Gutters and downspouts
Downspout extension calculator
Estimate downspout extension length. Discharge away from foundation.
Try the calculation
Add the measured segments from one outlet to its discharge point.
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 extension quantity adds route segments and rounds the total to stock pieces.
Steps
- Enter one or two measured route segments and the stock-piece length.
- Enter zero for the second segment when unused.
The form starts with the worked example below. Change its values and selections to match your task; Reset restores the example.
Formula
total length = segment 1 + segment 2; pieces = ceiling(total length รท piece length).
Variables and units
- Route segment 1 (ft)
- Required. Use a positive value. Starting example: 20. Replace it for your task.
- Route segment 2 (ft)
- Required. Zero is accepted. Starting example: 4. Replace it for your task.
- Stock-piece length (ft)
- Required. Use a positive value. Starting example: 10. Replace it for your task.
What the result means
It estimates extension stock only. It does not design discharge or infiltration.
Worked example
Inputs: Route segment 1 = 20; Route segment 2 = 4; Stock-piece length = 10.
Calculation: 20 ft + 4 ft = 24 ft; at 10 ft stock pieces, 3 pieces.
Result: 24 linear ft.
Frequently asked questions
Can the second segment be zero?
Yes.
Does it include elbows?
No.
Should the route reach a safe discharge point?
The physical layout still requires that decision.