Gutters and downspouts
Gutter maximum roof area calculator
Estimate maximum roof area per gutter. Comparing gutter options.
Try the calculation
Convert capacity and rainfall into maximum plan catchment area.
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
Maximum plan area converts a candidate gutter capacity into catchment area at a rainfall intensity.
Steps
- Enter capacity and rainfall intensity.
- Use a capacity value supported by the product or design reference.
The form starts with the worked example below. Change its values and selections to match your task; Reset restores the example.
Formula
maximum area = capacity × 43,200 ÷ intensity.
Variables and units
- Gutter capacity (cfs)
- Required. Use a positive value. Starting example: 0.18. Replace it for your task.
- Rainfall intensity (in/hr)
- Required. Use a positive value. Starting example: 6. Replace it for your task.
What the result means
It is a theoretical plan-area comparison under the entered intensity and capacity.
Worked example
Inputs: Gutter capacity = 0.18; Rainfall intensity = 6.
Calculation: 0.18 cfs × 43,200 ÷ 6 in/hr = 1,296 sq ft.
Result: 1,296 sq ft max plan area.
Frequently asked questions
Does it include a safety factor?
No.
Can intensity be zero?
No, the conversion requires a positive intensity.
Is the result sloped roof area?
No, it is plan catchment area.