WHOLE-PROPERTY RUNOFF ESTIMATOR
Rain falls on more than your roof.
Explore how your roof, paved areas and lawn can add to the water a property sheds. Move the controls to compare an illustrative runoff rate.
ONE RAINSTORM. THREE SURFACES.
Watch the water add up.
Change the rain or an area. See how each surface contributes to the estimated flow.
The layout is a schematic. The bars below show the calculated contributions; arrows identify sources, not drainage routes.
Where the runoff comes from
Shared scale: 0–200 GPM55.2% of this example’s total
19.9% of this example’s total
24.9% of this example’s total
ILLUSTRATIVE RUNOFF RATE
276.87 GPM
Gallons per minute, for the rain and areas above.
Effective collection area: 3,620 sq ft of the 8,800 sq ft entered.
This example assumes roof 1.00, hardscape 0.90 and lawn 0.15. These are illustration settings, not verified values for your property.
THE CALCULATION
Each surface gets a share.
A runoff coefficient represents the fraction of rainfall included in the runoff estimate. Multiply each area by its assumed coefficient, add those contributions, then apply the rainfall intensity.
GPM = rain intensity ×
(roof + 0.90 × hardscape + 0.15 × lawn) ÷ 96.23
Areas are in square feet and intensity is in inches per hour.
Formula: NDS, Principles of Exterior Drainage, printed page 26. The guide’s coefficients depend on surface and soil conditions; it does not establish this example’s 0.15 lawn setting.
A WORKED EXAMPLE
Try the starting values.
2,000 sq ft of roof + 0.90 × 800 sq ft of hardscape + 0.15 × 6,000 sq ft of lawn = 3,620 effective sq ft.
At the illustrative intensity of 7.36 inches per hour:
7.36 × 3,620 ÷ 96.23 = 276.87 GPM.
This estimates a runoff rate, not storm volume, ponding depth or drainage performance. It does not choose a pipe, pump or system. Property conditions and an appropriate local rainfall intensity need assessment before design decisions.
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