Flood FactorFind frequently asked questions about Flood Factor®, articles about the flood methodology and details on flood solutions.
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Flood Factor scores are based on the depth and likelihood of floodwater reaching the physical structure of a home.
The data used to calculate a property's Flood Factor comes from the First Street Foundation Flood Model, a nationwide flood model that calculates the likelihood of a flood based on a location’s history and geographic information, such as elevation, climate, changes in the environment, proximity to water, and adaptation measures. Use Risk Factor to find property-specific flood risk assessments for any U.S. address.
Calculation of Flood Factor scores
A property's Flood Factor is an indicator of its comprehensive, thirty-year risk of flooding from rainfall, overflowing rivers and streams, high tides, and storm surge, ranging from 1–10. Properties with higher Flood Factors are either more likely to flood, more likely to experience high floods, or both.
The First Street Foundation Flood Model is used to determine the likelihood of a flood occurring within a given year and the projected depth of a flood. A property’s Flood Factor is determined by its likelihood of flooding and the potential depth of that flood. Because flood risks accumulate over time, it specifically looks at the likelihood of 1 inch of water reaching the building footprint of a home at least once within the next 30 years. Flood Factor scores increase as the likelihood of flooding increases, the projected depth of flooding, of both over the lifetime of a 30-year mortgage.
Flood Factor Matrix
Properties more likely to flood at a greater depth have higher Flood Factors.
Properties with less than 0.2% chance of flood water reaching the building footprint in every year analyzed are considered to have minimal risk or a Flood Factor of 1. While it is still possible for properties with a Flood Factor of 1 to flood, they are not included in the Flood Factor matrix. In the Flood Factor Matrix below you can visualize how Flood Factors increase as the depth and likelihood of flooding increases. The specific depth and likelihood of flooding expected to reach a property is used to determine the boundaries for each level of flood risk. These boundaries produce a set of rules that can be used to understand why one property may have a Flood Factor of 4 while another has a Flood Factor of 7.
- Properties with at least an 6% chance of flooding over 30 years will have a Flood Factor of 2 or higher
- Properties with at least a 26% chance of flooding over 30 years with have a Flood Factor of 4 or higher
- Properties with at least a 80% chance of flooding over 30 years will have a Flood Factor of 5 or higher
- Properties with at least a 99% chance of flooding over 30 years will have a Flood Factor of 6 or higher
For example, a property that has a 6% chance of at least 1 inch of floodwater reaching the home will have a moderate Flood Factor of 4. In contrast, a property that has a 47% chance of at least 1 inch of floodwater reaching the home will have a major Flood Factor of 6.
Please note that only FEMA maps are used to determine flood insurance and building code requirements, not Flood Factor scores. Additionally, FEMA zone designations are not a factor in calculations used to determine a Flood Factor score.
Ensuring scientific accuracy
The creation of the First Street Foundation Flood Model required an unprecedented partnership of more than 80 world-renowned scientists, technologists and analysts. Methods used in the Flood Model have undergone an expert academic panel review and have been submitted to scientific peer-review journals. Where possible, data has been validated against historic flood reports and FEMA flood claims.The data the model produces undergoes multiple reviews and must pass comprehensive check-points before being made publicly available.
Risk Factor shows a property's annual and cumulative chance of flooding. Annual probability is the likelihood of flooding in a single year. Cumulative probability is the chance of flooding over multiple years.
Risk Factor is a free online tool created by the nonprofit First Street Foundation that makes it easy for Americans to find their property’s risk of flooding and understand how flood risks are changing because of a changing environment. Use Risk Factor to learn the annual and cumulative likelihood of water reaching your home so you can prepare for and mitigate risks before they become a reality.
A probability is a measure of how likely something is to happen. It is often expressed as a percentage, with 100% meaning something will definitely happen, and 0% meaning something will definitely not happen.
Probability of an event = # of ways it can happen total number of outcomes
For example, flipping a coin has two possible outcomes: heads or tails. There is a 50% chance a flipped coin will land on heads, and a 50% chance it will land on tails.
Annual and cumulative probabilities
An annual probability is the chance of something happening at least once within a specific, singular year. Flood Factor includes three annual probabilities: the current year, 15 years in the future, and 30 years in the future.
A cumulative probability is the chance of something happening at least once over the course of multiple years. Flood Factor includes two cumulative probabilities: the next 15 years and the next 30 years.
For example, imagine each coin flip represented a year, and you wanted to know the chance of the coin landing on heads once within two flips. That would be its two-year cumulative probability. To calculate this, you have to find all possible outcomes and identify how many outcomes include the coin landing on heads. In this case, there is a 50% annual chance of the coin landing on heads, and a 75% cumulative chance of the coin landing on heads at least once within two “years” (coin flips).
Probabilities on Risk Factor
Flood Factor uses data from the First Street Foundation Flood Model to determine a location’s annual and cumulative likelihood of flooding.
The model considers inputs such as a location’s flood history and its geographic information such as elevation, climate, proximity to water, and adaptation measures, to identify its likelihood of flooding in the current year, an annual probability. The model then incorporates projected environmental changes to forecast its risk of flooding in future years, which are also annual probabilities. Using these annual probabilities, the model is then able to calculate a location’s chance of flooding over multiple years, known as its cumulative probability.
Using the example above, if a home had a 50% annual chance of flooding, it would have a 75% chance of flooding at least once within 2 years, a 96.9% chance of flooding within 5 years, and a 99.9% chance of flooding within 10 years.
To provide a complete flood risk analysis, Flood Factor calculates multiple annual probabilities, 0.2%, 1%, 10%, 20%, 50%, which consider different depth scenarios. Lower probabilities represent deeper floods that are less likely to occur, but more likely to reach a building and cause greater damage.
In an earlier example, there was a 50% annual chance that a flood of some depth would occur and reach the home. But perhaps this flood is very shallow, and while it can damage the yard, it does not pose a significant threat to the home. That same home might have a 10% chance of experiencing a more extreme flood that is less likely to occur but more likely to cause damage.
Defining a low or high probability
Although the probabilities on Flood Factor may seem small or unlikely, having any chance of flooding is significant, especially given how risk accumulates over time. A home that only has a 1% chance of flooding this year, for example, has a 26% chance of flooding over 30 years– the average life of a mortgage.
About as likely as
Over 5 years
Over 15 years
Over 30 years
coin landing on heads
food poisoning in a year
being born prematurely
being audited by the IRS
being born with 11 fingers
See how other annual probabilities accumulate over time on weather.gov.
FEMA and Flood Factor are different risk assessments. Flood Factor may show higher risk because it accounts for flooding from rain and the effects of climate change.
Your FEMA zone and Flood Factor are independent risk assessments. While the findings of these models do not diverge starkly across the US, in some areas Flood Factor may show more or less flood risk than FEMA, simply because of differences in the methodologies employed. FEMA determines flood risk on the community level, their risk projections capture risk from a single 1-in-100 or 1-in-500 event from storm surge and overflowing rivers and streams. Flood Factor calculates flood risk on the property-level, accounts for changing climate conditions, and considers the risk of flooding due to high-intensity rainfall.
As a result, Flood Factor shows a more nuanced, property-specific flood risk, as opposed to a binary in-or-out of floodplain analysis from FEMA maps. Flood Factor uses publicly available flood risk information that shows how risk will accumulate and change over time. It is most powerful when used in conjunction with FEMA flood maps and other available state and local flood risk resources, and should be viewed as complementary to FEMA flood maps, which need to be used for building and permitting purposes. Individuals can search their address on Risk Factor to find their property’s flood risk as well as their FEMA zone.
Flood risk on Risk Factor
The methods used to create the First Street Foundation Flood Model bring together a number of resources and techniques already developed as inputs to understanding one’s flood risk. By combining these resources, the model represents flooding from multiple risks, such as rain, river, tidal and storm surge flooding, while also integrating current and future environmental considerations all at a property level. Adding to the full picture of flood risk, the model also recreated more than one hundred coastal and inland flood events over a 20 year period (2000-2019) to provide estimates of past flooding likelihoods. This combination of precision and national scope allow for a more comprehensive flood risk tool than currently available.
FEMA flood risk
To date, the standard national comparison for flood risk is the Federal Emergency Management Agency’s (FEMA) Special Flood Hazard Areas (SFHA). The FEMA SFHA models are currently the most popular flood-risk identification tool and are widely used throughout government, research, and private companies as a way to understand flood risk, price insurance premiums, and prepare for potential hazards. A property's FEMA zone designation is estimated on Flood Factor.
The initial FEMA SFHA models are made for individual communities and are generally very high-quality, have been built to explicitly understand the risks to a standard 1-in-100 or 1-in-500 year flood event, and have been developed over many years to identify community flood risk based on local historical context. However, these models do not exist for the entire country, and their output also undergoes a series of adjustments and revisions based on local stakeholder feedback. That means that the FEMA SFHA maps that get instituted are very rarely the same as the initial risk identified in the area. Couple that with the fact that the model is created to capture risk from a single 1-in-100 or 1-in-500 event, reliance on historical trends, and the lack of national scale, and you have opportunities for improvement in the communication of national flood risk.
Flood Factor vs FEMA
In comparison, the method used to create the flood risk visualized on Risk Factor expands flood risk mapping to the entire country by relying on an innovative Regionalized Flood Frequency Analysis (RFFA) approach. This approach makes use of traditional statistical matching techniques to model the characteristics of ungauged streams and river reaches across the country with known gauged characteristics to produce likely flow conditions with high confidence. Additionally, a core component of the model is the ability to also include pluvial (rainfall) events as probabilistic flood risks with depths and associated return periods in areas that are outside the modeled extents of FEMA studies. Both the RFFA and pluvial flooding integrations have allowed for a model that captures risk that is generally not captured in FEMA SFHAs.
To highlight the additional coverage of the risk identified by the First Street Foundation Flood Model, the 1-in-100 hazard layer, representing a 1% annual risk of occurrence, was compared to the same probability zones outlined by the FEMA SFHA zones. The results indicate that the Flood Model generally captures about 3 times the risk as the FEMA SFHA. Interestingly, the two models align well along gauged river channels and FEMA SFHAs help provide a source of validation for the fluvial risk identified in the model. However, the discrepancies indicate the practical need for the more comprehensive approach that was used to produce the statistics in this report.
The alignment of the Flood Model results with SFHAs in areas that are gauged and mapped by FEMA is important. This highlights the utility of the Flood Model as being in agreement with standard indicators of flood risk by building on trusted models, reports, and open data resources. To that point, this model relies heavily on data from the National Oceanic and Atmospheric Administration (NOAA), FEMA, the United States Geological Survey (USGS), the World Climate Research Programme (WCRP), and hundreds of local government resources. As such, the Flood Factor risk results should be seen as an extension of existing resources developed with innovative and creative methods to comprehensively include geographic areas that may have been left out of alternative-risk models.
Use Risk Factor to understand your risk
FEMA flood maps identify over 1.1 million miles of flood hazard areas, and while the maps can provide detailed information for homeowners on their flood risks, they are not available everywhere. Risk Factor’s national flood model shows that flood risk is more widespread in the United States, with over 26 million properties at risk over the next 30 years. Risk Factor also includes flood risk from extended periods of high-intensity rainfall, leading to urban stormwater flooding and storm surge. Additionally, future conditions are considered, such as sea level rise.
Without free access to accurate flood information, more Americans will unknowingly be at risk from the nation’s costliest natural disaster. Risk Factor aims to quantify and communicate America’s flood risk. By making flood risk data freely available for all, individuals and communities can prepare for and mitigate risks before they become a reality.
The New York Times - Climate Threats Could Mean Big Jumps in Insurance Costs This Year
We are committed to accuracy and continue to grow, adapt, and change our Flood and Wildfire Models as new and better information becomes available.
Quarterly Data Updates
At First Street Foundation, we aim to quantify and communicate America’s climate risk. By making flood and wildfire risk data freely available for all, individuals and communities can prepare for and mitigate risks before they become a reality.
We are committed to the accuracy of our data and will continue to grow, adapt, and change our Flood and Wildfire Models as new and better information becomes available. As such, improvements to our models are included in quarterly data updates, while both the Flood Model and Wildfire Model is updated annually. Sign up for our newsletter to learn about updates as they become available by clicking or tapping here.
Wildfire Model Updates
May 16, 2022
- Launched wildfire risk from the First Street Foundation Wildfire Model on the newly created Risk Factor™ site, which is a free online tool that makes it easy for anyone to find their property’s past, present, and future flood and wildfire risk.
Flood Model Updates
May 16, 2022
- Expanded the First Street Foundation Model to include Hawaii, Alaska, and Puerto Rico
- Improve address matching to allow for more flexibility in showing flood risk to properties other than single family residential homes.
- Updated our scoring methodology. The specific depth and likelihood of flooding expected to reach a property is now used to determine the boundaries for each level of flood risk.
April 23, 2021
- Updated building characteristics (i.e. # of stories, units, basements, ground floor elevation) in some areas to more accurately reflect current findings which improved accuracy for default annual loss calculations for those properties.
- Updated property FEMA zones based on user-provided documentation
- Expanded historic flood methodologies to improve accuracy of 3 historic hurricanes (Harvey, Florence, and Matthew). The updated models now include additional flooding caused by rainfall.
- Incorporated new high-resolution LiDAR elevation data provided by local officials in the Dallas-Fort Worth metro area
- Updated statistics for 1,600 previously unavailable properties in Georgia
February 22, 2021
- Added annual flood damage estimates for residential properties at risk
- Added total annual flood damage estimates for neighborhoods, zip codes, cities, and counties
January 18, 2021
- Updated Flood Factors that better represent wetland conditions and incorporate the latest geographic data.
- Improved the accuracy and extents of flooding for several historic floods, including Hurricane Katrina.
- Analyzed and added millions of new properties to Flood Factor with improved statistical methods and issues identified by users.
- Updated FEMA designations to reflect LOMAs and LOMCs submitted by users in addition to more accurate FEMA zone data obtained from FEMA.
Oct 30, 2020
- Improved our Flood Model by determining a property’s Flood Factor is most accurate when calculated based on the building’s footprint, or outline. As a result the center of map pins have been moved from the center of a property to the building footprint.
- Added new property addresses via Mapbox in order to provide Flood Factors for as many properties as possible.
- Widened the boundary for coastal properties in order to fully capture all possible flood risk scenarios for these areas.
- Adjusted Flood Factor for properties in South East Florida to reflect all flood risks scenarios a property faces.
- Updated 2035 map flood layers for some areas.
September 16, 2020
- Added 2 million new properties on Flood Factor.
- Updated Flood Factor scores for some properties based on new and improved data on flood risk.
- Made additional coastal cities and counties with cleaner coastal flood layer visuals available.
- Removed areas that are currently under review based on new and improved flood risk data.
Ongoing site enhancements aim to provide our users with the best experience possible on riskfactor.com.
May 16, 2022
- Created Risk Factor™ is a free, online tool that makes it easy for anyone to find their property’s past, present, and future flood and wildfire risk
- Launched Fire Factor™, the first public, peer-reviewed wildfire model to show how property-level flood risks change over time because of the environment
March 19, 2021
- Improved site and search speeds across Flood Factor.
- Added new videos explaining Flood Factor fundamentals and the differences between Flood Factor and FEMA.
- Modified the content on locality pages to highlight the percent of properties at risk of flooding.
- Modeled risk on the community level to show the severity of risk threatening access to utilities, emergency services, and transportation within a neighborhood, zip code, city, and county.
February 22, 2021
- Added the average annual cost of all potential flood damages this year and 30 years from now, based on a property's flood projections and the accumulation of those expected costs over time.
- Included the “adjust building details” tool in order for users to change the various inputs, such as the estimated value of a home, presence of a basement, number of stories, and ground floor elevation, to see how these factors affect the costs associated with flood damage
- Added the total annual flood damage estimates this year, and in 30 years to neighborhood, zip code, city, county, and state pages
January 18, 2021
- Enhanced maps on desktop to expand for a fully immersive experience.
December 3, 2020
- Included a zoom bar for the score map to allow users to view Flood Factors on a neighborhood and city level.
- Improved the Flood Factor experience on mobile devices.
November 13, 2020
- Added a pin on score maps so users can easily identify their property at any zoom level.
- Optimized Flood Factor’s load time for an even faster experience.
- Updated some coastal areas to include 50% probability.
October 30, 2020
- Updated address information in order to improve the search experience.
- Updated map pins to be shown on top of building outlines (when possible).
- Added new and updated coastal cities and counties with cleaner coastal flood layer visuals on the Flood risk explorer in 2035 for properties in southeastern Florida.
Sep 16, 2020
- Added local Flood Factor score maps on property pages.
- Made copy updates in partnership with The Association of State Floodplain Management.
August 24, 2020
- Added the 5% likelihood of a property flooding to the flood risk explorer.
- Improved address search function on Flood Factor.
Want access to our data?
You can get access to the most comprehensive property-level flood and wildfire risk data set in existence. For commercial use or non-profits, academics, or public sector professionals needing data at a property-level, First Street Foundation provides access to property specific flood risk statistics for purchase. Learn more.
For nonprofits, academics, and the public sector, aggregated flood risk summary statistics are now available for public, noncommercial use through the AWS Registry of Open Data and on ESRI’s Living Atlas and Maps for Public Policy sites. Learn more.
If you believe your property’s FEMA zone is incorrect, a flood control feature is missing, or you have more information about a historic flood please reach out to us!
Flood Factor is a free online tool created by the nonprofit First Street Foundation that makes it easy for Americans to find their property’s risk of flooding and understand how flood risks are changing because of a changing environment. First Street Foundation’s flood model is, built upon the excellent work of FEMA, NOAA, NASA, USGS, and many others, as well as the established and peer-reviewed work of all of our flood model partners. Please let us know if RiskFactor.com is displaying an incorrect FEMA zone, a flood control feature is missing, or if you have additional information regarding the impact of a historic flood. We are continuously working to refine our flood model methodology and incorporate new information into Flood Factor to ensure we are providing the best, most accurate picture of flood risk to our users.
The projections shown on Flood Factor come from the First Street Foundation Flood Model. We have made our flood model’s full technical methodology available to the public because we support scientific collaboration and data transparency. Like all flood models, its projections are intended to help provide a fuller understanding of flood risks, and we encourage you to supplement this information with additional data. We are committed to the accuracy of our data and will continue to grow, adapt, and incorporate feedback and expand our model over time, including an annual data update.
Data that will help improve the Flood Model
You know your property better than anyone! If you have information you think might be helpful to improve our Flood Model, please let us know by submitting a request through our contact forms. Please note that this information will not change your property's Flood Factor, but it can help improve the accuracy of the First Street Foundation Flood Model, and the information shown on Flood Factor. In order to improve our Flood Model, please reach out if you notice any of the following issues:
Incorrect FEMA Zone
If you believe Flood Factor has listed the incorrect FEMA flood zone for your property, please submit a Letter of Map Amendment (LOMA). The FEMA zone information listed on Risk Factor is currently an estimated zone. We recommend working directly with an outside company to acquire the Flood Determination Certificate for a property or to get the determinations reversed with a Letter of Map Amendment. Once submitted through our help center form, we can correct this information.
Missing flood control features
Our team is continuing to conduct research on flood control projects around the country and will be continually updating the model based on new information. While our database contains 23,000 features today, we acknowledge there are more to include, and value your input!
If you have knowledge of specific completed projects, precise service area extents-of-flood adaptation infrastructure, or spatial data on flood adaptation infrastructure, please help the team by providing any level of detail you can here. Certain information about flood control projects are particularly useful, such as:
- Name of the flood control project
- Entity in charge of the project (local government, FEMA, USACE, etc.)
- Address or specific location
- Links to local, state, or federal documents outlining project details, including the flood event it protects for
- A map showing the geographic area the project protects, including the physical location of the flood control project
New projects can be included for our next update to improve the model! If you would like to read about examples of flood control projects and how gray infrastructure works to alleviate flooding please review this article.
Example of a levee; an embankment built to prevent the overflow of a river. Levees can also be built along low-lying coastal areas.
Missing historic flooding event
Flood Factor uses data from the First Street Foundation, which has modeled 57 major river and storm surge floods in the U.S. from 2001 on, and is continually adding more. Historic events in the First Street Foundation Flood Model are recreated using hydraulic models and public data sources. Historic river flooding events are recreated by analyzing historical USGS stream gauge readings during identified flood events and then compared to recorded high water marks and insurance claims of flooding made during that event. Coastal flooding events are recreated by analyzing storm surge, tides, coastal circulation, wind-generated waves, and geospatial analysis for specific storms, such as Hurricane Katrina. The recreation is then compared to recorded high water marks during the event.
Aftermath of Hurricane Gustav - August 2008
If you have knowledge of specific historic flooding, we'd love to hear from you. We will investigate historic flooding this area further and incorporate any new events determined into our future update.
In order to include historic events in our Flood Model please reach out to us through our Help Center and provide:
- Dates the event took place
- Location of historic flooding
- Any pictures that may show the depth and damage of the flood
- News articles that describe the flooding your property experienced
We understand you may have questions regarding names of locations related to your property. The First Street Foundation Flood Model matches parcel data from official government sources to address data collected from commercial sources. Naming conventions attached to specific parcels and addresses may not exactly match, resulting in naming discrepancies. Here is a link to the methodology behind our research.
If you have knowledge of specific location names, we'd love to hear from you. Please submit a ticket through the Risk Factor Help Center that includes:
- A corrected address that includes the house number, street address, zip code, city, and state
- The incorrect address listed on FloodFactor.com
We will review the issue further and be sure to update any issues found during a future update. Please take a look at how we update Risk Factor data.