Calculator Inputs
Enter the selected IDF model, local coefficients, return period, and storm duration. Optional runoff values help estimate peak flow for design screening.
Formula Used
IDF equations relate rainfall intensity to storm duration and return period. This page supports four common forms so you can match your local design standard or calibrated dataset.
Generalized IDF
i = (A × Tm) / (d + B)n
Flexible for many calibrated municipal and highway drainage datasets.
Talbot
i = (A × Tm) / (d + B)
Useful when intensity falls nearly linearly with inverse duration.
Sherman
i = (A × Tm) / dn
A classic power relationship with no duration offset term.
Bernard
i = (A × Tm) / (dn + B)
Adds extra control when short-duration values need moderation.
Rainfall depth: Depth = i × (d / 60)
Metric peak runoff: Q = 0.00278 × C × i × A
Imperial peak runoff: Q = 1.008 × C × i × A
How to Use This Calculator
- Select the equation form that matches your local standard or fitted rainfall model.
- Choose the coefficient unit system used by your source coefficients.
- Enter storm duration and return period for the design event.
- Input coefficients A, B, m, and n from your study, standard, or curve fitting.
- Add runoff coefficient and drainage area to estimate peak discharge.
- Click Calculate IDF to show the result block above the form.
- Review the chart, summary values, and intensity table.
- Export the results using the CSV or PDF buttons.
Example Data Table
Illustrative example using the generalized model with A = 980, B = 8, m = 0.17, n = 0.76, and T = 10 years.
| Duration (min) | Intensity (mm/hr) | Depth (mm) |
|---|---|---|
| 5 | 206.36 | 17.20 |
| 10 | 161.15 | 26.86 |
| 15 | 133.76 | 33.44 |
| 30 | 91.33 | 45.66 |
| 60 | 58.68 | 58.68 |
| 120 | 36.29 | 72.57 |
Frequently Asked Questions
1) What does IDF mean in rainfall design?
IDF means intensity, duration, and frequency. It describes how hard rain falls, how long it lasts, and how often that storm severity is expected to occur.
2) Which equation model should I choose?
Use the model specified by your local drainage manual, stormwater criteria, or fitted station data. The best model is the one calibrated to the rainfall record used for design.
3) Why does intensity decrease for longer durations?
Short storms can produce sharp peaks. Longer storms average rainfall over more time, so the equivalent average intensity usually becomes smaller.
4) Can I use this for culverts and roadside drains?
Yes. It is useful for preliminary culvert, gutter, ditch, and inlet sizing when paired with the proper runoff coefficient, drainage area, and local design standard.
5) Do coefficients change by city or region?
Yes. Coefficients are location specific because climate, record length, station fitting, and governing standards vary from place to place.
6) Are the runoff equations included exact for final design?
They are standard screening equations. Final design should still check local criteria, time of concentration, allowable spread, storage effects, and regulatory requirements.
7) What units should I enter?
Enter coefficients in the same unit system as the chosen intensity output. Metric runoff uses hectares. Imperial runoff uses acres.
8) Why export to CSV or PDF?
CSV is useful for spreadsheets, checking, and documentation logs. PDF is useful for sharing a clean design snapshot with reviewers, clients, or field teams.