Free calculator
Return Period & Flood Risk Calculator
"1-in-100-year" sounds rare — until you convert it to the chance of it happening during the life of your building. This tool does that conversion for any return period.
Return Period & Flood Risk Calculator
About this tool: Converts a return period into its annual exceedance probability (AEP), and computes the probability of experiencing at least one such event over a chosen design life — the number that shows what a '1-in-100-year' label really means for a building.
Annual exceedance probability
Risk over the design life
A 1:100-year event has a 1% chance of being equalled or exceeded in any single year — and a ≈ 39.5% chance of occurring at least once over 50 years.
The formulas
Annual exceedance probability: AEP = 1 / T — a 1:100-year event has AEP = 1/100 = 1% in any given year, independently of what happened last year.
Risk over a design life: R = 1 − (1 − 1/T)n — the probability of at least one exceedance in n independent years.
Worked example
For the 1:100-year flood over a 50-year design life: R = 1 − (1 − 0.01)50 = 1 − 0.9950 = 1 − 0.605 ≈ 39.5%. Far from "once in a lifetime" — closer to a two-in-five chance.
For what these probabilities mean for floodlines and planning, see our return periods explainer.
Disclaimer
This tool produces screening-level estimates from standard published formulas. It is not a design deliverable, does not account for site-specific conditions, and must not be used as the basis for design, construction, or regulatory submissions without professional review. Delta Hydro Engineers (Pty) Ltd accepts no liability for decisions taken on unverified calculator output.
Need a defensible number for a real project — reviewed, documented, and ready for a regulator? That's what we do.
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