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
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Catchment area
Enter this project assumption in ft². The current imperial default is 2000 ft².
Storm rainfall depth
Enter this project assumption in in. The current imperial default is 2 in.
Runoff coefficient
Enter this project assumption in 0–1. The current imperial default is 0.9 0–1. Minimum: 0. Maximum: 1.
Storm duration
Enter this project assumption in minutes. The current imperial default is 60 minutes.
Target capture
Enter this project assumption in %. The current imperial default is 80 %. Minimum: 0. Maximum: 100.
Capacity per storage tank
Enter this project assumption in gal. The current imperial default is 1000 gal.
Price per tank
Enter this project assumption in a unitless value. The current imperial default is 850. Minimum: 0.
How to read the outputs
Average runoff flow
storm-average, not peak flow. Default preview: 37.4 gal/min.
Target captured volume
80% target. Default preview: 1,795 gal.
Storage tanks
1,000 gal each. Default preview: 2.
Purchased storage
full tank capacity. Default preview: 2,000 gal.
Unstored overflow
runoff beyond purchased capacity. Default preview: 244 gal.
Storage cost
tanks only. Default preview: $1,700.00.
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.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 29, 2026
This calculator estimates runoff flow or storm volume from drainage area, rainfall input and an editable runoff coefficient. It supports preliminary planning but does not replace local hydrologic design criteria.
Calculation steps
- 1Define the truly connected drainage area.
- 2Use rainfall intensity for peak flow or rainfall depth for storm volume.
- 3Select a runoff coefficient appropriate to the surface.
- 4Compare the result with downstream conveyance and storage.
Worked numerical example
For a 2,000 ft² roof under 2 in/hr rainfall with a 0.95 runoff coefficient, the rational-method peak flow is approximately 39.5 gallons per minute.
Purchasing and rounding
This calculator does not produce a product order directly. Use its flow or volume as an input to gutter, downspout, tank, swale or drainage-system planning.
Interpret the result
The coefficient represents losses and surface response. A higher coefficient produces more runoff from the same rainfall and area.
Page-specific assumptions
- Uniform rainfall across the area.
- The drainage area contributes simultaneously.
- A simplified coefficient is appropriate.
- No inlet blockage or backwater.
Common mistakes
- Using rainfall depth where intensity is required.
- Including disconnected areas.
- Selecting a coefficient without regard to surface type.
- Treating preliminary flow as pipe sizing approval.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
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.