Stormwater Drainage

Stormwater Runoff Calculator

Modified Rational Method

Calculate peak flow rates and total runoff volume based on local rainfall data, drainage area, and surface conditions.

1
Input Parameters

Select the design storm length (often matched to time of concentration); NOAA intensities below are shown as in/hr for this duration, and shorter durations raise the intensity.

2
Results

Peak Flow Rate

0cfs

Total Volume

0.02cf

How to use this result

Peak flow is the planning discharge for checking inlet, channel, and pipe capacity. Total volume is the runoff generated during the selected duration and is the starting point for detention or storage sizing. Your current area converts to 0.0000 acres; both results update immediately when you change C, I, A, or duration.

Check open-channel capacitySize the trench and outlets

About the Modified Rational Method

The Modified Rational Method estimates peak discharge and runoff volume from small drainage areas, extending the traditional Rational Method by incorporating storm duration for volume calculations.

Q = CIA

  • Q = Peak flow rate (cfs)
  • C = Runoff coefficient
  • I = Rainfall intensity (in/hr)
  • A = Drainage area (acres)

Worked Rational Method example

For a 1-acre paved drainage area (C = 0.95) with a 3.0 in/hr design intensity, Q = 0.95 × 3.0 × 1.0 = 2.85 cfs. If that intensity lasts 30 minutes, the estimated runoff volume is about 5,173 cubic feet.

This example demonstrates the equation only; use the design storm, time of concentration, coefficients, and criteria required by the project jurisdiction.

Inputs, data, and design limits

  • Rainfall: enter a project intensity directly or use the ZIP lookup as a starting point. Confirm the selected duration and return period against current local design criteria.
  • Runoff coefficient: the surface presets are screening values. Mixed drainage areas should use a weighted coefficient based on actual cover and site conditions.
  • Method: the Rational Method assumes a reasonably uniform design intensity and is generally used for relatively small drainage areas. Confirm applicability with the reviewing authority.
  • Pipe suggestion: the displayed pipe size is a preliminary Manning check at 1% slope and n = 0.013, not a final hydraulic design or code approval.