Gutters and downspouts
Downspout capacity/count calculator
Estimate downspouts by hydraulic capacity. Sizing a gutter system.
Try the calculation
Compare runoff with one downspout 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
Downspout capacity count divides modeled runoff by one downspout capacity.
Steps
- Enter runoff in cfs and the capacity of one downspout.
- Use the same flow basis for both.
The form starts with the worked example below. Change its values and selections to match your task; Reset restores the example.
Formula
downspouts = ceiling(runoff ÷ capacity).
Variables and units
- Runoff (cfs)
- Required. Zero is accepted. Starting example: 0.167. Replace it for your task.
- Downspout capacity (cfs)
- Required. Use a positive value. Starting example: 0.11. Replace it for your task.
What the result means
The result is a capacity comparison, not a placement or overflow design.
Worked example
Inputs: Runoff = 0.167; Downspout capacity = 0.11.
Calculation: 0.167 cfs ÷ 0.11 cfs = 1.518, so 2 downspouts.
Result: 2 downspouts.
Frequently asked questions
Can runoff be zero?
Yes, the arithmetic allows a zero demand.
Does it choose downspout diameter?
No.
Should the result be rounded down?
No, a partial required route still needs another downspout.