Nantucket County, MA: flood risk, in FEMA's own numbers
On FEMA's National Risk Index, Nantucket County scores at the 70th percentile for flood hazard, rated "Relatively Moderate" — a middling flood hazard by national standards. The hazard here is driven by coastal storm surge rather than inland rivers — FEMA's model puts a surge event here at about 3.7 times a year. That's a county-wide figure, not a per-address one — FEMA's parcel maps are a live lookup, not a bulk file.
Nantucket County's flood-insurance compliance gap
56.2% of the homes FEMA requires to carry flood insurance in Nantucket County — those inside the mapped Special Flood Hazard Area — actually have a policy in force: 300 policies against 534 mandated structures, which means a meaningful share of legally-required coverage simply isn't there. Looking beyond the mandated zone, 7.5% of all residential structures in Nantucket County carry flood coverage voluntarily or otherwise — relevant because most NFIP claims nationally come from outside the mapped SFHA.
Nantucket County against the rest of the country
Among the 3144 US counties FEMA scores for flood hazard, Nantucket County ranks 986 — 1 being the highest hazard nationally — and 2110 by population among all 3144 counties covered here, where 1 is the largest. About 14,670 people live in Nantucket County, Massachusetts, spread over 46.192 square miles.
Your street may differ from Nantucket County's average
Across all of Nantucket County's 9,533 residential structures, 714 carry an NFIP policy today — a county-wide count that still can't tell you one address's own mapped zone.
Where Nantucket County's figures come from
Every figure on this page traces to a named FEMA dataset, documented in the methodology guide. Nantucket County's coastal exposure is covered in the coastal vs. inland flood risk guide, including how storm surge risk differs from river flooding.