Rainfall Intensity IDF Calculator

Example Data Table

This table demonstrates intensities and depths for common durations, using the same return period and coefficients you entered.

Formula Used

This calculator uses a common intensity-duration-frequency relationship:

i = (a · T^b) / (t + c)^d

• i is rainfall intensity (mm/hr).
• T is return period (years).
• t is storm duration (minutes).
• a, b, c, d are fitted coefficients from local IDF data.

Professional Notes on IDF Design Use

Intensity–Duration–Frequency (IDF) relationships translate long‑term rainfall records into practical design intensities for stormwater and drainage infrastructure. In construction planning, IDF values help size roof drains, gutters, site inlets, pipes, channels, detention features, and temporary erosion controls. The intent is not to predict a specific storm on a calendar date; instead, IDF curves describe the expected magnitude of short‑duration rainfall associated with a selected return period.

This calculator uses a widely applied power‑law form that relates intensity to both duration and return period. The coefficients a, b, c, and d must come from a local source such as a municipal standard, a regional hydrology study, or a published dataset. Because rainfall climate varies sharply by location, avoid reusing coefficients from distant stations. When your project crosses elevations or coastal influences, confirm that the chosen dataset matches the project’s rainfall regime.

Practical workflow typically starts by selecting a return period aligned with code or client risk tolerance. Short return periods are common for minor drainage, while critical infrastructure may require larger values. Next, choose a duration that reflects the time of concentration or the controlling drainage path. When unsure, evaluate several durations and adopt the governing case for peak intensity.

Example data: using duration 15 minutes and return period 10 years with coefficients a=1000, b=0.2, c=10, d=0.75, the computed intensity is 141.757 mm/hr and the storm depth is 35.439 mm. You can compare that depth against inlet capacity, surface storage, or allowable ponding limits.

Finally, document assumptions. Record the source of coefficients, the return period rationale, and the duration basis. Use the CSV/PDF exports to keep a clear audit trail across design iterations, peer reviews, and construction submittals. Consistent documentation reduces rework and improves coordination between civil, architectural, and MEP teams.

FAQs

1) What is an IDF curve used for?

An IDF curve estimates design rainfall intensity for a chosen storm duration and return period. Engineers use it to size drainage elements like inlets, pipes, gutters, and detention features.

2) Where do I get the coefficients a, b, c, and d?

Use coefficients published by your municipality, highway agency, watershed study, or local hydrology report. They should be derived from regional rain gauge records and updated standards.

3) What does the return period mean in practice?

A 10‑year return period means a storm of that intensity has about a 10% chance of being equaled or exceeded in any year. It does not mean it occurs exactly once per decade.

4) How should I choose the storm duration?

Select a duration close to the time of concentration or the controlling drainage travel time. If uncertain, test multiple durations and adopt the one producing the most critical design.

5) Why does intensity decrease as duration increases?

Short bursts can be very intense, but sustaining that rate over longer periods is less likely. IDF curves capture this statistical behavior in observed rainfall records.

6) Is storm depth the same as rainfall intensity?

No. Intensity is a rate (mm/hr or in/hr). Depth is the accumulated rainfall for the selected duration, computed as intensity multiplied by duration expressed in hours.

7) Can I use this calculator without local coefficients?

You can explore sensitivity using example values, but final design should use coefficients from an approved local source. Using non‑local parameters can significantly over‑ or under‑size drainage systems.