Rainfall Intensity Calculator: mm/hr, IDF & Runoff Discharge

Convert rainfall depth and storm duration into mm/hr and in/hr, then estimate runoff volume and peak discharge with the Rational Method and an IDF curve.

Quick 1-Click Storm Presets

Meteorological & Watershed Parameters

Rainfall Intensity
25.0mm / hr
0.98 in/hr (Heavy Rain)
Peak Discharge (Q)
5.90m³ / s
Rational Method peak flow
Total Runoff Volume
85000m³
Per 1 km² basin area
Annual Exceedance Prob.
10.0% / year
10-Year Design Recurrence

Intensity-Duration-Frequency (IDF) Curve

Intensity decay over storm duration for a 10-year return interval.

Storm Hyetograph & Depth Profile

Synthetic bell distribution of instantaneous intensity over the 4h storm.

Worked Example: 100 mm Storm Over 4 Hours in an Urban Catchment (1 km²)

Estimating mean intensity, runoff volume, and peak stormwater discharge for an urban basin:

Rainfall Depth
100 mm (Total Accumulated Precipitation)
Storm Duration
4.0 Hours
Catchment Area
1.0 km² (1,000,000 m²)
Area Runoff Coefficient (C)
0.85 (Urban Impervious Pavement/Roofs)
Hydrological Calculations:
•1. Mean Rainfall Intensity (i): 100 mm / 4.0 h = 25.0 mm/hr (0.984 in/hr).
•2. Runoff Volume (V): Depth (m) × Area × C = (100 / 1000) × 1,000,000 × 0.85 = 85,000 m³.
•3. Rational Method Peak Discharge (Q): Q = (C × i × A) / (3.6 × 10⁶) = (0.85 × 25 × 1,000,000) / 3,600,000 = 5.90 m³/s.
•4. Return Period Exceedance: A 10-year storm has an annual exceedance probability (AEP) of 1 / 10 = 10.0%.
•5. Sherman IDF Equation: i = a · T^b / (t + c)^d evaluates how short-duration peak intensity diverges from storm mean.

💡 Civil Engineering Insight: A 100 mm storm in 4 hours generates an intense mean rate of 25 mm/hr, creating 85,000 m³ of runoff and a peak runoff crest of 5.90 m³/s requiring substantial municipal drainage capacity.

Frequently Asked Questions

Authoritative Hydrology & Stormwater Engineering References

NOAA National Weather Service - Precipitation Frequency Data Server (Atlas 14)

Official United States precipitation frequency estimates, point recurrence intervals, and regional IDF curve matrices.

United States Geological Survey (USGS) - Surface-Water Runoff & Modeling

National hydrology monitoring data, streamflow gauging, and peak flood frequency evaluation methodologies.

ASCE - Design and Construction of Urban Stormwater Management Systems

Standard American Society of Civil Engineers manual of practice for stormwater drainage and Rational Method application.

US EPA - National Stormwater Calculator & Green Infrastructure Modeling

Environmental Protection Agency hydrologic modeling software estimating runoff from local soil and meteorological datasets.

Related Calculators

Wind Chill Calculator

Meteorology

Heat Index Calculator

Meteorology

Dew Point Calculator

Meteorology

Relative Humidity Calculator

Meteorology

Rainfall Intensity Calculator

Meteorology

Air Pressure Calculator

Meteorology

Cloud Base Height Calculator

Meteorology
View All Calculators

Quick Tips

  • 1Use consistent units throughout calculations
  • 2Consider significant figures in results
  • 3Document experimental conditions
  • 4Verify results with control calculations