Rainfall Runoff Calculator
Estimate stormwater runoff volume, average discharge, captured water, overflow and storage-tank quantity from catchment area and rainfall depth.
Default project preview
2,244 gallons
Average runoff flow
37.4 gal/min
Target captured volume
1,795 gal
Storage tanks
2
Purchased storage
2,000 gal
Calculator
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Enter the project details and calculate a rounded material estimate.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Runoff volume = catchment area × rainfall depth × runoff coefficient. Average flow divides volume by storm duration. Storage tanks are rounded up to the target captured volume.
FAQ
Rainfall Runoff Calculator questions
What does the runoff coefficient represent?+
It estimates the fraction of rainfall that becomes runoff. Smooth roofs are high; landscaped or permeable surfaces are lower.
Is average flow enough to size drains?+
No. Drainage systems are commonly sized for peak rainfall intensity, not storm-average flow.
Should storage include freeboard?+
Operational reserve, first-flush diversion and overflow space may require more capacity than the simple captured volume.