Roof runoff formula
Gallons = roof footprint x rainfall inches x 0.623 x runoff factor. Use the horizontal footprint of the roof section, not the sloped roof surface.
RainYard Tools
Downspout volume
Roof Runoff Calculator
Convert roof area and rainfall into gallons and cubic feet of runoff.
Before you calculate
Use it when sizing a barrel, cistern, dry well, or rain garden for one roof face.
- Use horizontal roof footprint, not roof slope.
- One inch on 1,000 sq ft is about 623 gallons.
- Metal and shingle roofs usually shed most rainfall.
Gallons710 galEstimated water volume.
Cubic feet94.9 cu ftUseful for storage sizing.
55-gal barrels12.9Rain barrel equivalent.
1
Measure length times width under the roof section.
2
Enter storm depth in inches.
3
Use the gallons result for storage planning.
Example
A 1,200 sq ft roof section in a 1 inch storm produces about 710 gallons at a 95% runoff factor.
Split by downspout
If one roof has several downspouts, estimate the area feeding each one. Storage and overflow are usually planned downspout by downspout.
Why roof runoff matters
Roof water is often the easiest source to capture, redirect, or spread into a planted area before it becomes a yard problem.
Questions
Common checks.
Do I include roof pitch?
For rainfall volume, use the horizontal footprint. Rain falls on the projected area.
What runoff factor should I use?
For early roof planning, 90-95% is a reasonable screen.