Free tool
Rainfall & Runoff Calculator
Turn roof, driveway and yard area into gallons per storm and peak flow, with a pipe size suggestion.
Four quick steps.
- 1
Enter your areas
Roof, driveway, patio and lawn square footage.
- 2
Choose the storm
Total rainfall for volume, and hourly intensity for peak flow.
- 3
Read the gallons
See how much water each surface sends your way.
- 4
Check the pipe
A suggested pipe size for a given slope.
About this tool.
What formula is this?
Volume is area times rainfall times 0.623 gallons, adjusted by a runoff coefficient. Peak flow uses the rational method, and pipe capacity uses Manning’s equation.
What rainfall should I use?
A heavy local storm is often around 3 to 4 inches per hour in short bursts. We use sensible local values when we design.
Does this replace an engineer?
No. It is a planning tool.
Related services.
YardDownspout DrainsBury your downspouts and send roof water far from the foundation to a safe discharge.Learn more 
YardChannel DrainsA long, slim grate that catches surface water across driveways, patios, walkways and garage doors.Learn more 
YardDry WellsAn underground storage pit that holds storm water and releases it slowly into the soil.Learn more