Gutters and downspouts
Gutter capacity calculator
Check gutter flow capacity. Existing or proposed gutter.
Try the calculation
Compare runoff demand with the candidate capacity.
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
Gutter capacity compares runoff demand with a candidate flow capacity.
Steps
- Enter plan area, rainfall intensity and candidate capacity in cubic feet per second.
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; utilization = Q ÷ capacity × 100.
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.
- Gutter capacity (cfs)
- Required. Use a positive value. Starting example: 0.18. Replace it for your task.
What the result means
It compares arithmetic demand to a supplied capacity; it does not verify the full drainage assembly.
Worked example
Inputs: Plan roof area = 1200; Rainfall intensity = 6; Gutter capacity = 0.18.
Calculation: 1,200 sq ft × 6 in/hr ÷ 43,200 = 0.1667 cfs; against 0.18 cfs, utilization is 92.6%.
Result: 0.17 cfs runoff.
Frequently asked questions
What does “within” mean?
The calculated demand does not exceed the entered candidate capacity.
Can rainfall intensity be zero?
Yes for a base arithmetic check, but it is not a design storm.
Does this check downspouts?
No, use the downspout capacity page separately.