How to use the flood return period calculator
Estimate the return period of a flood from its rank in a record of past floods. Enter years of record, rank of flood (1 = largest) in the fields above; the return period (weibull), annual exceedance probability are computed instantly and plotted on the chart so you can see how the answer changes.
Common searches this calculator answers include “flood return period”, “100 year flood calculation”, “Weibull plotting position”, “calculate flood return period”, “find return period”.
Formula
Variables
- n — Years of record
- m — Rank of flood (1 = largest)
Worked example
With years of record = 50, rank of flood (1 = largest) = 5, the calculator gives return period (weibull) ≈ 10.2 years; annual exceedance probability ≈ 9.803922 %.
Frequently asked questions
What is the flood return period calculator used for?
Estimate the return period of a flood from its rank in a record of past floods. It helps with questions such as “flood return period”, “100 year flood calculation”, “Weibull plotting position”.
What is the formula for the flood return period calculation?
Return period (Weibull) (years) = (n+1) / m. Annual exceedance probability (%) = 100 × m / (n+1). Here n is years of record, m is rank of flood (1 = largest).
What inputs do I need for the flood return period calculator?
You need years of record, rank of flood (1 = largest). 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 return 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 return 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.