Flood Return Period Calculator
Convert between a flood's return period and its annual probability — and see the chance it happens at least once over a mortgage, a design life or a lifetime, which is the number that actually matters.
A "100-year flood" has a 1% chance in any given year.
How the flood return period calculator works
A "100-year flood" has a 1% chance in any given year. It is not a flood that comes every century. Over 30 years the chance of at least one is 26%; over a 70-year building life it is more than half.
That conversion — from annual probability to the probability over a span of years — is what people misread, and it is a one-line formula: 1 − (1 − p)ⁿ.
Formula: p = 1 / T; P(at least one in n years) = 1 − (1 − p)ⁿ
Worked examples
| Inputs | Chance of at least one flood in that time | Note |
|---|---|---|
| A 100-year flood over 30 years | 26.03% | 26% — one in four |
| Over a 70-year life | 50.52% | more likely than not |
| A 20-year flood | 78.54% | 79% over 30 years |
FAQFrequently asked questions
What is a 100-year flood?
A flood with a 1% chance of being equalled or exceeded in any year. It can happen two years running; the name describes probability, not schedule.
What is AEP?
Annual exceedance probability — the chance in a single year. Planners are moving to it from return periods precisely because "100-year" misleads.
Why does the 30-year figure matter?
Because that is a mortgage. A one-in-four chance of flooding during the loan is a different proposition from "once a century".
Does a flood reset the clock?
No. Floods are independent events; having one does not make the next less likely. That is the whole point of the probability framing.
What return period should a building be designed for?
Floor levels are commonly set to the 100-year level plus freeboard; critical infrastructure to 500 or 1,000. Check the local flood study.
Where these figures come from
- UN-Habitat — Urban Indicators Guidelines — density, green space and public space benchmarks
- World Health Organization — Urban green spaces — the 9 m² per person minimum
- ITE — Parking Generation Manual — parking demand rates by land use
- Australian Bureau of Statistics — Regional population — Australia's national statistics agency
Last checked: September 2026. Default per-capita rates are mid-range figures from national statistical agencies and utility reports; every one is an input you should replace with the local figure.