What Is a 100-Year Flood?

Most people hear "100-year flood" and assume it means a flood that happens once every 100 years. If one just happened, you might think you are safe for the next 99 years. This interpretation is wrong, and it leads to dangerous underestimation of flood risk.

The term does not describe a schedule or a cycle. It describes a probability. A 100-year flood has a 1% chance of occurring in any given year. That 1% chance resets every single year, regardless of whether a flood happened the year before. Two 100-year floods can occur in back-to-back years, or even in the same year on different rivers.

FEMA and hydrologists describe the 100-year flood as the "1-percent-annual-chance flood." While 1% sounds small in any single year, the cumulative probability over time is significant.

Over a typical 30-year mortgage, a property in the 1% annual chance floodplain has a 26% chance of experiencing at least one major flood. By comparison, the chance of a house fire during the same period is approximately 9%. Flood risk in these areas is nearly three times more likely than fire, yet most homeowners carry fire insurance and skip flood insurance.

FEMA and many hydrologists have acknowledged that the term "100-year flood" is misleading. The agency increasingly uses the more accurate phrase "1-percent-annual-chance flood" in official documents. However, the older term is deeply embedded in public language, real estate, insurance, and regulation.

The confusion is not just academic. Studies have shown that homeowners in the 100-year floodplain are less likely to purchase flood insurance after a recent flood event, believing the risk has been "used up." In reality, the probability is the same every year, regardless of what happened before.

The 100-year flood is the foundation of FEMA's flood zone system. Areas with a 1% or greater annual chance of flooding are designated as Special Flood Hazard Areas (SFHAs). All A and V zones fall within this boundary. Under the Flood Disaster Protection Act of 1973 (42 U.S.C. § 4012a), properties in SFHAs with federally regulated or government-backed mortgages are required to carry flood insurance.

The elevation that floodwater is expected to reach during a 100-year flood is called the Base Flood Elevation (BFE). Building codes in the floodplain are tied to the BFE. New construction must have the lowest floor at or above this elevation per NFIP floodplain management standards (44 CFR 60.3).