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Flood Return Period Calculator

Work out flood return period instantly with clear inputs, formula shown and shareable results.

About Flood Return Period Calculator

A 100 year flood has a 1% chance in any year, but over a 30 year mortgage the chance of at least one is 1 - 0.99^30 = 26%. The return period is an average recurrence interval, not a schedule, which is why two hundred-year floods can occur in consecutive years without contradiction.

Formulas

Annual exceedance probability

p = 1 / return period

Risk over n years

risk = 1 - (1 - p)^n

Worked example

With the default values below, Flood Return Period Calculator returns Probability Of At Least One Event of 26.03%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Return periodminimum 1 years
100 years
Period of interestminimum 0 years
30 years

Results

Probability Of At Least One Event
26.03%
Annual Exceedance Probability
1.000%
Expected Number Of Events
0.300
Expressed As Odds
About a 1 in 3.8 chance over 30 years

Step by step

  1. 1Values used
  2. 2Annual exceedance probability
  3. 3Risk over n years
  4. 4Probability Of At Least One Event= 26.03
  5. 5Annual Exceedance Probability= 1.000
  6. 6Expected Number Of Events= 0.300
  7. 7Expressed As Odds= About a 1 in 3.8 chance over 30 years

Assumptions and limits

  • The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or fees or charges into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Can a 100 year flood happen twice in a decade?

Yes. Each year is independent, so consecutive occurrences are unlikely but entirely possible - the term describes long-run frequency.

Why is the 30 year risk so much higher than 1%?

Because the risk compounds across years. Over 70 years the chance of at least one 100 year event reaches 50%.

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