Updated May 2026

Methodology

FloodRiskData computes a 0–100 composite flood risk score for 3,277 U.S. counties using public-domain FEMA flood-mapping program data. Every input is open, every formula is documented below, and every figure can be reproduced from the underlying OpenFEMA endpoints.

Data Source

All data comes from the OpenFEMA API, FEMA's public access layer for federal disaster and insurance datasets. We use two primary endpoints:

Disaster Declarations Summaries (v2) — 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. The OpenFEMA API has no auth requirement but does apply rate limits, so the fetch script uses retry-with-backoff. FEMA typically updates NFIP claims on a quarterly cycle; we refresh from the API on a recurring cadence and publish the freshness date in the page footer.

We deliberately do not include private flood-insurance data, real-estate listings, or proprietary risk scores. Every figure on the site can be regenerated by querying the same public endpoints. Real-time stream-gauge readings — useful complementary data when a storm is active — are available from USGS Water Data; NOAA storm-surge models complement the FEMA picture for coastal counties.

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 factor is scaled to a 0–100 range against the national distribution before the weighted sum is taken, so a score of 50 on any individual factor represents the national median.

40%

Claims Density

Total NFIP flood-insurance claims filed in the county, normalized against the national distribution. This is the strongest predictor of future flood risk in actuarial research because it directly measures realized insured loss frequency. The 40% weight reflects that empirical primacy.

25%

Disaster Frequency

Number of federally declared flood-related disasters in the county. Federal declarations capture large-loss events that exceed state and local response capacity — a different and complementary signal from raw NFIP claim volume.

20%

Claim Severity

Average payout per NFIP claim. Higher average payouts distinguish counties hit by catastrophic single events (Katrina, Sandy, Harvey, Ida, Ian) from those that experience frequent nuisance flooding — both important, but with very different insurance and mitigation implications.

15%

Trend

Year-over-year change in claims volume. Counties where claims are increasing receive higher trend scores, capturing emerging risk that historical averages miss. The 15% weight reflects that trend is the only forward-looking signal in the formula but is also the noisiest.

Grade Scale

The composite 0–100 score is converted to a letter grade for fast reference:

Grade	Score Range	Risk Level	Counties
A	0 – 20	Lowest risk	3,121
B	21 – 40	Moderate risk	153
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 (3,121 of 3,277) receive an A grade, reflecting that severe flood risk is heavily concentrated along the Gulf Coast, Atlantic seaboard, and major river floodplains. That distribution is not a methodological artifact — it is a real feature of the insured-loss data, and it is one of the more robust findings in U.S. flood-loss research.

Limitations

Several important limitations apply. Reading these before relying on the score for any consequential decision is essential.

County-level granularity — Flood risk varies sharply within a single county. A county-level score cannot capture neighborhood- or parcel-level differences. For property-specific assessments use FEMA flood maps at fema.gov/flood-maps and a recent USGS gauge crest history.
NFIP-participation bias — Claims data only reflects insured losses through the National Flood Insurance Program. Uninsured flooding and private flood-insurance claims are not captured, which can understate risk in low-NFIP-participation regions.
Historical bias — Scores reflect past patterns. Climate-driven shifts in precipitation and storm intensity, new development extending into flood-prone areas, and infrastructure improvements may alter forward risk in ways the historical data does not yet show. The trend factor is the only forward-looking signal.
Disaster-declaration threshold — Not all significant floods result in federal disaster declarations. Some counties experience repeated flooding that falls below the federal threshold and therefore does not register on the disaster-frequency factor.
FEMA publication lag — NFIP claims are published roughly quarterly, so very recent storm seasons may not yet be reflected in the score.

Frequently Asked Questions

Why these four factors and these weights?

Claims density (40%) is the strongest single predictor of forward flood risk in actuarial research, because it directly measures the rate at which insured loss has occurred. Disaster frequency (25%) captures large-loss events that exceed the federal disaster threshold — a different and complementary signal from raw claim volume. Claim severity (20%) separates places with frequent nuisance flooding from those hit by catastrophic single events, which carry very different insurance and mitigation implications. Trend (15%) is the smallest factor on purpose: it can be noisy year-to-year, but it is the only forward-looking signal in the formula.

How does the score relate to FEMA flood maps?

The composite score is a county-wide aggregate; FEMA Flood Insurance Rate Maps (FIRMs) are parcel-level zone designations. The two answer different questions. A county-wide score tells you how often the surrounding region actually experiences damaging floods (a useful relocation, real-estate, or insurance signal); a FIRM tells you whether your specific lot is in a Special Flood Hazard Area subject to mandatory insurance under federal law. Use both. FIRMs are at fema.gov/flood-maps.

How current is the data?

The pipeline pulls fresh data from the OpenFEMA API on a recurring cadence; FEMA itself republishes NFIP claims roughly every quarter, so figures here can lag actual events by up to three months. The current dataset was last updated on 2026-05-16 and covers 3,277 counties, 99,729 flood-insurance claims, and 1,403 federal disaster declarations.

Why does the dataset have so many A-grade counties?

Of 3,277 counties, 3,121 carry an A grade — about 95% of the country. That is a real and important feature of the U.S. flood-loss distribution: severe flood risk is sharply concentrated along the Gulf Coast, Atlantic seaboard, and major river floodplains, while most of the inland country sees little measurable insured flood loss.

Can I cite these numbers in a paper, article, or report?

Yes. Cite as: "FloodRiskData (floodriskdata.org), drawn from FEMA OpenFEMA NFIP claims and disaster declarations, accessed [date]." Where exact accuracy matters, link directly to the underlying FEMA datasets — both are public-domain U.S. government work. The composite score and grade are derived metrics computed by FloodRiskData, not official FEMA designations.

What should I do if my county has a high score?

Three steps. First, look up your specific parcel's FEMA Flood Insurance Rate Map zone — the county-wide score does not tell you whether your individual lot is in a Special Flood Hazard Area. Second, get an NFIP or private flood-insurance quote; standard homeowners policies do not cover flood damage and the NFIP 30-day waiting period means coverage cannot be added the day a storm forms. Third, consult our Guides page for property-level mitigation steps.