The methodology behind Fire Factor

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Fire Factor uses the First Street Foundation Wildfire Model to find wildfire risks across the United States.

The First Street Foundation National Wildfire Model is a first-of-its-kind, nationwide, behavioral wildfire model that shows a specific location’s probabilistic risk of wildfire based on the vegetation, topography, and fire weather in the surrounding area. It builds off of decades of peer-reviewed research and forecasts how wildfire risks will change over time due to changes in the environment. Use Risk Factor™ to find property-specific wildfire risk assessments for any U.S. address.

Calculating and mapping wildfire probabilities

The First Street Foundation Wildfire Model is a behavioral wildfire model, which means it considers the real-world conditions that create wildfires and then outputs the likelihood of wildfire at any given location based on over 100 million simulated wildfire events. 

The process starts by asking the question: “What would burn if a wildfire were to occur?” The First Street Foundation Wildfire Model uses data from the United States Forest Service to identify the type, quantity, age, and condition of the vegetation across the Continental US that would provide “fuel” for a potential wildfire. This also includes the location of wildfire prevention measures such as prescribed burns, vegetation thinning, fire breaks, and other forest management practices, as well as the location of previous wildfires. One of the things that makes the First Street Foundation Wildfire Model unique is that it also incorporates homes as potential fuels for wildfire spread to other homes based on patterns observed in 550 historic wildfires.

The model then considers the past weather patterns that impact fuels by making them hotter and drier, as well as wind and weather conditions that help spread fires further. The National Oceanic and Atmospheric Administration data used to simulate these conditions is based on 10 years of hourly resolution weather data that includes air temperature, relative humidity, and precipitation trends for the Continental US, and gives a realistic representation of surface weather conditions that drive wildfire behavior.


The model then simulates the ignition of wildfires based on historic fire locations. It then observes how wildfires spread given the fuel and fire weather conditions, and tracks fires that grow to a sufficient size, noting the distance, location, and duration of these fires. 

Determining future wildfire risks

As with any First Street Foundation® risk model, the inclusion of environmental changes that impact future wildfire risk, such as temperatures and precipitation patterns, is an essential trademark of the First Street Foundation Wildfire Model and wildfire methodology. The model uses 2011-2021 to establish baseline weather conditions and analyzes multiple environmental possibilities under the RCP 4.5 carbon emissions scenario to forecast conditions from 2041-2050 that would impact future wildfire risk. This allows the First Street Foundation Wildfire Model to predict wildfire risk 30 years into the future in a way that meets the rigorous standard of scientific peer review.

Fire Factor scores

In order to provide the most accurate current and future wildfire risk assessments, the Model simulates over 100 million wildfires each for the 2022 and 2052 simulations,  to see which of these wildfires grow and become damaging. It outputs over 8 million significant fires per simulation that are then used to calculate risk. Publicly available and 3rd party data is used to identify property boundaries and buildings. The building’s location is then used to determine the likelihood of the home being in a wildfire based on how many times the 8 million simulated fires reached the home

Properties with no risk of being in a wildfire based on our fire simulations and with no modeled exposure to embers are considered to have minimal risk or a Fire Factor® of 1. The probability of a home being in a wildfire or its exposure to embers is used to determine the boundaries for each level of fire risk. These boundaries produce a set of rules that can be used to understand why one property may have a Fire Factor of 4 while another has a Fire Factor of 7. 

  • Fire Factors of 2 represent properties located in areas indirectly exposed to wildfires from embers or in an area with less than a 1% chance of burning over 30 years
  • Fire Factors of 3 represent properties with 1-3% chance of burning over 30 years
  • Fire Factors of 4 represent properties with a 3-6% chance of burning over 30 years
  • Fire Factors of 5 represent properties with a 6-9% chance of burning over 30 years
  • Fire Factors of 6 represent properties with a 9-14% chance of burning over 30 years
  • Fire Factors of 7 represent properties with a 14-19% chance of burning over 30 years
  • Fire Factors of 8 represent properties with a 19-26% chance of burning over 30 years
  • Fire Factors of 9 represent properties with a 26-36% chance of burning over 30 years
  • Fire Factors of 10 represent properties with more than a 36% chance of burning over 30 years

Calculating home vulnerability

A property’s vulnerability refers to the amount of structural damage to a home in the event of a wildfire and considers the most significant characteristic related to a building surviving a wildfire. These estimates are based on a property’s probability of burning, the height of flames, and exposure to embers.

Significant building characteristics related to building survival in a wildfire were determined through historic data on buildings lost or damaged in historic fires. From this analysis, significant building characteristics were identified:

  • Slope of property: The steepness of a property. It refers to the number of feet the land rises, or falls, across a property. The slope is an important indicator of the ability of fire to reach a building.
  • Exterior Wall Type: The material the external walls of a home are made of. Certain materials are more likely to ignite, while others may be ignition resistant or non-combustible. 
  • Defensible space: The cleared area between the home and surrounding vegetation such as grass, trees, shrubs, or any wildland area that surrounds it. This clear area protects the home from catching fire by distancing it from vegetation and fuel sources. 
  • Roof material: The material the roof of a building is made out of such as concrete, slate, gravel and rock, tile, and wood. A key factor for structure survival indicates a structure's ability to prevent ember or flame penetration.

How wildfire will impact a home depends on the vulnerability of that home to the fire’s intensity and embers. The slope of the property and distance to potential wildfire locations determine a property's risk from fire embers. The building materials such as the type of roof and siding, as well as the distance to fuel sources impact the building's likelihood of igniting. 

The amount of damage a home may sustain in a wildfire is based on its unique building characteristics. These characteristics are then compared to a property’s probability of burning, the height of flames, and exposure to embers to determine its vulnerability.


“Fragility curves” developed by ARUP Corporation were applied to the First Street Foundation Wildfire Model to determine the amount of damage a home may sustain based on a property’s wildfire risk, ignition likelihood, fire intensity, and exposure to embers; and building characteristics such as the size of the structure, slope of the property, materials, and building value. 

Ensuring scientific accuracy

The creation of the peer-reviewed First Street Foundation Wildfire Model required an unprecedented partnership with top climate scientists and modelers from leading organizations along with top data scientists and technologists to ensure the data is accessible, accurate, and actionable. 

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