What is a 100-year flood?

Checked against FEMA and other official sources · Last reviewed October 8, 2026

What "1-percent annual chance" means

Federal flood insurance rules define the base flood as "the flood having a one percent chance of being equalled or exceeded in any given year." FEMA says this 1-percent-annual-chance flood "is also referred to as the base flood or 100-year flood."

The "100" comes from statistics. USGS explains that the 100-year flood is "an estimate of the long-term average recurrence interval." That "does not mean that we really have 100 years between each flood of greater or equal magnitude."

The chance resets every year. USGS notes that "a rare flood does not reduce the chances of another rare flood within a short time period." A 100-year flood last year doesn't buy you 99 safe years.

USGS itself admits that "use of such terms as the '100-year flood' can confuse or unintentionally mislead those unfamiliar with flood science." USGS uses the more exact term "1-percent AEP flood," where AEP stands for annual exceedance probability. [1][2][3][4][5]

The odds over a 30-year mortgage

A 1-percent chance in one year adds up over time. USGS puts it this way: "during the span of a 30-year mortgage, a home in the 1-percent AEP (100-year) floodplain has a 26-percent chance of being flooded at least once during those 30 years."

USGS says the 26 percent figure "is based on probability theory that accounts for each of the 30 years having a 1-percent chance of flooding."

FEMA gives the same number: "A home located within an SFHA has a 26 percent chance of suffering flood damage during the term of a 30-year mortgage." FloodSmart, the NFIP's official site, rounds it to a "1 in 4 chance of flooding at least once" over a 30-year mortgage. [4][6][7]

The 500-year flood (0.2-percent annual chance)

USGS says the "500-year flood" "corresponds to an AEP of 0.2 percent, which means a flood of that size or greater has a 0.2-percent chance (or 1 in 500 chance) of occurring in a given year." The 1,000-year flood has a 0.1 percent chance in any given year.

On FEMA flood maps, moderate flood hazard areas are labeled zone B or shaded zone X. They are "the areas between the limits of the base flood and the 0.2-percent-annual-chance (or 500-year) flood." Minimal hazard areas are labeled zone C or unshaded zone X. [3][4][5]

Floodplain, 100-year floodplain and base flood elevation

In federal rules, "flood plain" is broad: "any land area susceptible to being inundated by water from any source."

The "100-year floodplain" is narrower. It is the area FEMA maps as the Special Flood Hazard Area (SFHA): "the land in the flood plain within a community subject to a 1 percent or greater chance of flooding in any given year." FEMA notes SFHAs "are also known as 1%-annual-chance floodplains, base floodplains or 100-year floodplains." High-risk areas begin with the letters A or V on FEMA flood maps, such as zone AE, zone A, and zone VE.

The base flood elevation (BFE) is "the computed elevation to which floodwater is anticipated to rise during the base (1-percent-annual-chance) flood event." To get a Letter of Map Amendment, for example, the lowest ground touching your building must be at or above the BFE.

The federal flood insurance purchase requirement applies to structures in SFHAs with a mortgage from a federally regulated lender or servicer. See do I need flood insurance? [1][7][8][9]

Floodplain vs. floodway

The regulatory floodway is the part of a river's floodplain that must be kept open to carry flood water. Federal rules define it as "the channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than a designated height."

Building in the floodway is tightly limited. Where FEMA has provided floodway data, communities in the NFIP must prohibit "encroachments, including fill, new construction, substantial improvements, and other development" in the floodway, unless engineering analysis shows the project "would not result in any increase in flood levels within the community during the occurrence of the base flood discharge."

If you plan to build or add on near a river or stream, check with your local floodplain administrator first. [1][10]

Why the 100-year line can move

The 100-year flood is an estimate, not a fixed fact. USGS explains that the 100-year flood level is computed from past data, and that as more data come in, or when a river basin is changed in a way that alters the flow of water, "scientists re-evaluate the frequency of flooding." USGS says "dams and urban development are examples of some man-made changes in a basin that affect floods."

Estimates also carry uncertainty. USGS describes 90-percent confidence limits around its flood frequency curves, meaning "there is a 10-percent chance that the true value lies somewhere outside the confidence interval lines."

Flood maps change too. FEMA warns that effective map information "may change or become superseded by new maps over time." To see your current map, follow what flood zone am I in? [2][3][4][11]

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