Gutters and downspouts
Roof runoff rate calculator
Estimate runoff rate from roof area. Gutter sizing pre-check.
Try the calculation
Convert roof catchment and rainfall into flow.
Adjustable example. Change these illustrative inputs to match your task, then recalculate. Reset restores the example and its result.
Understand the result
How this calculation works
Roof runoff rate converts plan area and rainfall intensity to flow.
Steps
- Enter plan roof area and rainfall intensity.
- Use the result as a flow comparison for a stated condition.
The form starts with the worked example below. Change its values and selections to match your task; Reset restores the example.
Formula
Q = area × intensity ÷ 43,200 cfs; gpm = Q × 448.8.
Variables and units
- Plan roof area (sq ft)
- Required. Use a positive value. Starting example: 1200. Replace it for your task.
- Rainfall intensity (in/hr)
- Required. Zero is accepted. Starting example: 6. Replace it for your task.
What the result means
It is a rainfall-flow conversion. It does not size the complete gutter and downspout system.
Worked example
Inputs: Plan roof area = 1200; Rainfall intensity = 6.
Calculation: 1,200 sq ft × 6 in/hr ÷ 43,200 = 0.1667 cfs; × 448.8 = 74.8 gpm.
Result: 0.17 cfs.
Frequently asked questions
Why is plan area used?
The conversion is based on horizontal catchment area.
Can intensity be zero?
Yes for a zero-flow arithmetic check.
Does this prove a gutter will carry the flow?
No, compare with a documented capacity and layout.