How to use the flood risk over a period calculator
Find the chance of at least one flood of a given return period during a time period such as a mortgage or building life. Enter return period, time period of interest in the fields above; the annual chance of flood, chance of at least one flood in period are computed instantly and plotted on the chart so you can see how the answer changes.
Common searches this calculator answers include “flood risk over lifetime”, “chance of 100 year flood”, “probability of flood in 30 years”, “calculate flood risk over a period”, “find annual chance of flood”.
Formula
Variables
- T — Return period (years)
- yrs — Time period of interest (years)
Worked example
With return period = 100 years, time period of interest = 30 years, the calculator gives annual chance of flood ≈ 1 %; chance of at least one flood in period ≈ 26.02996 %.
Frequently asked questions
What is the flood risk over a period calculator used for?
Find the chance of at least one flood of a given return period during a time period such as a mortgage or building life. It helps with questions such as “flood risk over lifetime”, “chance of 100 year flood”, “probability of flood in 30 years”.
What is the formula for the flood risk over a period calculation?
Annual chance of flood (%) = 100 / T. Chance of at least one flood in period (%) = 100 × (1-(1-1 / T)^yrs). Here T is return period (years), yrs is time period of interest (years).
What inputs do I need for the flood risk over a period calculator?
You need return period in years, time period of interest in years. Each field is pre-filled with a typical example so you can see how the result responds as you edit the numbers.
How do I use the flood risk over a period calculator?
Type values into the input fields. Results and the chart update instantly. Use Reset to restore the example values, Copy results to paste them elsewhere, or Share to copy a link that preserves your inputs.
Is the flood risk over a period result exact?
Calculations use standard formulas and double-precision arithmetic, then display about seven significant figures. Real-world measurements carry uncertainty, so treat results as estimates unless your inputs are exact.