate of spread is the fastest. See the Standard Fire Orders (F-I-R-E-O-R-D-E-R-S) in the Fireline Handbook, PMS 410-1 or NFES 0065.
 

USING A SCALED RULER FOR SPREAD MEASUREMENTS

When drawing or measuring distances, a scaled ruler is sometimes necessary for direct measurements. A scaled ruler, or engineer's ruler, has 1-inch increments subdivided into 1/10-inch increments, convenient for ground measurements. Ground measurements are usually given in fractions of multiples of 10. Consider a map distance of 3.7 inches, where 1 inch equals 100 feet. In this case, each tenth of an inch will equal 10 feet. Consequently, 3.7 inches equals 370 feet.

image of engineer's ruler

MAP SPREAD WORKSHEET

All data collected and calculated related to spread distance and rate of spread can be put into a map spread worksheet.

Example 3 - Using a ruler scaled in 1/10 inch or 0.1 inch increments, draw a sketch of the burn in Example 2. The effective windspeed is 3 miles/hour. Complete the map spread worksheet. See the steps and illustrations below.

Step 1. Draw a line of 0.5 inches from point A up the slope to point B.
A - B

Step 2. See the fire shapes in the fire shape figure below to determine how wide to make the ellipse.

Step 3. Draw an ellipse around the line drawn.

ellipse around line
 

fire shape wind speed dependent

Fire shape depends on effective windspeeds.

NWCG Latest Announcements

NEW! S-390, Introduction to Wildland Fire Behavior Calculations (Blended) Available Now

Date: December 22, 2025
Questions about RT-130? 
Please contact: Fire Behavior Subcommittee

NWCG is excited to announce that the new S-390, Introduction to Wildland Fire Behavior Calculations (Blended) training is now available on the Wildland Fire Learning Portal.

This third course in the series combines online and instructor-led training components aimed at individuals who are involved in planning, managing, and executing wildland fire and prescribed burn operations; who require a thorough understanding of fire behavior calculations to enhance effectiveness and safety. This includes students who require the knowledge and skill necessary to perform the duties of a Type 3 Incident Commander (ICT3), Division/Group Supervisor (DIVS), or Prescribed Fire Burn Boss Type 2 (RXB2).

Students are required to be qualified as any Single Resource Boss position and complete the prerequisite S-290, Intermediate Wildland Fire behavior (Blended) course, before enrolling in S-390.

References:

S-390, Introduction to Wildland Fire Behavior Calculations (Blended)

Wildland Fire Learning Portal

NEW! S-320, Introduction to Incident Management Teams (Blended) Available Now

Date: December 18, 2025
Questions about RT-130? 
Please contact: Incident and Position Standards Committee

NWCG is excited to announce that S-320, Introduction to Incident Management Teams (Blended) is now available on the Wildland Fire Learning Portal!

This blended course combines online learning with instructor-led training, designed for individuals seeking to build leadership skills and gain experience in incident management.

Students are required to complete the prerequisite trainings ICS-100, Introduction to Incident Command System (ICS), ICS-200, Basic ICS for Initial Response, and ICS-700, An Introduction to the National Incident Management System (NIMS) prior to attending S-320.

References:

S-320, Introduction to Incident Management Teams (Blended)

Wildland Fire Learning Portal

New! Ransom Road WFSTAR Module

Date: December 16, 2025
Questions about RT-130? 
Please contact: NWCG Audiovisual Specialist

NWCG is excited to announce the release of the new Ransom Road Fire Module to RT-130, Wildland Fire Safety Training Annual Refresher (WFSTAR).

This module features a firsthand account from Rob Lee, official reports, and animated maps of the Ransom Road Fire which occurred on June 8, 1981, in Florida's Merritt Island National Wildlife Refuge.

The module is available now in NWCG's RT-130, WFSTAR Catalog.

References:

WFSTAR Catalog

Wildland Fire Lessons Learned Center

Call for Nominations: Paul Gleason Lead by Example Award

Date: December 11, 2025
Questions? Please contact:
 Leadership Committee 

Do you know someone working in wildland fire who strives to make positive change and is undeterred by obstacles or setbacks? Now is your chance to give that person the acknowledgment they deserve by nominating them for a Paul Gleason Lead by Example Award.

This award, is presented by the NWCG Leadership Committee to remember Paul Gleason's contributions to the wildland fire community and to recognize individuals or groups that exhibit the same spirit and dedication to leadership – those who lead by example.

Nominations can be submitted via email with an attached Lead by Example Form to BLM_FA_Leadership_Feedback@blm.gov or through the online form. 

Tips for successful nominations and more information can be found on the Lead by Example webpage. All nominations must be submitted by December 31, 2025.

References:

Paul Gleason Lead By Example Award 

Paul Gleason Lead by Example Award Nomination Form 2025 

Leadership Committee