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Determining Risk

In the National Risk Index, risk is defined as the potential for negative impacts as a result of a natural hazard.

The risk equation behind the Risk Index includes three components: a natural hazards component (Expected Annual Loss), a consequence enhancing component (Social Vulnerability), and a consequence reduction component (Community Resilience). The dataset supporting the natural hazards component provides estimates measured in 2022 U.S. dollars. The datasets supporting the consequence enhancing and consequence reduction component have been standardized using a minimum-maximum normalization approach prior to being incorporated into the National Risk Index risk calculation.

Using these three components, composite Risk Index values and hazard type Risk Index values are calculated for each community (county and Census tract) included in the Index. Risk Index values form an absolute basis for measuring Risk within the National Risk Index, and they are used to generate Risk Index percentiles and ratings across communities.


A graphic depicting how the four components of the National Risk Index equation are used to actually calculate the National Risk Index. The Risk Index represents the potential for negative impacts resulting from natural hazards. The risk index equation is expected annual loss, which is a natural hazard component that represents the average economic loss in dollars resulting from natural hazards each year, times a community risk factor, which is a scaling factor that incorporates Social Vulnerability and Community Resilience into the National Risk Index. The Community Risk Factor is calculated as a function that divides a Social Vulnerability factor, which is a consequence enhancing risk component and community risk factor that represents the susceptibility of social groups to the adverse impacts of natural hazards, by a Community Resilience factor, which is a consequence reduction risk component and community risk factor that represents the ability of a community to prepare for anticipated natural hazards, adapt to changing conditions, and withstand and recover rapidly from disruptions.

Each community has a composite Risk Index score and up to 18 hazard type Risk Index scores (e.g., Tornado Risk Index score). A composite Risk Index percentile measures the relative risk of a community based on all 18 natural hazards included in the Index by comparing its composite Risk Index value with other communities at the same level. A hazard type Risk Index percentile measures the relative risk of a community for a specific hazard type by comparing its hazard type Risk Index value with other communities at the same level.

Risk Value

Risk Index values form an absolute basis for measuring risk within the National Risk Index. They are used to generate all Risk Index percentiles and ratings.

To calculate Risk Index values, the National Risk Index generates a Community Risk Adjustment to scale Expected Annual Loss values up or down, depending on their community risk factors. The Community Risk Adjustment increases with Social Vulnerability and decreases with Community Resilience. So, higher Social Vulnerability for a community results in a higher Risk Index value while higher Community Resilience for a community results in a lower Risk Index value.

For comprehensive details about how risk is calculated for the Risk Index, see the National Risk Index Technical Documentation.

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