Returnperiod: free online flood frequency and IDF curve tool

Returnperiod fits flood and rainfall frequency curves in your browser. Paste annual peaks, choose a distribution and a fitting method, and read the quantile for any return period with confidence limits and a probability-paper plot.

It follows the Expected Moments Algorithm of USGS Bulletin 17C for log-Pearson III, with the Multiple Grubbs-Beck low-outlier test, historical and missing-record information and regional skew weighting. It also fits GEV, Gumbel and lognormal distributions by moments, L-moments or maximum likelihood, builds IDF curves and design storms from your own rainfall maxima, and sizes reservoir storage by the sequent peak method.

What Returnperiod can do

Annual maximum and partial duration series
Log-Pearson III with the Expected Moments Algorithm, GEV, Gumbel and lognormal
Moments, L-moments and maximum likelihood fitting
Confidence limits, goodness of fit and probability-paper plots
IDF equations, alternating-block and NRCS-style 24-hour design storms
Reservoir storage and yield by the sequent peak method; exports as CSV, SVG and PNG

How we verify Returnperiod

Returnperiod is checked against 141 independent reference cases. Where a reference answer exists you can run every check yourself on the Verify page, and the Returnperiod user guide documents every feature and limitation.

The Verify page runs 141 reference cases in your browser and shows, for each, what is checked, the independent reference, the value the tool computes, the difference and the tolerance. The same cases run in the test suite before every release, so what you see is what is tested.

Returnperiod is part of Open Tools, the free engineering tools that run in your browser.

Professional use. Results are for education and preliminary design. They must be checked by a qualified professional, and they are not a substitute for code-compliant design or licensed engineering judgement.