Updated April 2026

Methodology

How we calculate flood risk scores for 3,271 U.S. counties using publicly available FEMA data.

Data Source

All data comes from the OpenFEMA API, which provides public access to FEMA datasets. We use two primary datasets:

Disaster Declarations Summaries (v2), Records of every federally declared disaster, filtered to flood-related incident types (flood, hurricane, severe storm, coastal storm, typhoon)
FIMA NFIP Claims, Individual flood insurance claims filed through the National Flood Insurance Program, aggregated by county FIPS code

Both datasets are U.S. government public domain works. We refresh our data monthly from the OpenFEMA API. FEMA typically updates NFIP claims on a quarterly cycle, so some figures may lag by up to three months.

Composite Score

Each county receives a composite flood risk score from 0 to 100. The score is a weighted average of four normalized factors, each scaled independently before combining:

40%

Claims Density

Total NFIP flood insurance claims filed in the county, normalized by the number of counties in the dataset. Counties with more claims per capita receive higher scores. This is the strongest predictor of future flood risk.

25%

Disaster Frequency

The number of federal disaster declarations involving flooding in the county. More frequent declarations indicate a pattern of recurring flood events severe enough to warrant federal assistance.

20%

Claim Severity

The average payout per NFIP claim in the county. Higher average payouts suggest more severe flooding events that cause greater property damage per incident.

15%

Trend

Year-over-year change in claims volume. Counties where claims are increasing over time receive higher trend scores, reflecting worsening flood conditions. This factor ensures the score captures emerging risk, not just historical patterns.

Grade Scale

The composite score is converted to a letter grade for quick reference:

Grade	Score Range	Risk Level	Counties
A	0 – 20	Lowest risk	3,132
B	21 – 40	Moderate risk	136
C	41 – 60	Elevated risk	2
D	61 – 80	High risk	1
F	81 – 100	Extreme risk	0

The vast majority of U.S. counties (3,132 of 3,271) receive an A grade, reflecting that severe flood risk is concentrated in a relatively small number of areas, particularly along the Gulf Coast, Atlantic seaboard, and major river floodplains.

Limitations

This scoring methodology has several important limitations:

County-level granularity, Flood risk varies significantly within a county. A county-level score cannot capture neighborhood or parcel-level differences. Use FEMA flood maps for property-specific assessments.
NFIP participation bias, Claims data only reflects insured losses through the NFIP. Uninsured flooding and private flood insurance claims are not captured.
Historical bias, Scores reflect historical patterns. Climate change, new development, and infrastructure improvements may alter future risk in ways not yet reflected in the data.
Disaster declaration threshold, Not all significant floods result in federal disaster declarations. Some counties may experience repeated flooding that falls below the federal threshold.