Methodology

Data Source

All risk data on RiskByCounty comes from the FEMA National Risk Index (NRI), a publicly available dataset produced by the Federal Emergency Management Agency. The NRI provides a nationally consistent, county-level assessment of natural hazard risk across the United States.

The NRI is designed to help communities understand their relative risk compared to other counties nationwide. FEMA updates the dataset periodically as new hazard, exposure, and vulnerability data become available.

Risk Metrics

Each county is evaluated on several key metrics from the FEMA NRI:

Natural Hazard Types

The FEMA NRI covers 18 natural hazard types. RiskByCounty focuses on the five most impactful hazards that affect the largest number of US counties:

• Flood — riverine and coastal flooding risk, the most widespread natural hazard in the US.
• Wildfire — risk of uncontrolled fires in wildland-urban interface areas.
• Tornado — risk from rotating columns of air, most common in the central US.
• Earthquake — seismic hazard risk, concentrated along fault lines and in the Pacific Northwest.
• Hurricane — tropical cyclone risk, primarily affecting Gulf and Atlantic coastal counties.

Additional hazard types in the FEMA NRI include avalanche, cold wave, drought, hail, heat wave, ice storm, landslide, lightning, strong wind, tsunami, volcanic activity, and winter weather. These contribute to the overall composite risk score but are not broken out individually on RiskByCounty.

How Risk Scores Are Calculated

RiskByCounty uses percentile-rank scoring to convert raw FEMA NRI data into a 0-100 scale:

1. Raw FEMA NRI composite risk values are collected for all 3,100+ US counties.
2. Counties are ranked against all other counties nationwide for each metric.
3. Percentile ranks are calculated on a 0-100 scale, where 100 represents the highest risk.
4. Individual hazard scores (flood, wildfire, tornado, earthquake, hurricane) follow the same percentile methodology.

Risk ratings are assigned based on FEMA's official rating bands:

Geographic Coverage

RiskByCounty covers all 3,100+ counties and county-equivalents across all 50 US states and the District of Columbia. This includes parishes (Louisiana), boroughs and census areas (Alaska), and independent cities (Virginia). Coverage is determined by the FEMA NRI dataset, which provides risk assessments for every county-level jurisdiction recognized by the US Census Bureau.

AI-Generated Content Disclosure

Some descriptive text on RiskByCounty is generated or assisted by artificial intelligence (Claude by Anthropic). This includes county and state narrative summaries that contextualize the raw data. All AI-generated content is reviewed for accuracy and is based on the underlying FEMA NRI data.

All numerical data, scores, and rankings are computed directly from FEMA source data — they are never generated or estimated by AI. The AI is used solely to produce readable explanations of what the numbers mean in context.

Data Freshness

The FEMA National Risk Index is updated periodically as new hazard modeling, Census data, and loss estimates become available. RiskByCounty refreshes its data after each new NRI release.

Current data vintage: FEMA National Risk Index (most recent release). County-level scores on this site reflect the latest available NRI data at the time of our most recent data pipeline run.

Because natural hazard risk is based on historical patterns, exposure models, and long-term climate data, risk scores do not change dramatically from year to year. However, significant events (e.g., new flood maps, updated seismic models) can cause meaningful shifts for specific counties.