Roof Drainage Calculator
Estimate peak roof runoff, downspout quantity, flow per outlet and storm-water volume from roof area, rainfall intensity and runoff coefficient.
Default project preview
53.3 gal/min
Minimum downspouts
2
Flow per downspout
26.65 gal/min
Storm rainfall depth
1.5 in
Storm runoff volume
1,599 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.
Horizontal roof area
Enter this project assumption in ft². The current imperial default is 1800 ft².
Design rainfall intensity
Enter this project assumption in in/hour. The current imperial default is 3 in/hour.
Runoff coefficient
Enter this project assumption in 0–1. The current imperial default is 0.95 0–1. Minimum: 0.1. Maximum: 1.
Capacity per downspout
Enter this project assumption in gal/min. The current imperial default is 35 gal/min.
Storm duration
Enter this project assumption in minutes. The current imperial default is 30 minutes. Minimum: 1.
Runoff directed to storage
Enter this project assumption in %. The current imperial default is 50 %. Minimum: 0. Maximum: 100.
How to read the outputs
Minimum downspouts
based on entered capacity. Default preview: 2.
Flow per downspout
equal distribution. Default preview: 26.65 gal/min.
Storm rainfall depth
30 minutes. Default preview: 1.5 in.
Storm runoff volume
coefficient applied. Default preview: 1,599 gal.
Directed to storage
50% entered. Default preview: 799 gal.
Runoff coefficient
imperviousness assumption. Default preview: 0.95.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Imperial flow uses roof area × rainfall intensity × 0.0103896 × runoff coefficient in gallons per minute. Metric flow uses area × intensity ÷ 3600 in litres per second.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 29, 2026
Use this calculator to estimate roof runoff flow from projected drainage area, rainfall intensity and runoff coefficient. It supports preliminary gutter, drain and storage comparisons but not final hydraulic sizing.
Calculation steps
- 1Determine the plan area draining to each outlet.
- 2Use local design rainfall intensity for the required duration.
- 3Apply an appropriate runoff coefficient.
- 4Compare flow with gutter, downspout, drain and overflow capacity.
Worked numerical example
A 3,000 ft² roof under 3 in/hr rainfall with a 0.95 coefficient produces about 88.8 gpm of runoff.
Purchasing and rounding
Flow results can inform the number of outlets or components, but product capacity must come from manufacturer data and applicable design criteria.
Interpret the result
Dividing the total roof among more outlets reduces flow per outlet only when drainage paths are actually separated.
Page-specific assumptions
- Uniform rainfall.
- All mapped roof area contributes.
- No blockage or ponding.
- Coefficient represents the roof surface.
Common mistakes
- Using sloped roof surface instead of horizontal projected area when the method requires plan area.
- Using rainfall depth instead of intensity.
- Ignoring overflow paths.
- Selecting drain size from flow alone.
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
Roof Drainage Calculator questions
Should sloped roof area or horizontal area be used?+
Rainfall is normally collected over the horizontal projected area because rain depth is measured vertically.
Why include a runoff coefficient?+
It accounts for small losses such as wetting and minor retention before water reaches the drainage system.
Does this size gutters?+
It estimates peak flow and downspout count. Gutter profile, slope, outlets and local design rules still need verification.