How is runoff estimated?
This calculator uses the rational method, the standard quick estimate for small sites. It multiplies the area by how hard it is raining and by a runoff coefficient: the share of rain that runs off instead of soaking in.
Peak flow = coefficient × rainfall intensity × area
A 1,500 ft² roof (coefficient 0.95) in a 2 in per hour storm sheds about 29 gallons per minute. A 4 in pipe at a 1% fall carries roughly 75 gallons per minute, so it has room to spare.
Planning the rest of the job? Use the French drain calculator and the Landscape fabric calculator too.
Runoff coefficients
How do I choose a rainfall intensity?
Use the heaviest rain you expect over a short period. Many US regions plan for 2 to 4 in per hour, which is 50 to 100 mm per hour. Your local building department or weather service can give the design figure for your area.
Pipe sizes here assume smooth plastic pipe laid at a 1% fall. They are a starting point, not an engineering design. For anything near a house foundation, get a professional opinion.