Downspout Drainage Calculator

Overview

Multiplies number of downspouts by run length for pipe, fittings, outlet hardware, and the tool’s planning drop.

Assumptions

• Each run has the same length.
• The tool calculates drop at 1/8 in per foot; local requirements or site design may differ.

Worked example

Two 20-ft runs need 40 ft of pipe; each run needs 20 × 0.125 = 2.5 in of drop.

Common mistakes

• Combining roof water with a perforated collection run.
• Pointing an outlet toward a neighbor or foundation.

Limitations

Roof area, rainfall intensity, soil capacity, and outlet approval are outside the estimate.

Safety and local requirements

Call before digging and check local rules on stormwater discharge and connections.

Sources

• EPA — Soak Up the Rain