Return period

A return period is the average time expected between two events that reach or exceed a given severity, estimated from historical frequency or a hazard model. A "100-year flood" is a flood level with a return period of 100 years, meaning it has historically occurred, or is modeled to occur, on average once every 100 years at that location.

The clearer, less misleading way to express the same idea is the annual exceedance probability: probability per year = 1 / return period, so a 100-year event has a 1% chance of being equaled or exceeded in any single year, and roughly a 26% chance over a 30-year mortgage term, not a 1-in-100 chance restricted to exactly year 100. Return periods are the standard axis output by a catastrophe model or coastal flood modeling simulation, expressed as a loss or flood depth at each of several return periods.

Return periods set flood-insurance zones, building codes, and reinsurance attachment points, and the single most common misreading is treating "100-year event" as meaning the next one is 100 years away, or that a location cannot have two in the same decade; independent years each carry the same 1% probability, so clustering is expected, not anomalous. A second pitfall is quoting a return period without noting that climate change and updated exposure data shift the underlying probabilities over time, so historical return periods understate current risk in many regions.